Biological Science, 6th Edition Scott Freeman Solution Manual.pdf (22)

Give the students about 10 minutes to come up with their answers. The answers should be relatively basic. Ask each group to present its explanations to the class.

• Nitrogen (N): Plant growth is stunted, particularly the growth of lateral shoots, so the plants become spindly. All leaves become light green to yellowish (chlorosis), and older leaves fall off early.

Sample answer: Nitrogen is a necessary component of amino acids and is therefore a prerequisite for new protein synthesis. Lack of nitrogen will mean that the plant cannot make new proteins, including enzymes. This will result in an overall slowing of plant growth.

The leaves experience chlorosis because without enzymes chlorophyll cannot be made.

• Phosphorus (P): Plant growth is stunted, particularly the growth of lateral shoots, so the plants become spindly. Leaves are dull dark green to bluish-green with purple petioles and veins.

• Potassium (K): The leaves show chlorosis (loss of green color) and brown/black spots on their edges. These symptoms appear first on older leaves and then progress to younger leaves as the deficiency becomes more severe.

• Calcium (Ca): New growth begins to die back. Leaves show slight chlorosis (loss of green color, or change in color), and new leaves may show brown or black scorch marks.

• Magnesium (Mg): Leaves look marbled since they are experiencing chlorosis (loss of green color) between the leaf veins. These yellow patches start in the center and move to the edges. When the deficiency is severe, black or brown scorch marks appear on the leaves.

• Sulfur (S): Older leaves appear yellowish green, and stems become thin and hard. Some plants show orange and red tints rather than yellowing. The stems, though hard and woody, increase in length but not in diameter.

Note: All of the answers are similar to the sample answer provided for nitrogen. Students should realize that, no matter what specific role each nutrient plays, all ultimately allow for normal cellular function; therefore, without them, growth and photosynthesis ultimately suffer.

Debating the Benefits of Genetically Modified Foods Estimated duration of activity: 15–20 minutes Protein starvation is the most common type of malnutrition in the world. Agricultural research has brought about hybrid forms of rice, corn, and wheat that have increased amounts of protein. These crops require more nitrogen for growth, which is usually provided via fertilizers. Making these fertilizers is costly and requires large amounts of energy in the form of fossil fuels. One alternative to using fertilizers is enabling plants to fix nitrogen on their own.

Researchers have identified the gene for nitrogenase and are working to genetically engineer plants that can make nitrogenase and fix nitrogen on their own. Have students debate the ethics of using such genetically engineered plants to feed starving populations in developing nations. Students will likely need a few days to research the issue on the Internet and decide what position to take. If students decide to argue against the use of genetically modified plants for this application, ask them to come to the debate ready to discuss alternatives.

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In-class Challenge Questions Discussion Ideas for All Class Sizes Here are three suggestions for in-class challenge questions.

• What is acid rain, and how does it influence the availability of soil nutrients?

Answer: Acid rain is atmospheric moisture that has a pH considerably lower than that of unpolluted rain. The burning of fossil fuels releases sulfur dioxide and nitrogen oxides, which react with water vapor to form sulfuric and nitric acids that are deposited in rain, snow, and fog. Acid rain can affect plants in several ways—direct damage to exposed plant parts (leaves, flowers, fruits, and so forth) and indirect damage due to the changing availability of soil nutrients. Direct damage to leaves, for example, can occur when acid solutions enter through stomata or thin cuticle and cause decreased photosynthesis and cell death. As the soil acidity (H+ concentration) increases due to acidic rainwater, the exchange of cations (calcium, magnesium, potassium) from soil particles occurs, and they are more readily leached from the soil. Removal of these important mineral nutrients results in plant deficiencies. Furthermore, increased acidity alters the solubility of other soil components (particularly heavy metals such as aluminum, lead, and mercury), increasing their concentration to potentially toxic levels. The effects of altered nutrients in soil are loss or damage to foliage, reduced root growth, death of roots, decrease in mycorrhizae, inhibition of seed germination, and so forth.

Note: This topic could lead to a much broader discussion on the buffering effects of soil ions, and how altering the pH leads to cation exchange and alteration of ion solubility. It could also lead to an even broader discussion about the outcome of acid rain on ecosystems downstream of forests—increased levels of leached cations can lead to eutrophication in lakes and streams, to subsequent effects on algal growth, to declines in fish populations in those bodies of water, and so on.

• The diversity in root systems enables plants living in the same environment to tap different nutrient sources. Plants with shallow, fibrous roots obtain nutrients on the upper surface of the soil; those with deeper taproots can obtain nutrients farther down in the soil layers.

How would you determine whether plants draw on different nutrient sources in soil?

Answer: In the laboratory, germinate seeds of plants from the same habitat that have different types of root systems. Grow the seedlings in soil from their native habitat. Inject radioactive phosphorus into the lowest soil layer where roots are growing. Measure the concentration of accumulated isotopes in the different species. Repeat the experiment using different isotopes and injecting them into different soil layers and measuring uptake.

This should tell you whether the plants draw on different sources in soil.

• Part 1. Design an experiment to determine the concentrations of nitrogen, magnesium, and calcium in a plant’s leaves, twigs, and wood. Would you expect the concentrations to vary in the different parts of the plant body? Why? Answer: Obtain a plant to sample, and determine the total dry weight of the intact plant including both root and shoot systems. Take another sample; separate the leaves, twigs, and wood; and determine the percentage of the total dry weight of nitrogen, magnesium, and calcium in each of the three plant parts. This will tell you whether there is a difference in the concentration of these nutrients in different parts of the plant.

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Yes, you would expect differences to occur in each of these nutrients because the plant parts have different functions. For example, because leaves carry out photosynthesis and Mg++ is needed as a component of the chlorophyll molecule, you would expect that concentrations of Mg++ would be higher in leaves than in twigs or wood.

• Part 2. Using the experimental design you developed in part 1, calculate the concentrations of these nutrients in three different plant species that inhabit the same environment.

Then calculate their percentages in the leaves, twigs, and wood. Assume that one is a deciduous canopy tree, one an understory evergreen tree species, and the third a deciduous shrub. Would you expect the nutrient concentrations to vary in the different plant species?

Why? Answer: You would anticipate that the concentrations will vary depending on the type of plant. The canopy tree has a larger mass and would be expected to have a higher concentration of nitrogen. However, the deciduous shrub may be expected to have a higher concentration of Mg++ because it has a high concentration of chlorophyll in its short-lived leaves.

Note: Discussion could lead into topics such as nutrient cycling in ecosystems, competition among organisms for limited resources, the role of fire in succession, and so forth.

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CHAPTER

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Plant Sensory Systems, Signals, and Responses

Learning Objectives: Students should be able to…

• Describe how plants detect sensory information.

• Explain the connection between signal transduction and hormone production in plants.

• Summarize how hormones regulate plant growth and responses to the environment.

• Explain how plants defend against pathogens and herbivores.

Lecture Outline I. Information Processing in Plants A. Plants gather, process, and respond to the information that they monitor in three steps: (Fig. 37.1) 1. A receptor cell receives an external signal and transduces it to an internal signal.

2. The receptor cell sends a signal to cells in another part of the plant body that can respond to the information.

3. Responder cells receive the long-distance signal, transduce it to an intracellular signal, and change their activity in response to it.

B. How do cells receive and process an external signal?

1. In a process called signal transduction, a receptor changes shape in response to a stimulus and then the information changes form—from an external signal to an intracellular signal. (Fig. 37.2) 2. Target cells receive information from several different hormones at the same time, so it is common for different types of hormones to interact with one another and modulate the cell’s response.

3. Plant hormones have common properties. a. They can elicit a response only if the target cell has the appropriate receptor. b. They are active at extremely small concentrations.

C. How do cells respond to cell-cell signals?

1. A receptor, usually a protein on the cell membrane, binds to the hormone and changes shape.

2. The change in shape of the receptor triggers the signal transduction pathway inside the cell that ultimately changes the cell’s activity.

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3. There are two basic types of transduction pathways: a. Phosphorylation cascades involve the serial activation of protein kinases. b. Second messengers are intracellular signals that are released upon hormone binding.

4. Signal transduction cascades also amplify the signal.

5. Activation of signal transduction cascades may result in changes in cell activity: a. Activation of membrane transport proteins b. The translation machinery c. Changes in gene expression—transcriptional activators or repressors 6. When cells respond to a hormone, the change in their activity helps the individual cope with the environmental change sensed by the receptor cell.

II. Blue Light: The Phototropic Response A. Phototropism is a plant’s directed movement in response to light.

1. Plants sense and respond to only a few wavelengths of light.

2. Charles and Francis Darwin studied phototropism in canary grass.

3. They found that young shoots bend toward the light. (Fig. 37.3) a. Grass shoots grown behind a solution of potassium dichromate did not show phototropism. b. Potassium dichromate filters out wavelengths in the blue part of the spectrum. c. Plants move toward blue light because it is important for photosynthesis.

B. Phototropins as blue-light receptors 1. Researchers assumed that the blue-light receptor must be a pigment that absorbs blue light.

2. A membrane protein was discovered that is phosphorylated in the presence of blue light.

3. A blue-light receptor gene was isolated in Arabidopsis thaliana. a. Experiments with nonphototrophic hypocotyl mutants of A. thaliana led researchers to the gene. b. The gene is called PHOT1 and a mutant plant without PHOT1 was produced, which does not respond to light. (Fig. 37.4) 4. More recent studies have found more blue-light receptors that are related to PHOT1, collectively called phototropins.

5. Other blue-light receptors mediate other responses to sunlight: a. Stem elongation in shady conditions and flower induction b. The opening of stomata in the presence of light C. Auxin as the phototropic hormone 1. The Darwins cut the tips off of the coleoptiles they studied and found that phototropism stopped. (Fig. 37.5) a. Putting opaque covers on coleoptile tips also eliminates phototropism, but opaque collars do not. b. Their hypothesis was that a substance produced at the tip of the coleoptile acts as a signal and is transported to the area of bending.

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2. Peter Boysen-Jensen tested the hormone theory. a. He cut off the tips of oat shoots and placed porous gelatin or nonporous mica between the tip and shoot. b. Only shoots treated with the porous gelatin showed normal phototropism.

(Fig. 37.6a) c. He concluded that the phototropic signal is a diffusible water-soluble chemical.

3. Frits Went tested whether the hormone can cause bending in darkness. a. He collected hormone from oat shoots in agar blocks. b. Agar blocks with hormone were placed asymmetrically on decapitated shoots in darkness. c. The shoots bent away from the side where the agar block, the source of hormone, was placed. (Fig. 37.6b) d. Follow-up research illustrated that phototropism is the result of hormone-induced cell elongation in shoot cells on the side opposite a light source. e. Went named this phototropic hormone auxin. f. Auxin was characterized and found to be indole acetic acid (IAA). g. IAA is found in very low concentrations in plant tissue.

4. The Cholodny-Went hypothesis a. Auxin produced in shoot tips moves from the sunny side to the shady side of the shoot in response to light. b. Auxin is transported down the shoot. c. The auxin concentration is higher in cells on the shady side than in cells on the sunny side. d. Cells with a higher auxin concentration elongate more than those with a lower concentration, and bending toward light occurs. (Fig. 37.6c) e. Winslow Briggs confirmed the auxin redistribution hypothesis.

(1) Shoot tips were divided completely or partially, with nonporous mica placed on agar blocks and exposed to light from one direction. f. The results support the hypothesis that light leads to the redistribution of auxin and increased bending on the shady side of the shoot.

5. The cell-elongation response a. Auxin-receptor binding leads to water influx and cell-wall expansion, which result in elongation. b. The acid-growth hypothesis proposes that auxin triggers the production or activation of proton pumps, H+-ATPases. (Fig. 37.7) (1) Protons are pumped out of the cell; positively charged ions, such as K+, enter the cell.

(2) The ion concentration inside the cell increases; water enters by osmosis.

(3) As water enters the cell, the turgor pressure increases and cell-wall expansion occurs via proteins called expansins. c. Cells do not move water directly; instead, they move ions and create an osmotic gradient that favors the movement of water.

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III. Red and Far-Red Light: Germination, Stem Elongation, and Flowering A. The red/far-red “switch” 1. Red light, like blue light, drives photosynthesis.

2. Far-red light is not absorbed strongly by photosynthetic pigments. a. Far-red light passes through leaves instead of being absorbed. b. Therefore, far-red light is prominent in light that is filtered through tree leaves before it reaches the forest floor. c. Far-red light is an indicator of shade.

3. Red and far-red light act like an on-off switch for lettuce seed germination.

(Table 37.1) a. Red light triggers germination. b. Far-red light inhibits germination.

B. Phytochrome is a red/far-red receptor.

1. Researchers hypothesized that one pigment sensed both red and far-red light.

2. They hypothesized that the receptor exists in two forms: one that absorbs red light and one that absorbs far-red light.

3. This ability to switch sensitivity to different wavelengths is called photoreversibility.

4. Phytochrome, a light-receptor molecule, was hypothesized to be the pigment responsible for the on-off effect of the red/far-red switch. a. Phytochrome is a protein that exists in two forms: (Fig. 37.8) (1) Pr absorbs red light.

(2) Pfr absorbs far-red light. b. Photoreversibility between the two forms occurs.

5. Phytochrome plays a role in other plant responses controlled by light including circadian rhythms and etiolation.

6. Etiolation is the process by which plants become pale yellow and lengthen their stem.

(Fig. 37.9) C. Signals that promote flowering 1. Photoperiodism is plants’ response to the relative lengths of day and night.

2. Responding to changes in photoperiod a. Plants sense photoperiod in order to flower when pollinators are available.

(1) Long-day plants bloom in midsummer, when days are longer than nights.

(2) Short-day plants bloom in spring, late summer, or fall, when days are shorter than a species-specific critical length.

(3) Day-neutral plants flower without regard to photoperiod. b. In photoperiod/flowering experiments, night interruptions with red light disrupted the photoperiod; however, subsequent interruptions with far-red light erased the effect. (Fig. 37.10) c. Clock proteins rise during the day and trigger the expression of a gene called CONSTANS (CO).

(1) In long-day plants, high levels of CO stimulate the production of the flowering hormone.

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(2) In short-day plants, high levels of CO inhibit the production of the flowering hormone.

3. Discovery of the flowering hormone a. Grafting experiments show that a plant can be made to flower during the incorrect time by grafting a plant with a different photoperiod. (Fig. 37.11) b. The FLOWERING LOCUS T (FT) gene is known to promote flowering. c. When researchers exposed leaves of Arabidopsis thaliana to short nights, they found that this gene was expressed in vascular tissue. d. Researchers have also found that the protein product of the FT gene is transported from leaves to the shoot apical meristem.

IV. Gravity: The Gravitropic Response A. Experimental background 1. Charles and Francis Darwin conducted experiments on gravitropism in 1881. a. They removed the root caps and found that the roots no longer responded to gravity. b. They concluded that the gravity sensor is in the cap.

2. Recent studies have identified specific root cap cells that sense gravity. a. These studies used the laser to identify specific root cap cells that sense gravity in Arabidopsis. b. The data indicate that cells directly under the epidermal cells of the root tip are responsible for gravitropism. (Fig. 37.12) B. The statolith hypothesis 1. Many plant cells contain amyloplasts, starch-containing plastids.

2. Researchers hypothesize that the amyloplasts are pulled to the bottom of cells by gravity. (Fig. 37.13) a. The force of amyloplasts on the cell membrane or distention of the amyloplast membrane activates receptors that initiate the gravity response. b. This mechanism is similar to the sand-grain statoliths in animals.

3. Although recent experimental evidence strongly supports the statolith hypothesis, the search for the gravity receptor continues.

C. Auxin as the gravitropic signal 1. Root cap cells that sense changes in the direction of gravitational pull respond by changing the distribution of auxin in the root tip.

2. Auxin flows down the middle of the root and then toward the perimeter and away from the root cap. (Fig. 37.14) 3. There is more auxin at the bottom, causing the top cells to elongate and bend the root tip downward.

V. How Do Plants Respond to Wind and Touch?

A. Changes in growth patterns 1. Plants respond to wind or touch by increasing stem thickness and not growing as tall—thigmomorphogenesis. (Fig. 37.15) B. Movement responses 1. The response to touch, called thigmotropism, can happen quickly. (Fig. 37.16)

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2. Touch is converted to an electrical signal. a. Proton pumps create a charge difference between the interior and exterior of plant cells.

(1) The interior has a negative charge compared to the exterior.

(2) The charge difference across the cell membrane is membrane polarization. b. The charge difference across the membrane creates membrane voltage, a form of potential energy.

3. In the Venus flytrap, action potentials reach the outer surface and the cells change shape to push the trap shut—thigmonastic movements. (Fig. 37.17)

VI. Youth, Maturity, and Aging: The Growth Responses A. Auxin and apical dominance 1. Apical dominance refers to stem elongation at the apical meristem of the main shoot, while the meristems in the lateral buds below are inhibited. (Fig. 37.18) a. When the apical meristem of the main shoot is removed, branches grow out from the axillary buds. b. If auxin is added to the end of a decapitated shoot, lateral branch growth is inhibited. c. Auxin is a chemical signal from the shoot tips that determines the direction of growth. d. Disruption of the chemical signal when the tip is removed indicates an interruption of growth, and lateral branches take over for the main shoot.

2. Polar transport of auxin a. Auxin is transported in one direction from the shoot tip downward to the root tip. b. The unidirectional movement of auxin is known as polar transport.

(1) Polar transport sets up a strong concentration gradient for auxin in the plant body.

(2) Auxin is much more concentrated in shoots than in roots.

3. What is auxin’s overall role? a. Auxin plays an important role in phototropism, gravitropism, and apical dominance. b. Auxin is also involved in other processes.

(1) Seeds produce auxin to stimulate fruit development.

(2) Decreasing concentrations of auxin are involved in the falling of leaves (abscission) associated with aging (senescence).

(3) Auxin is responsible for the proper differentiation of xylem and phloem in vascular tissue.

(4) Auxin stimulates the development of adventitious roots in cuttings. c. Auxin concentration identifies cell location in relation to the long axis of the plant.

B. Cytokinins and cell division 1. Cytokinins are a group of plant hormones that promote cell division.

2. The discovery of cytokinins a. Coconut milk was shown to promote the growth of cells and plant embryos in culture.

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b. Later experiments with corn and apples showed that a molecule derived from the amino acid adenine could stimulate growth. c. Cytokinins are synthesized in root tips, young fruits, seeds, growing buds, and other developing structures. d. Cytokinins can be used to promote the growth and differentiation of cells in culture.

(BioSkills 11) 3. How do cytokinins promote cell division? a. Because cytokinins stimulate mitosis, scientists hypothesized that they may be activating cyclins and cyclin-dependent kinases that initiate mitosis. b. Arabidopsis cells were cultured so that their growing environment could be controlled.

(1) The cells were starved of cytokinins for a day and then treated with a hormone.

(2) The presence of the hormone induced an increase in the CycD3 gene that encodes for a cyclin.

(3) This is all strong evidence that cytokinins regulate growth by activating genes that keep the cell cycle going.

C. Gibberellins and ABA: growth and dormancy 1. Many plants initiate growth in the spring, grow during the summer and fall, but lie dormant during the winter.

2. Two hormones control dormancy and growth: a. Gibberellins stimulate growth. b. Abscisic acid inhibits growth and promotes dormancy.

3. The discovery of gibberellins a. Rice seedlings infected with the fungus Gibberella fujikuroi elongate abnormally. b. Fungal extract, named gibberellin, induces elongation. c. Plants naturally produce gibberellin, although they elongate excessively when additional gibberellin is applied.

4. Defective gibberellin genes cause dwarfing a. Forward genetics was used to identify the gene locus responsible for the mutant dwarf phenotype. (Fig. 37.19) b. Mendel’s studies of dwarf and tall pea plants indicated that a single locus was involved. c. Mutants at the Le locus grow to normal size when treated with gibberellin GA1.

(1) This suggested that, although dwarf plants respond to GA1, they are unable to make their own.

(2) Dwarf plants treated with the radioactive precursor to GA1 do not synthesize GA1.

(3) The Le locus appears to be involved in GA synthesis.

5. Gibberellins and ABA interact during seed dormancy and germination. a. The enzyme α-amylase is released from the aleurone layer of germinating oat and barley seeds. b. α-Amylase diffuses to the endosperm tissue, where it breaks bonds in the starch molecules and releases sugars. (Fig. 37.20)

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c. Gibberellic acid (GA) added to the aleurone layer increases α-amylase production, thus promoting seed germination. d. Abscisic acid (ABA) decreases α-amylase levels when added to the aleurone layer, thus inhibiting seed germination.

6. ABA closes guard cells in stomata. a. Guard cells close when plant roots are unable to replace the water lost from the leaves. b. Studies with ABA indicate that guard cells close when ABA is applied to their exterior. c. Researchers designed an experiment to determine whether ABA regulates stomatal closing. (Fig. 37.21) (1) Experimental plants were grown in a pot with roots divided into two parts—half the roots received water, and the other half was allowed to dry.

(2) The water potential in the leaves of both control and experimental plants was the same.

(3) The stomata of experimental plants closed, even though the leaves were not experiencing a water shortage.

(4) Roots from the dry side of the pot signaled a decrease in available water.

(5) ABA levels in roots on the dry side of the pot were high, as was ABA concentration in the leaves of experimental plants compared to control plant leaves.

(6) Conclusion: ABA is the signal carried from roots to leaves, where the response to water shortage results in guard cell closure.

7. The molecular mechanism of guard cell closure a. Stomata consist of a pore surrounded by two guard cells that can change shape to open or close the pore. b. When guard cell vacuoles are full of water, they are turgid and take on a curved shape that opens the pore, allowing for gas exchange and transpiration. c. When the vacuoles of guard cells lose water and are flaccid, they close the pore. d. Activation of PHOT causes water to enter guard cells. (Fig. 37.22a) (1) Activation of PHOT by blue light leads to an increase in activity of an H+ATPase that pumps protons out of each guard cell.

(2) This creates a strong electrochemical gradient that triggers the entry of potassium and chloride into the cell.

(3) This, in turn, lowers the water potential in the guard cell, and water enters via osmosis. e. Activation of ABA receptors on guard cells leads to water flowing out of the cell.

(Fig. 37.22b) (1) In response to ABA, calcium ions are released from intracellular storage.

(2) This inhibits the H+-ATPases and potassium channels.

(3) It also creates an electrochemical gradient that sends chloride and other anions outside the cell.

(4) When the anions leave, the membrane potential changes, potassium channels open, and potassium leaves the cell.

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(5) This lowers the water potential outside the guard cell and water flows out, making the cells flaccid.

D. Brassinosteroids and body size 1. Growth spurts are triggered by surges in steroid hormones called brassinosteroids.

2. These hormones were originally discovered in Brassica napus, a crop plant that is the source of canola oil.

3. They are steroids.

4. They promote growth and regulate overall body size. Plants that cannot synthesize brassinosteroids are dwarfed. (BioSkills 11) 5. In contrast to other steroid molecules that bind to intracellular receptors, brassinosteroids bind to receptors on the membrane surface.

6. The genes for the synthesis of brassinosteroids appear to be homologous with genes required for steroid hormones in animals. They are derived from a similar gene in a common ancestor of plants and animals.

E. Ethylene and senescence 1. Senescence is the process of aging, decline, and eventual death of any part of a plant.

2. Senescence is triggered by interactions among several different hormones in response to changes in light, temperature, and other conditions.

3. Ethylene is the hormone most closely associated with senescence. a. It is a gas that is active at small concentrations. b. Ethylene is synthesized from the amino acid methionine.

4. Ethylene is involved in many plant processes: a. Fruit ripening, which at its extreme leads to fruit rotting and death b. Fading of flowers c. The abscission or detachment of leaves d. A plant’s stress reaction in response to drought 5. The discovery of ethylene a. Chinese fruit growers noticed that burning incense caused pears to ripen faster. b. People noticed that plants growing near gas street lamps lost their leaves faster. c. Biologists found sharp increases in ethylene production during the ripening of the fruits of many different plants.

6. Ethylene and fruit ripening a. Ethylene triggers fruit ripening by inducing the production of enzymes that: (1) Convert stored starch to sugar (2) Remove protective toxins (3) Break down and degrade cellulose to soften cell walls (4) Break down chlorophyll (5) Produce pigments and aromas that signal ripeness b. Fruit growers manipulate ethylene levels to control the life of fruit. c. Green bananas were picked and transported, then ripened quickly by exposure to ethylene. (Fig. 37.23) d. Apples are picked ripe and stored in warehouses that have low oxygen levels to prevent the production of ethylene, thus extending their shelf life. .

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7. Ethylene and leaf abscission a. Leaf senescence and abscission involve interactions between auxin and ethylene.

(Fig. 37.24) b. Healthy leaves produce auxin until temperature, day length, or plant age inhibits its production. c. When auxin concentrations drop, the cells in the abscission zone of the leaf petiole become more sensitive to ethylene. d. Ethylene activates enzymes in these cells that weaken the cell walls at the base of the petiole to the point where it eventually falls from the plant. e. The application of cytokinins can extend the life of leaves.

F. An overview of plant growth regulators 1. Current research focuses on understanding plant hormone receptors.

2. Two key observations about the major plant hormones: (Table 37.2) a. It is common for a single hormone to affect many different target tissues. This means there can be an array of responses to the same cell-cell signal. b. Several hormones can affect the same response; hormones do not work independently but interact with one another.

VII. Pathogens and Herbivores: The Defense Responses A. Plants must be able to sense attacks by pathogens and stop those attacks.

1. Secondary metabolites are closely related compounds to key synthetic pathways and are commonly used for defense.

B. How do plants sense and respond to pathogens?

1. An evolutionary arms race a. If a pathogen evolves a mechanism to exploit its host more effectively, the host may evolve new abilities to detect the pathogen and defend itself from attack. In response, the pathogen may evolve a new strategy, and so on.

2. The hypersensitive response a. The events that take place within hours of an infection are termed the hypersensitive response (HR) b. The hypersensitive response protects plants from pathogens by triggering a cascade of signals that lead to: (Fig. 37.25) a. Stomatal closure—prevents pathogens from entering b. Production of toxin molecules c. Reinforcement of neighboring plant cell walls to contain pathogen d. Rapid suicide of cells in the infected region c. Students should be able to explain the adaptive value of the hypersensitive response despite death of plant cells.

3. An alarm hormone extends the HR. a. The HR triggers the release of a hormone that turns on a longer-lasting defense system called systemic acquired resistance (SAR). b. These events prepare the root and shoot cells for assault by the pathogen, even if they are not yet infected.

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c. The interaction between the HR and SAR involves pathogenesis-related (PR) genes.

(Fig. 37.26) (1) This signaling molecule is likely methyl salicylate (MeSA)—a molecule derived from salicylic acid.

C. How do plants sense and respond to herbivore attack?

1. The role of protease inhibitors a. Many different types of plants make proteinase inhibitors that block proteindigesting enzymes in animal mouths and ultimately make the animal sick.

2. The discovery of systemin a. Insect attack in one part of a plant induces the formation of proteinase inhibitors in other parts of the plant. (Fig. 37.27) b. The inducing hormone has been identified and named systemin. c. Systemin binds to a receptor on the membrane of an uninjured cell, inducing the production of proteinase inhibitors.

3. “Talking trees”: Responses from nearby plants a. When an insect munches on a leaf, volatile compounds are released and travel to nearby plants. b. Evidence is supporting the hypothesis that nearby plants sense these volatiles and produce defense compounds.

4. Pheromones released from plant wounds recruit help from wasps. a. Caterpillars may be infected with parasitoids (wasp larvae) that infect the caterpillar, hatch, and eat the caterpillar from the inside out. (Fig. 37.28) b. Parasitoid infections are common in herbivore outbreaks in croplands. c. Researchers predicted that plants release a substance that recruits parasitoids when a herbivore is present.

(1) They found that insect-damaged leaves produced 11 volatile chemicals that undamaged leaves did not produce.

(2) Other experiments indicated that wasps tended to fly toward insect-damaged leaves.

5. Plants, though stationary, constantly monitor a wide array of information about their environment. (Table 37.3)

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text.

ABP1 abscisic acid abscission abscission zone acid-growth hypothesis action potential

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aleurone layer amyloplast apical dominance auxin avirulence loci callus

circadian rhythm coleoptile cytokinins day-neutral plants dephosphorylation dormancy

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electrical signaling amyloplast ethylene expansins gene duplication gene families gene-for-gene hypothesis gibberellins gravitropism guard cells hypersensitive response inducible defences lateral buds lateral buds electrical signaling Le genes long-day plants membrane H+-ATPases membrane potential methyl salicylate Myb proteins

Northern blotting parasitoid pathogens Pfr pheromones phosphorylation phosphorylation cascade PHOT1 photoperiodism photoreceptor photoreversibility Pr phototropins phototropism PHYA/PHYB genes phytoalexins phytochrome pigments plasmodesmata polar transport polar transport action

protein kinase proteinase inhibitors reactive oxygen intermediates red/far-red root cap second messengers senescence senescence apical dominance short-day plants signal transduction statolith hypothesis statolith gravitropism statolith hypothesis stomata systemic acquired resistance systemin thigmomorphogenesis thigmonastic movements thigmotropism virulence

potential

Lecture Activities NASA Botanists for a Day This activity requires students to apply what they have learned about plant sensory systems to design a plant-growing chamber that astronauts could use to grow crops in space. Ask the students to get into groups of two to five students, depending on class size. Present the following challenges, and give students 10 to 15 minutes to draft a simple design. Then ask them to present and explain their design to the class. If you teach a very large class, you may find it more efficient to ask the students to present the design on a class website. Another time-saving alternative is simply to do the exercise together as a class with you facilitating the design.

Mission Space Garden You are a team of botanists hired by NASA to design a plant-growing chamber for use on the international space station. A new U.S. mission will launch soon to take a fresh crew of American astronauts to live on the space station. They are interested in growing some fresh vegetables there. Keep in mind that plants use cues from their earthly environment to grow.

These cues will be different or nonexistent on the space station. Your plant-growing apparatus must compensate for these differences.

Note: Give students the following hints and reminders before they begin.

1. There is no gravity in space; therefore, the direction of root growth could be affected, dirt and water will not stay in open containers, and so on.

2. The lights on the space station may not be enriched with the types of wavelengths that plants need to grow and photosynthesize.

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3. Plant hormones normally produced in response to light and gravity may not be produced in the same amounts or be transported through the plants as they would have been on Earth.

4. Ethylene, a gas hormone released by aging plants, will accumulate in an enclosed chamber.

5. Plants need a constant source of fresh carbon dioxide for photosynthesis.

The Ethylene Scrubber The following article discusses a technology invented to aid in the growth of plants in space that is now widely used in greenhouses, produce storage facilities, and florists. Post the link for this article on the class website and have the students read it and write a short essay describing how this technology has been used for applications beyond botany. Alternatively, you could ask the students to read the article and come to class prepared to discuss what the ethylene scrubber is, what its original application was, what it is widely used for now, and what applications it may be used for in the future. www.sciencedaily.com/releases/2002/04/020403030627.htm www.spaceref.com/news/viewpr.html?pid=7724 Note: These two links contain the same article.

The Biology of Gardening Estimated duration of activity: 5–10 minutes Students may find this chapter (and the previous chapter) interesting, since it may help them understand common gardening practices. You can run this as a class discussion, or you can make it a game by dividing the class into two teams and keeping score. A bag of candy is an easy and fun reward for the winning team. Explain why each of the following actions would be appropriate.

• Pruning: Cutting off the tip of a shoot will make the plant grow bushier.

Answer: The auxin produced in the shoot tip inhibits lateral bud growth. If the shoot tip is removed, then lateral growth will occur and the plant will be bushier.

• Forcing bulbs: Bulbs must be kept in a cool place for 12 weeks before they can be forced to bloom inside.

Answer: The bulbs have to have an extended period of cold to break dormancy.

• Growing vegetables from seed: Seeds must be planted in soil or another medium that can drain effectively. Then they must be watered thoroughly and allowed to drain.

Answer: The seed must be watered and allowed to drain in order to wash away the abscisic acid in the seed coat that is inhibiting germination.

• Keeping fruit from ripening: Fruit must be kept in a space that has a constant flow of oxygen.

Answer: Oxygen inhibits ethylene production by the fruit, which triggers ripening.

• Growing a tree from seed: Buckeye seeds must be dried until the seed coat cracks before they are planted.

Answer: The seed coat must be broken for water to enter the seed so that inhibition and germination can occur.

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In-Class Challenge Questions Discussion Ideas for All Class Sizes Here are some ideas for in-class discussion questions.

• When you plant seeds in the ground, they are not oriented in any particular direction. Why isn’t it important to position them with the basal portion of the seed down?

Answer: Because the roots are positively gravitropic, they will grow from a horizontal position down toward gravity. Also, because the shoot system is negatively gravitropic, it will grow away from gravity and will grow in an upward direction. So, it doesn’t matter how the seed is oriented when planted.

• California poppy flowers open during the day and close at night. Design an experiment to test whether this behavior is due to an internal clock or to light.

Answer: Grow poppy plants in darkness and see whether flower opening continues in a daily cycle. If it does, the effect is due to an endogenous clock. If this result is obtained, look for mutants in which flower opening is not on the same cycle as that of the wild-type poppies. If you find these mutants, try to isolate proteins that may have a role in setting the opening clock. If flower opening ceases in darkness, this suggests that light has an effect.

You may then ask: Is the effect due to varying the light intensity? Is the effect due to the absorption of specific light wavelengths? Both concepts can be readily tested in a lab.

• Some biologists refer to the organic substances produced in one plant part that are transported to another part where they have an effect as “plant growth regulators” rather than hormones. Why do you suppose they make such a distinction? How do plant hormones differ from animal hormones? What are the similarities between them?

Answer: Animal hormones are chemicals synthesized in a tissue or organ specialized for hormone production. They are transported by the bloodstream or endocrine system to another location where they have a unique effect. In plants, there are no specialized tissues or organs whose function is hormone synthesis. Plant hormones have multiple effects, rather than a single specific effect. For example, auxin plays a role in phototropism, gravitropism, cell elongation, apical dominance, promotion of lateral roots, promotion of fruit development, and so on. This is why plant hormones differ from animal hormones. They are similar in that both are organic chemical messengers produced in minute quantities and transported to a target site where they interact with receptor proteins.

• Why are synthetic auxins used as defoliants?

Answer: Since hormones create their effects at very low concentrations, plants must be able to carefully regulate the concentration of naturally produced hormones. They do so in various ways: by regulating their synthesis, breaking them down to an inert form, or temporarily inactivating them. But in the case of synthetic auxins, many plants cannot metabolize these substances as readily as naturally occurring ones, so the levels of synthetic auxins can be very high and can cause cell death at elevated concentrations.

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• Why are plant hormones essential for life in a terrestrial environment?

Answer: This question leads into a discussion of the differences between aquatic and terrestrial environments. Aquatic environments tend to be fairly stable in temperature, oxygen concentration, and acidity, whereas terrestrial environments tend to be variable in these conditions as well as in the availability of water, local temperature changes, variations in light quality and intensity, available nutrients, and so forth. Plant hormones enable plants to adjust their growth and differentiation to these continuously changing external conditions while the plants themselves remain fixed in position.

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Plant Reproduction and Development

Learning Objectives: Students should be able to…

• Use examples to explain the concept of alternation of generations.

• Describe the function of gametophyte structures in angiosperms.

• Describe how flowers are adapted to being pollinated by animals or wind.

• Explain how seeds transformed plant reproduction.

• Describe the process of embryogenesis in plants.

• Explain the function and importance of meristems, and the role of regulatory genes in controlling leaf shape.

• Evaluate evidence from flower homeotic mutants in support of the ABC model of flower formation.

Lecture Outline I. An Introduction to Plant Reproduction A. Asexual reproduction 1. Asexual reproduction leads to new individuals without meiosis and fertilization.

2. The offspring produced (clones) are genetically identical to the parent plant.

3. Plants reproduce asexually in various ways. a. Rhizomes are horizontal underground stems with nodes that produce shoots and roots. (Fig. 38.1a) b. Stolons, or runners, are aboveground stems that form new individuals. Example: corms (Fig. 38.1b) c. Plantlets are formed from meristematic tissue on the leaf margins of some plants.

(Fig. 38.1c) d. Some plants (including the dandelion) can generate seeds without undergoing fertilization (apomixis). (Fig. 38.1d) 4. The genetic similarity of asexually produced individuals makes them more vulnerable to disease-causing agents.

B. Sexual reproduction and the plant life cycle 1. Sexual reproduction involves two processes: a. Meiosis—a reduction division that generates haploid gametes 384

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b. Fertilization—fusion of gametes (1) Male gametes are sperm.

(2) Female gametes are eggs. c. Gametes are formed in flowers.

2. Sexual reproduction produces offspring that are genetically different from their parents. a. Even products of self-fertilization are different. b. However, outcrossing generates even more genetic diversity in offspring.

3. Two multicellular generations make up the plant life cycle. (Fig. 38.2) a. The generation comprised of diploid cells is the sporophyte.

(1) Cells in the sporophyte generation undergo meiosis and produce haploid spores; this occurs in sporangia.

(2) Spores divide by mitosis to form the multicellular gametophyte generation; this occurs in gametangia. b. The generation comprised of haploid cells is the gametophyte.

(1) Cells in the gametophyte generation produce gametes by mitosis.

(2) One gamete must fuse with another gamete to form a diploid zygote that develops into the sporophyte generation.

4. Students should be able to (1) identify the male spore and female spore in Figure 38.2, and (2) provide evidence to support the statement that in angiosperms, female gametophytes don’t leave their parent plant.

II. Reproductive Structures A. The general structure of the flower 1. Flowers are made up of four basic parts. (Fig. 38.3) a. Sepals form an outer, protective whorl.

(1) Green, slightly thickened, leaflike structures (2) May protect young buds from damage (3) The entire group of sepals in the flower is collectively called the calyx. b. Petals furnish a visual advertisement.

(1) Brightly colored parts arranged in whorl interior to sepals (2) Advertise flower to animals (Fig. 38.4) (3) May have a nectary with sugary nectar gathered by animals (4) The entire group of petals in the flower is collectively called the corolla. c. Stamens produce pollen.

(1) Anther—terminal portion of the stamen where meiosis occurs and male gametophytes form (2) Filament—stalk that supports the anther d. Carpels produce ovules.

(1) Stigma—terminal portion of the pistil (2) Style—stalk of the pistil (3) Ovary with one or more ovules inside e. The “sex” of flowers varies.

(1) Flowers with both male and female organs are perfect. .

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(2) Flowers with either male or female organs are imperfect.

(3) In monoecious plants, separate male and female flowers are located on the same plant. (Fig. 38.5a) (4) In dioecious plants, male and female flowers are located on different plants.

(Fig. 38.5b) B. How are female gametophytes produced? (Fig. 38.6) 1. The ovule is the location of meiosis and female gametophyte formation.

2. Steps in the production of the female gametophyte a. A progenitor cell divides by meiosis to form four haploid megaspores. b. Three megaspores degenerate, leaving one haploid megaspore. c. The single remaining megaspore divides mitotically to form the eight-nucleate, seven-celled female gametophyte, also called the embryo sac. d. The egg cell is located at the base of the embryo sac near an opening, the micropyle. e. Two polar nuclei are located at the center of the embryo sac.

C. How are male gametophytes produced? (Fig. 38.7) 1. The products of meiosis in the anther are haploid microspores.

2. Microspores divide once to form a two-celled pollen grain.

3. The pollen grain is the male gametophyte. a. A pollen grain is composed of a generative cell and a tube cell. b. The generative cell divides to generate two sperm nuclei.

III. Pollination and Fertilization A. Pollination 1. The male and female gametophytes are brought together—transfer of pollen grains to a stigma.

2. Selfing versus outcrossing: costs and benefits a. Self-pollination, a type of inbreeding, is the fusion of gametes from the same individual. b. Pollination is virtually assured, but offspring is less diverse. c. Outcrossing occurs when gametes from different individuals of the same species fuse. d. Self-incompatibility eliminates inbred offspring, which have poor fitness. e. Plants have elaborate mechanisms to prevent selfing: (1) Temporal avoidance describes the flowering of male and female parts at different times.

(2) Spatial avoidance means that the male and female gametes are separated (either on the same plant or on separate plants) to ensure that outcrossing occurs.

(3) Self-pollination can also be blocked in species that normally outcross (molecular matching).

3. Pollination syndromes a. The relationship between animal pollinators and flowers is mutualistic. b. Pollinators obtain rewards in the form of pollen, nectar, or both.

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c. The probability of outcrossing is increased in plants whose male gametophytes are transferred to different individuals. d. Pollination syndromes are close correlations between the shape of the flower and the size, shape, and behavior of its pollinator. (Fig. 38.8) 4. Why did pollination evolve? a. The first lineages of plants to evolve had flagellated male gametes that swam to the female gamete. (Fig. 38.9) b. Extant mosses, liverworts, ferns, and other plant groups that require water for fertilization are dependent on wet habitats. c. The evolution of pollination enabled plants to colonize and survive in drier environments—seed plants do not require external water for reproduction to occur. d. Pollination by animals made sexual reproduction a more precise and efficient process.

5. Does pollination by animals encourage speciation? a. Alpine skypilots that grow at or below the timberline have small flowers, short stalks, a “skunky” odor, and are pollinated by flies. b. Alpine skypilot populations that grow above the timberline have large flowers, long stalks, and a sweet odor and are pollinated by bumblebees. c. Skypilot populations are evolving distinctive characteristics as a result of the pollinators’ preferences. d. Skypilot populations are moving toward becoming distinct species.

B. Fertilization (Fig. 38.10) 1. Compatible pollen germinates on the stigma and forms a pollen tube.

2. A generative cell within the pollen tube divides by mitosis to form two sperm nuclei.

3. The pollen tube enters the ovule at the micropyle and releases sperm nuclei.

4. Double fertilization occurs. a. One sperm nucleus fuses with the egg nucleus to form a diploid zygote. The zygote divides by mitosis to form an embryo. b. One sperm nucleus fuses with polar nuclei to form a triploid cell. The triploid cell divides by mitosis to form the endosperm, a nutritive tissue.

5. Students should be able to identify a female gametophyte immediately after fertilization occurs and identify which cells are haploid, diploid, and triploid.

IV. Seeds and Fruits A. The role of drying in seed maturation 1. As the seed matures, the embryo and endosperm are surrounded by a covering—the seed coat.

2. Cotyledons function as nutrient storage in mature seeds.

3. Monocots have one cotyledon, whereas dicots have two. (Fig. 38.11) 4. Water loss prevents seeds from germinating while they are on the parent plant.

5. Drying ensures that germination will not occur without available water.

6. Research demonstrates that the cell components of dry seeds are protected by a viscous, glassy sugar coating.

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B. Fruit development and seed dispersal 1. A mature pericarp with seeds inside is the fruit. (Fig. 38.12) 2. Ovary cells surrounding the developing seeds form the pericarp.

3. Fruits come in three basic types: simple, aggregate, and multiple. (Fig. 38.13) 4. Fruit has two functions: a. To protect seeds from mechanical damage and seed predators b. To aid in the dispersal of seeds from the parent plant (1) Wind, animals, and propulsion facilitate dispersal.

(2) Dispersal of fleshy fruits by animals is another example of mutualism associated with plant reproduction. (a) Fruit provides sugars and other nutrients to the animal. (b) Animal dispersal results in a wider distribution of plant species and provides a supply of fertilizer when seeds are excreted.

5. Mice tend to destroy seeds, whereas birds tend to swallow fruit whole. Some plants deter mice from eating seeds with chemicals, such as capsaicin. (Fig. 38.14) C. Seed dormancy 1. Dormancy occurs in the seeds of plants that grow in seasonal environments. a. Seeds can remain viable until conditions are favorable for growth. b. No single factor is responsible for initiating and maintaining dormancy.

(1) Desert seeds with high levels of abscisic acid (ABA) in seed coats remain dormant until rains leach out the ABA. (2) In some seeds, sensitivity to ABA levels changes during seed maturation.

2. Numerous environmental cues break dormancy when a seed is in an environment where germination can be successful. a. Scarification (breaking) of the seed coat (1) Scarification results from fire, passage through the digestive tracts of animals, or abrasion against soil particles. b. Cold, wet conditions (1) This involves the seeds of plant species native to northern or alpine habitats.

(2) It is unknown how plants perceive the change in conditions and what molecular mechanisms are involved. c. Red-light exposure for small seeds d. Chemical requirement—a component in smoke from fires D. Seed germination 1. Germination occurs when dormancy is broken and environmental conditions are favorable.

2. Three phases of water uptake occur during germination: (1) Phase 1: Water uptake occurs, oxygen is consumed, proteins are synthesized, no mRNA synthesis occurs.

(2) Phase 2: Water uptake ceases, new mRNAs are synthesized, mitochondria increase in number.

(3) Phase 3: Water uptake resumes, embryonic cells enlarge, radicle emerges through the seed coat.

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3. Monocot and eudicot seedlings germinate differently. (Fig. 38.15) 4. A seedling is established when it is photosynthetically independent and no longer relies on stored food reserves in the seed.

V. Embryogenesis and Vegetative Development A. Vegetative development produces the nonreproductive portions of the plant body, whereas reproductive development produces the gametes and their supporting structures.

B. Embryogenesis (Fig. 38.16) 1. Embryogenesis is a developmental process by which a single-celled zygote becomes a multicellular embryo.

2. The initial zygote divides to form two daughter cells.

3. The basal cell forms a row of cells called the suspensor that transport nutrients from the parent to the embryo.

4. The long axis of the embryo emerges. a. Cotyledons develop and take up nutrients from the endosperm. b. The hypocotyl, the stem region below the cotyledons, and the radicle, the first root, differentiate.

5. Students should be able to understand the basic steps in embryonic development and compare and contrast a cotyledon with a radicle in terms of their tissue composition and the progenitor cells from which they originated.

C. Meristem formation 1. As the cotelydons, hypocotyl, and root begin to develop, the shoot apical meristem (SAM) and root apical meristem (RAM) are specified. (Fig. 38.17) 2. During embryogenesis, these tissues are produced and arranged along the radial axis: a. Epidermis—the outer covering of cells that protects the individual b. Ground tissue—a mass of cells that may differentiate into cells specialized for photosynthesis, food storage, or other functions c. Vascular tissue—cells in the center of the plant specialized for transport of food and water D. Which genes determine body axes in the plant embryo?

1. Gerd Jürgens and colleagues (1990s) set out to identify genes in Arabidopsis that are responsible for establishing the apical−basal axis.

2. They hoped to identify individuals with defects involving the apical−basal axis in the seedling stage. (Fig. 38.18) a. Apical mutants lacked cotyledons. b. Central mutants lacked hypocotyls. c. Basal mutants lacked both hypocotyls and roots.

3. Identification of these mutants led to the hypothesis that each mutant had a defective gene that played a specific role in positioning cells along the apical−basal axis of the plant body.

4. Basal mutants have a mutation in the MONOPTEROS gene. a. The MONOPTEROS protein has a DNA-binding domain and is likely a transcription factor that regulates the expression of other genes.

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b. MONOPTEROS is expressed in response to the signal, auxin, a plant hormone made in the SAM and transported toward the basal parts of the plant. c. The concentration of auxin along the apical−basal axis of the plant gives cells positional information, much like the concentration of Bicoid sets up the anterior−posterior axis of a fruit-fly embryo.

5. Summary: Auxin and MONOPTEROS are parts of a regulatory transcription cascade that is specific to cells in the hypocotyls and roots.

E. Which genes and proteins determine leaf shape?

1. Leaf growth can be induced by applying auxin to a SAM. 2. Once the leaf begins to grow, its three structural axes need to be established.

(Fig. 38.19) a. Proximal−distal: near and far with respect to the stem b. Lateral: from middle to the margin of the leaf c. Upper−lower: top and bottom of the leaf (adaxial-abdaxial) 3. Leaf shape depends on the amount and direction of growth along these three axes.

4. Research has revealed the genes responsible for establishing these three axes of leaf growth. a. The gene PHANTASTICA (PHAN) establishes the upper−lower axis. b. PHAN has a DNA-binding domain and acts as a regulatory transcription factor. c. PHAN triggers the expression of genes that form cells found on the upper surface of the leaf (suppresses genes that form cells on the lower surface of the leaf).

5. Experimentally altering regulatory genes involved in leaf development can lead to striking changes in leaf shape—e.g., in tomato plants. (Fig. 38.20)

VI. Reproductive Development A. Differences between plants and animals 1. Plants do not produce germ cells—egg and sperm—early in development like animals do.

2. Flowering and gametogenesis occur when SAM converts from vegetative development into reproductive development.

3. Meristems that have divided many times to give rise to the mature plant can also give rise to the reproductive tissues that will produce egg and sperm.

B. The floral meristems and the flower 1. In Arabidopsis, specialized proteins sense changes in day length and trigger the production of signals that convert SAMs from vegetative growth into reproductive growth. The result is the formation of floral meristems that will produce flowers.

2. Floral meristems produce four organs that are arranged in concentric circles called whorls. (Fig. 38.21) a. Sepals are on the outside of the flower and help protect it. b. Petals are colorful and attract pollinators. c. Stamens are the pollen-producing organs. d. Carpels are the egg-producing organs.

3. Homeotic genes regulate the development of various body parts.

4. Homeotic mutant allowed us to understand the genetic control of flower structure.

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C. The genetic control of flower structures 1. Elliot Meyerowitz and colleagues studied homeotic mutants to characterize the genes involved in flower formation.

2. They identified three classes of mutants: (Fig. 38.22) a. Mutants with only carpels and stamens b. Mutants with only sepals and carpels c. Mutants with only petals and sepals 3. Meyerowitz hypothesized that each mutant had a mutation in a single gene. a. He thought each of these three genes interacted to establish the four whorls in a flower. b. He called the hypothetical genes A, B, and C. c. His model is called the ABC model.

4. Three basic ideas underlie the ABC model: (Fig. 38.23a) a. Each of the three genes is expressed in two adjacent whorls. b. If each gene is expressed in two adjacent whorls, four possible combinations of gene products can occur. c. Each of the four combinations of gene products triggers the formation of a specific whorl.

5. Specifically, the ABC model proposed: a. The A protein alone causes the formation of sepals. b. The A and B proteins together cause the formation of petals. c. The B and C proteins together cause the formation of stamens. d. The C protein alone causes the formation of carpels.

6. Gene interactions in the ABC model a. The A protein inhibits the production of the C protein. b. The C protein inhibits the production of the A protein. c. These interactions explain homeotic mutants. (Fig. 38.23b) (1) If gene A is mutated, then gene C can be expressed.

(2) As a result, carpels form where sepals should be and stamens form where petals should be.

7. Students should be able to explain why in mutants that lack C gene expression, A genes become expressed across all whorls.

8. Meyerowitz tested the ABC model directly using in situ hybridization. a. The mRNAs were expressed in the whorls as originally predicted. b. The A gene is expressed in the outer two whorls. c. The B gene is expressed in the middle two whorls. d. The C gene is expressed in the inner two whorls.

9. The ABC genes are now known as the floral identity genes and have been characterized. a. They are known as the MADS box that is a DNA-binding domain. b. They are thought to be transcription factors that control the expression of the genes required for flower parts.

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c. The MADS-box genes are similar to the Hox genes found in animals. d. Although different genes are involved, the logic of how to put a multicellular body together is similar in plants and animals.

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text.

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generative cell

pollination

abscisic acid alternation of generations angiosperms anther apical cell apical−basal axis Arabidopsis thaliana asexual reproduction basal cell carpels clones corms corolla cotyledons cross-pollination dioecious diploid dormancy double fertilization egg embryo sac embryogenesis endosperm epicotyl epidermis female gametophyte fertilization filament floral meristems florigen flowers fruit gametangia gamete gametophyte

germ cells germination globular stage ground tissue haploid hypocotyl imperfect inbreeding MADS box male gametophyte megasporangium megaspores megasporocyte meiosis meristems micropyle microsporangium microspores microsporocyte monoecious MONOPTEROS mutualism nectar nectary outcrossing ovary ovule perfect pericarp petals PHANTASTICA pistil plantlets pollen grain pollen tube

radial axis radicle receptacle reporter gene reproductive development rhizome root root apical meristem (RAM) runner scarify seed seed coat self-fertilization self-incompatible self-pollination sepals sexual reproduction shoot shoot apical meristem (SAM) sperm sporangia spore sporophyte stamen stigma stolon style suspensor vascular tissue vegetative cell vegetative development vegetative organs whorls zygote

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Lecture Activities Sexual Encounters of the Floral Kind Activity Ideas for all Class Sizes • Discuss pollination syndromes and the characteristics of flowers pollinated by different vectors, both biotic and abiotic.

Note: Show slides of different types of flowers; many may be downloaded from the Internet or are available as slides through supply houses. If the class is small, provide a variety of flowers purchased at supermarkets (large flowers are preferable), and have the students examine the flower modifications themselves. Have the students make a chart of characteristics of animal-pollinated flowers (birds, bees, bats, moths, butterflies, ants, flies, rodents, and any others) as well as flowers that are wind and water pollinated.

• Get a copy of the PBS Nature program video Sexual Encounters of the Floral Kind, filmed in 1983. Your students will thoroughly enjoy this classic; it has marvelous photography— not technical, but visually terrific.

• A video series covering several topics in this chapter is The Private Life of Plants, by David Attenborough (produced by BBC in 1995).

Activity Idea: Explaining Common Observations in Plant Development Estimated duration of activity: 10–20 minutes Divide class into groups of three students. Assign each group a letter designation (A, B, or C), along with one of the three observations listed below (with corresponding letter designations). There should be at least two groups A, two groups B, and two groups C. If there are fewer than 18 total students in the class, group sizes should be reduced (and the numbers during the rest of the activity scaled down accordingly) so that there are at least two groups of each type.

Give each group a few minutes to collectively formulate a suitable explanation for the assigned observation based on what they have learned from the chapter. Then have each group send two representatives to join another group assigned to the same topic (i.e., representatives from group A find another group A, etc.). If the class is large enough for three or more groups of each type, design the rotation so that the two representatives each go to a different group assigned to the same topic as their original group. In other words, each group now consists of one individual from the original group and two new members transplanted from two other groups assigned to the same topic.

Instruct the groups to share their answers to check each other for accuracy. Groups may use the textbook and/or class notes to reconcile differences. Be sure that you or a teaching assistant is available to settle disputes. Once everyone has reached a consensus, all students should return to their original groups. At this point, each group should feel confident in their explanation for their assigned observation.

Now have the groups go through a second rotation in which new three-member groups are formed. Each group now has one individual originating from a group A, one from a B, and one from a C. Have the individuals in the groups take turns sharing explanations from their original topic with each other.

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Observations: A. For many species of plants, one can take a cutting from the plant and place it in a cup of water. After several days, roots form in the water and the cutting may be planted.

B. For certain species of trees, pruning the central top branch causes the tree to broaden but prevents it from growing taller.

C. Many plants exhibit a behavior known as phototropism. For example, a bean plant growing in a pot will orient itself toward the sunlight. If the pot is rotated, the bean will reorient itself toward the sunlight within a couple of days. If the tip of the stem is removed or covered from the light, the bean plant loses its ability to orient toward the sunlight.

Activity Idea: Diagramming Flower Development Estimated duration of activity: 10 minutes This activity begins as an individual effort. Have students draw a diagram that teaches how the ABC hypothesis for flower development works. The diagram should contain arrows illustrating how different combinations of gene products lead to the formation of the various floral structures in Arabidopsis (sepal, petal, stamen, carpel). Students should also illustrate inhibitory pathways. Textbooks and notes should not be used at any point in this activity until the end, as indicated below.

After students have had several minutes to develop their diagrams, instruct them to share their diagram with another student. If groups discover differences in their understanding, they should attempt to resolve the issues through consultation with other students seated nearby.

Then remind students of the observations made by Meyerowitz’s research group. Instruct them to work in pairs to test whether their diagram is capable of explaining Meyerowitz’s results.

Critical-Thinking Questions Discussion Ideas for all Class Sizes Here are two ideas for in-class discussion questions.

• In the evolution of land plants, the most advanced plants have a dominant diploid generation, and the less advanced plants have a dominant gametophyte generation. What adaptations, both vegetative and reproductive, have influenced this evolutionary trend? Answer: In the bryophytes, the gametophyte generation is dominant and is the generation that produces gametes. The male gamete needs water to swim to the female gamete in these plants. Due to the requirement of water for fertilization and the absence of specialized conducting and supportive tissues, the gametophytes of these plants must grow in moist environments, closely pressed to the surface of soil, rocks, or trees. With the development of lignified vascular tissues in the sporophyte generation, early vascular plants were able to occupy somewhat drier habitats and could form new plants vegetatively, with underground stems. But sexual reproduction in these plants still required access to water.

These early plants gave rise to sporophytes with a shoot system containing a cuticular epidermis and functional stomata. The root system became more extensive, anchoring the plant and absorbing water and nutrients through specialized root epidermal cells. The gametophyte generation—lacking vascular tissue, a waxy cuticle, and functional stomata— became reduced and nutritionally dependent on the larger sporophyte. These overall trends 394

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in land-plant evolution reflect the vegetative adaptations of the sporophyte generation to life in dry environments and the reduction of the gametophyte. Also, with the specialization of reproductive parts into cones and flowers and the containment of the gametophytes in these reproductive structures, water was no longer needed to transport gametes. The sporophyte’s ability to survive in a dry environment enabled those plants with a dominant sporophyte generation to occupy more habitats and thrive in them.

• Three classes of genes have been identified in Arabidopsis that determine the normal development and sequence of appearance of the four floral whorls. Class A genes influence the development of the first two whorls, the sepals, and petals; Class B genes influence the formation of the second and third floral whorls, the petals, and stamens; and Class C genes affect the third and fourth whorls, the stamens, and carpels. A mutation in a Class A gene, for example, causes sepals to be converted into carpels and petals converted into stamens.

A Class B mutation leads to flowers with sepals and carpels, but no petals and no stamens.

Mutation in a Class C gene produces flowers with only sepals and petals, and no stamens or carpels. An Arabidopsis plant with mutations in all three classes, a so-called triple mutant, produces no floral organs at the floral meristems; instead, it produces green leaflike structures arranged in whorls at the apex. Why is this result not too surprising?

Answer: A flower is formed when an apical meristem that has been producing leaves undergoes a transition and forms flowers. Historically, biologists have hypothesized that the four floral whorls are modified leaves. The observation of Arabidopsis triple mutants in which all four floral organs are converted into whorled leaves at the apex directly supports this hypothesis.

Note: See the article by Elliot M. Meyerowitz, “The Genetics of Flower Development,” Scientific American, May 1994, pp. 56–65.

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Animal Form and Function

Learning Objectives: Students should be able to…

• Describe the relationship between the form and function of animal tissues, organs, and systems.

• Identify the key characteristics of the four tissue types of animals.

• Describe how body size affects metabolic rates and thermoregulation.

• Describe how feedback loops are used to maintain homeostasis.

• Compare and contrast the different methods by which animals thermoregulate.

Lecture Outline I. Form, Function, and Adaptation A. Animals differ in their anatomy (form) and physiology (function) and are adapted for the conditions in which they live.

1. Natural selection is the process that produces adaptations.

2. Natural selection occurs when individuals that carry certain alleles leave more offspring than individuals with different alleles of the same gene. a. The frequency of selected alleles increases from one generation to the next, which results in evolution. b. Evolution also occurs through genetic drift, migration, and mutation.

3. Natural selection is the only process that, over time, increases the ability of the organism to survive and reproduce.

B. The role of fitness trade-offs 1. Trade-offs, inescapable compromises between traits, may be the most important type of constraint on adaptation.

2. Trade-offs often involve expenditures of time or energy.

3. An example in crickets: (Fig. 39.1) a. Male crickets produce a spermatophore (sperm packet with a gelatinous mass) that is transferred to the female during copulation. (Fig. 39.1a) b. After copulation, the female consumes the mass (Fig. 39.1b). The larger the spermatophore, the more sperm that is transferred to the female, which increases the number of eggs fertilized by the male.

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c. Larger investment in spermatophore production would provide a reproductive benefit to the male, but at the potential cost of: (1) Time/other mating opportunities: mating is excluded while a new spermatophore is produced.

(2) Immune function: decreased ability to amount an immune response due to resource investment to the spermatophore 4. Experiments were conducted on the trade-off between reproductive and immune function in male crickets. (Fig. 39.2) a. Hypothesis: Male crickets need to make an energy trade-off between reproductive function and immune function. b. Null hypothesis: No energy trade-off between reproduction and immune function is required. c. Two experiments were conducted: 5. Experiment 1: Spermatophores were removed from male crickets. a. Three treatments (1) Low reproductive effort: no spermatophore removal (control) (2) Moderate reproductive effort: spermatophores removed every five days (3) High reproductive effort: spermatophores removed every day b. Hemolymph samples were taken from the crickets. c. Bacteria were added to the hemolymph samples, and lytic activity was recorded. d. Prediction: Hemolymph from males forced to produce more spermatophores will exhibit a lower lytic activity than controls. e. Null prediction: There will be no difference in the lytic activity between treated males and controls. f. Result: Treated males had lower lytic activity.

6. Experiment 2: Males were injected with lipopolysaccharides (LPS) to simulate bacterial infection. a. Two treatments (1) Males were injected with LPS. (2) Males were injected with placebo (control). b. Spermatophores were removed, and the size of the gelatinous mass was measured. c. Prediction: Spermatophores from LPS-injected males will have smaller gelatinous masses than those from control males. d. Null prediction: There will be no difference in gelatinous mass size between treated and control males. e. Results: Spermatophores from treated males had smaller gelatinous masses than controls.

7. Conclusion: Male crickets must make energy trade-offs to support increased reproductive effort. a. Trade-offs are common in nature, and organisms cannot be perfectly adapted to all aspects of their environment. b. Desert animals that sweat to cool off are in danger of dehydration. c. The eagle’s beak is highly adapted for tearing meat but not for building nests.

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C. Adaptation and acclimatization 1. All adaptations are compromises, constrained by genetic and historical factors.

2. Adaptations are not short-term, reversible responses to environmental fluctuations; those changes are called acclimatizations.

3. Adaptations occur due to changes in allele frequencies.

4. The ability to acclimatize is itself an adaptation.

II. Tissues, Organs, and Systems: How Does Structure Correlate with Function?

A. The function of an anatomical feature often correlates with its size, shape, or composition.

1. For example, beak size and shape in finches correlate with the type of seed they eat.

(Fig. 39.3) B. Structure-function relationships at the molecular and cellular levels 1. Correlations between structure and function exist at the levels of molecules and organelles. a. Protein shape correlates with whether the protein is an enzyme or performs a structural role. b. Molecular example: A transmembrane protein that serves as a channel has a hydrophobic exterior and a hydrophilic interior. c. Cellular example: Cells that transport material into or out of the body have a large surface area.

2. Structure-function relationships also exist at the levels of tissues, organs, and organ systems.

C. Tissues are groups of similar cells that function as a unit.

1. There are four adult tissue types: connective, nervous, muscle, and epithelial.

2. Connective tissue a. Connective tissue consists of cells arranged in an extracellular matrix. b. Each type of connective tissue secretes a different type of extracellular matrix. c. There are four major types of connective tissue: (Table 39.1) 1) Loose connective tissue cushions organs and is composed of fibrous proteins in a soft matrix.

2) Dense connective tissue is found in the tendons and ligaments that connect muscles, bone, and organs and is dominated by tough collagen fibers.

3) Supporting connective tissue such as cartilage and bone provides structure, support, and protection and is composed of firm extracellular matrix.

4) Fluid connective tissue, such as blood, transports gases, nutrients, and wastes to and from the body, consists of cells surrounded by a liquid matrix.

3. Nervous tissue (Fig. 39.4) a. Function—deliver electrical signals to other cells b. Structure—long projections that contact other cells and deliver signals by means of electrical impulses and chemical secretions c. Neurons have two distinct projections from the cell body: (1) Dendrites are short, branching, and receive messages (electrical signals) from other cells.

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(2) Axons are longer and send messages (electrical signals) to other cells.

4. Muscle tissue a. Types of muscle tissue (Fig. 39.5) (1) Skeletal muscle tissue consists of muscle fibers that contract in response to information from nerves. a. Function—voluntary body movements b. Structure—long cells with striated appearance (2) Cardiac muscle makes up the wall of the heart. a. Function—involuntary contractions b. Structure—branched cells (3) Smooth muscle lines the digestive tract and other organs. a. Function—involuntary contractions b. Structure—tapered cells 5. Epithelial tissues a. Epithelial tissue covers the outside of the body, lines the surfaces of organs, and forms glands. b. Function—barrier and protective layer (Fig. 39.6) c. Structure—layers of tightly packed cells that have many membrane proteins to regulate the exchange of molecules d. Epithelial cells are joined by tight junctions and desmosomes. e. Epithelia can be simple (one cell layer) or stratified (multiple cell layers). f. Students should be able to predict which is more likely to conatin stratified epithelium: the inside of the surface of the mouth or the gas exchange surface of the lungs. g. Epithelial tissue has polarity and apical and basolateral surfaces.

(1) The apical side faces away from other tissues or faces the environment.

(2) The basolateral surface faces the interior of the animal and is connected to other tissues. h. The apical and basolateral sides of epithelium have distinct structures and functions. i. Epithelial cells have short life spans because they are exposed to the environment and damaged. New epithelial cells are constantly manufactured via mitosis to replace the dying cells.

D. Organs and organ systems 1. Organs are structures with a specialized function; they consist of several tissues.

Example: small intestine (Fig. 39.7a) 2. Systems are groups of tissues and organs that work together.

3. Organ structure correlates with function, and system components work together in an integrated fashion. Example: digestive system (Fig. 39.7b) 4. The digestive system is one long tube divided into sections with different functions. a. The salivary glands secrete enzymes to begin to digest food. b. The esophagus transports food from the mouth to the stomach; it is a long muscular tube. c. The stomach is a muscular sac that contracts to break up food.

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d. The liver and pancreas are secretory; their cells contain abundant endoplasmic reticulum. e. The small intestine is a long, coiled tube where enzymes break down food and nutrients are absorbed. f. The large intestine is a long tube that reabsorbs water and compacts waste.

5. Each component in the body is integrated with other parts, and each level of organization is integrated with other levels; therefore, the organism as a whole is greater than the sum of its parts. (Fig. 39.8)

III. How Does Body Size Affect Animal Physiology?

A. The body size of an animal affects many aspects of its functions and behavior.

1. Larger bodies require more food, produce more waste, reproduce more slowly, take longer to mature, and live longer.

2. Smaller bodies lose heat and water faster and so are more susceptible to cold and dehydration.

B. Surface area/volume relationships: theory 1. How do changes in surface area and volume affect the form and function of cells? a. The rate at which molecules and ions diffuse into and out of a cell depends on the cell’s surface area. b. The rate at which nutrients are used and heat and waste products produced depends on the volume of the cell. c. As a cell gets larger, its volume increases much faster than its surface area.

(1) Surface area increases as a function of the square of the cell’s linear dimension, whereas the volume increases as a function of the cube of the cell’s linear dimension. (Fig. 39.9a) (2) Volume increases much more rapidly than surface area as the linear dimension increases. (Fig. 39.9b) C. Surface area/volume relationships: data 1. Comparing mice and elephants a. The metabolic rate is the overall rate of energy consumption by an individual and is most often measured in terms of oxygen consumption. b. To compare metabolic rates in different species, metabolic rate is divided by overall mass.

(1) Metabolic rate divided by mass is reported in units of mL O2/gram/hour.

(2) Because metabolic rate depends on activity level, the basal metabolic rate (BMR) is measured: the rate at rest with an empty stomach, in the absence of temperature or water stress. c. Do larger or smaller organisms have higher metabolic rates?

(1) Elephants are 200,000 times larger than mice, but their mass-specific BMR is 1/12 that of mice. (Fig. 39.10) (BioSkills 5) (2) Hypothesis: As the size of an organism increases, its mass-specific BMR must decrease. (a) Metabolism depends on the exchange of materials across surfaces. (b) As size increases, volume increases much faster than surface area.

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(c) The available surface area fails to keep up with the metabolic rate unless the BMR slows.

2. Changes during development a. Surface area/volume issues influence the development of an organism during its lifetime. b. Salmon hatchlings weigh a few milligrams, but adults grow to 50 kilograms or larger for a total size increase of a millionfold. c. Salmon exchange gases across their gills, skin, and yolk sacs.

(1) Research question: Do young salmon breathe predominantly through their gills or skin? (Fig. 39.11) (a) Hypothesis: The relative amount of gas exchange across gills and skin changes as a salmon grows. (b) Null hypothesis: The relative amount of gas exchange across gills and skin does not change as a salmon grows.

(2) Test: Fish were inserted into a device that placed their head in a different compartment from their body.

(3) One oxygen electrode measured oxygen uptake by their gills, and another measured oxygen uptake by their skin.

(4) Prediction: Juveniles will exchange a higher percentage of gas across skin than larvae.

(5) Null prediction: Juveniles and larvae will exchange the same percentage across skin and gills.

(6) Result: Newly hatched larvae take up O2 mostly through their skin; juveniles take up O2 mostly through their gills.

(7) Conclusions: Gills provide larger surface area relative to increasing body size, so the fish develop gills as the primary respiratory structure.

D. Adaptations that increase surface area 1. If the function of a cell or tissue depends on diffusion, its structure has a shape that increases its surface area relative to its volume.

2. Three major ways that structures have high surface area/volume ratios to facilitate diffusion of necessary: a. Flattening: Gills consist of thin sheets of epithelial cells called lamellae whose cells are flattened. (Fig. 39.12a) b. Folding: The lining of the small intestine has extensive folded surfaces called villi.

(Fig. 39.12b) c. Branching: The circulatory system is highly branched, with capillaries as the smallest elements. (Fig. 39.12c)

IV. Homeostasis A. Homeostasis: general principles 1. Although internal conditions may vary as an animal’s environment changes, chemical and physical states are kept within a tolerable range.

2. Homeostasis is the maintenance of relatively constant chemical and physical conditions for some quantity such as temperature, pH, or nutrient levels.

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3. Homeostasis is achieved via regulation. a. Many organisms are able to regulate their internal conditions—to actively maintain relatively constant internal conditions even when the environment fluctuates. b. This may be too energetically expensive in some cases—most fish conform to environmental water temperature. c. Regulation and conformation are two extreme ends of a spectrum.

4. The role of epithelia a. The epithelium plays a critical role in maintaining homeostasis. b. Epithelium exists at the interface between the internal and external environments; membrane proteins on the surfaces of epithelial cells regulate the transport of ions, water, nutrients, and wastes. c. Epithelium controls the exchange of materials across surfaces.

5. Why is homeostasis important? a. Enzymes, which are critical to all processes in living cells, function normally only within a narrow range of physical and chemical conditions. b. Changes in pH, temperature, and the chemical environment can dramatically alter enzyme structure and function. c. Temperature can also affect membrane permeability and rates of diffusion. d. In extreme situations, changes in temperature can cause tissue damage as the water in cells freezes and expands. e. Deviations from homeostasis can be adaptive in cases such as a fever in response to a pathogen.

B. The role of regulation and feedback 1. Regulatory systems monitor internal conditions and return them to set points. a. Set points are normal, or target, values for a controlled variable.

2. Each homeostatic system has three main components: (Fig. 39.13) a. Sensor—a structure that senses some aspect of the external or internal environment b. Integrator—a component of the nervous system that: (1) Evaluates incoming sensory information (2) “Decides” whether a response is required to achieve homeostasis c. Effector—any structure that helps restore the desired internal condition 3. Homeostatic systems are based on negative feedback. a. Negative feedback occurs when effectors reduce or oppose a change in internal conditions. b. For example, a rise in blood pH triggers effectors that reduce that rise in pH to normalize it.

4. Students should be able to explain how body temperature homeostasis in mammals might change as global temperature rises.

V. Thermoregulation: A Closer Look A. Mechanisms of heat exchange (Fig. 39.14) 1. Conduction is the direct transfer of heat between two physical bodies that are in contact with each other.

2. Convection is the transfer of heat between a solid and a liquid or gas.

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3. Radiation is the transfer of heat between two bodies that are not in direct physical contact.

4. Evaporation is the phase change that occurs when liquid water becomes a gas.

5. Heat exchange is critical in animal physiology because individuals that get too hot or too cold may die.

B. Thermoregulatory strategies 1. Animals fall into two general categories based on how they obtain heat: a. Endotherms produce heat within their own tissues. b. Ectotherms rely on heat gained from the environment.

2. Animals may also be categorized by whether their body temperature is held constant. a. Homeotherms keep their body temperature constant. b. Poikilotherms allow their body temperature to rise or fall, depending on environmental conditions.

3. These four categories represent extremes and there are many variations within these four categories. a. Humans and birds are endothermic homeotherms that produce their own heat and maintain it at a specific temperature. b. Freshwater and terrestrial invertebrates, fishes, amphibians and reptiles are ectothermic poikilotherms whose body temperature changes throughout the day and season. c. Some desert-adapted mammals allow their body heat to increase during the hottest part of the day, making them somewhat poikilothermic. d. Small mammals in cold climates lose heat, so they lower their metabolic rate and allow their body temperatures to drop—a condition called torpor or, if sustained, hibernation. e. Naked mole rats live in underground tunnels and allow their body temperatures to rise and fall to match the burrow temperatures. f. Japanese honeybees exhibit poikilothermy when defending their hives from predatory hornets (Fig. 39.15). When a hornet invades, the honeybees swarm the hornet and contract flight muscles repeatedly to produce heat endothermically. The increase in temperature is fatal to the hornet, but is tolerated by the honeybees.

C. Comparing endothermy and ectothermy 1. Endothermy and ectothermy are an example of trade-offs in adaptation. a. Advantages of endothermy (1) Enzymes are maintained at constant temperature at all times.

(2) Mammals and birds can remain active in winter and at night.

(3) The high metabolic rate of these animals enables them to sustain high levels of aerobic activity, such as running or flying. b. Disadvantages of endothermy (1) These animals must consume large quantities of energy-rich food.

(2) Energy used to produce heat is unavailable for growth, reproduction, and other processes. c. Advantages of ectothermy (1) These animals can survive on much less food. .

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(2) They can use a greater proportion of their energy to support reproduction. d. Disadvantages of ectothermy (1) Temperature-dependent chemical reactions slow down as body temperature drops.

(2) Muscular activity is slowed, making these animals more vulnerable to predation in the cold.

(3) They are less successful at inhabiting cold environments.

D. Countercurrent heat exchangers 1. An example: gray whale tongues a. A countercurrent heat exchanger is used to minimize heat lost through the tongue of whales during feeding. b. Arteries and veins in gray whale tongues are arranged antiparallel to one another.

(Fig. 39.16a) c. Heat from arteries (transporting warm blood) transfers directly into veins (transporting cool blood) returning to the body center. d. Loss of heat, which would occur when arterial blood arrives at body extremities, is reduced. e. Countercurrent heat exchangers are more efficient than concurrent flow systems because they maintain a gradient between the fluid along their entire length.

(Fig. 39.16b) 2. Countercurrent heat exchangers are also found in the flippers of whales and dolphins and in the legs of many mammals and birds that live in cold terrestrial environments.

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text.

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acclimatization

cardiac muscle

extracellular matrix

adaptations adaptive evolution adipose tissue anatomy anterior hypothalamus apical axon basal lamina basal metabolic rate (BMR) basolateral blood bone brown adipose tissue capillaries

cartilage conduction conformation connective tissue convection countercurrent dendrite dense connective tissue ectotherm effector endotherm epithelia/epithelium epithelial tissue esophagus evaporation

fluid connective tissue gene flow genetic drift gills gland heat exchange heat shock proteins hibernation homeostasis homeotherm integrator lamellae large intestine liver loose connective tissue

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lumen metabolic rate metabolic water multicellular muscle fibers muscle tissue mutation negative feedback nervous tissue neurons organ organ system

pancreas physiology poikilotherm posterior hypothalamus radiation regulation salivary gland sensor set point skeletal muscle small intestine smooth muscle

spermatophore stomach striated muscle supporting connective tissue system thermoregulation tissue torpor trade-off villi

Lecture Activities Illustrating Trade-offs Estimated duration of activity: 10–15 minutes Trade-offs are common in several aspects of biology, and no individuals are perfectly suited to their environment. As such, there are often constraints on adaptations. To emphasize this concept, this activity will ask students to allocate “points” to different attributes of their favorite organism. As in many biological cases, allocation of a resource is disconnected to the costs and benefits of those decisions. This activity can work with any class size, although modifications may be needed for larger class sizes.

Have each student take out a sheet of paper. On the sheet of paper, place five major categories: Armor, Speed, Camouflage, Weapons, Variability of habitats.

Students have a total of 24 “points” that represent energy invested into each attribute.

Students will allocate all 24 points to the attributes, and their success will be based on how well their organism performs under a variety of scenarios presented by the instructor. Please remind students that you cannot change your answers (highlight the difference between acclimatization and adaptation). The scenarios can be randomly selected or designed specifically to illustrate specific topics. Some sample scenarios are below: 1) Your organism must be able to defend itself from predators to survive through their first year of life. If you invested more than five into armor, you survive.

2) Your organism would need to outrun predators to survive as an adult. If you invested more than six into speed, you survive.

3) Being able to hide in the bushes allows you to obtain more prey as well as hide from your predators. If you invested more than five in camouflage, you survive.

4) Your organism must compete with other individuals of the same sex to obtain mating opportunities. If you invested more than three in weapons, you are able to secure a mate.

5) The environment is constantly changing, and your habitat experiences torrential rain followed by drought conditions. If you invested more than seven in variability of habitats, you would survive.

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Scenarios can also be negative, such as: A large shell weighs you down in the water, reducing mobility (those that invested more than five to armor would be less fit).

After each scenario is presented, individuals that survived/reproduced would proceed to the next round. For large classes, this is easily facilitated by having all individuals stand at the beginning and sit if they are eliminated. The order of scenarios presented and points needed to survive will influence which attributes were most important for survival or reproduction.

Points and scenarios can be easily modified to illustrate specific conditions or concepts or to illustrate that lasting conditions can have different impacts. For example, the first time a scenario is presented, individuals may have needed to invest a minimum of four points; however, if conditions continued, more points may be necessary (e.g., extreme conditions).

This exercise could also be modified to illustrate behavioral trade-offs using an example of a time budget, where the 24 points represent hours and students would allocate time to five major activities: Sleep, Eat, Study, Social interactions, Employment.

Sample scenarios could include: A pop quiz (benefits to those that studied) Networking for a new job (social interactions) Fighting off an illness (sleep) Energy to complete a paper (eat) Credit card bill comes due (employment) After the experience, students can use their observations of the exercise to develop a prediction for the type of complex environment (minimum of four scenarios) for which their organism is very well suited (likely to survive) or poorly suited (likely to die). Students can then work in pairs to evaluate each other’s organisms, situations, and environments. Students can also gain experience with graphing (pie charts), visualizing data, and interpretation.

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40

Water and Electrolyte Balance in Animals

Learning Objectives: Students should be able to…

• Describe the relationship between osmoregulation and excretion.

• Describe how cells, tissues, and organs in marine fishes function to remove salts from internal fluids.

• Describe how cells, tissues, and organs in freshwater fishes function to gain salts from the • environment.

• Explain how terrestrial insects maintain water and electrolyte balance.

Explain how terrestrial vertebrates maintain water and electrolyte balance.

Lecture Outline I. Osmoregulation and Excretion A. Electrolytes and water move by two processes: 1. Diffusion: net movement of substances from regions of higher concentration to lower concentration 2. Osmosis: a type of diffusion; specifically the movement of water from higher water concentration to lower concentration through a semipermeable membrane B. In both diffusion and osmosis, net movement of ions and molecules occurs along a gradient.

1. Diffusion involves movements of solutes along the gradient to areas with lower concentrations, with equilibrium established when the movements between the two regions occur at equal rates.

2. Osmosis involves movement of water along the gradient from areas of lower osmolarity (high solute concentration) to areas of higher osmolarity (low solute concentration), but the solutes do not move due to the selectively permeable membrane.

C. What is osmotic stress?

1. Osmotic stress occurs when the concentration of dissolved substances is abnormal.

2. Many organisms respond to osmotic stress by controlling the water and solutes in their bodies—osmoregulation.

3. Many animals (such as marine invertebrates) have internal solute concentrations very similar to the environment and are considered osmoconformers. Small changes .

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in the marine environment are therefore reflected in the internal composition of osmoconformers.

D. Osmotic stress in seawater, in freshwater, and on land 1. Marine fish in seawater a. Seawater is hyperosmotic to marine bony fish and they tend to lose water and gain electrolytes. (Fig. 40.1) b. They must osmoregulate by drinking water and excreting excess electrolytes. c. For gas exchange to occur, the gill epithelium must be in direct contact with seawater. The salinity difference between the cells within the gill epithelia and the seawater results in water loss due to osmosis and an increase in electrolytes due to diffusion. The extra electrolytes then must be actively pumped out. (Fig. 40.2a) 2. Freshwater animals a. Freshwater is hyposmotic to freshwater fishes, and they tend to gain water and lose electrolytes. b. They must osmoregulate by excreting excess water and obtaining electrolytes from their environment and food. c. For gas exchange to occur, the gill epithelium must be in direct contact with the water. The salinity difference between the cells within the gill epithelia and the freshwater results in excess water due to osmosis and a decrease in electrolytes due to diffusion. The electrolytes then must be actively pumped in to replace the ones that were lost. (Fig. 40.2b) 3. Terrestrial animals a. Terrestrial animals run the risk of dehydration due to evaporation of water from their body surfaces, including respiratory organs, as well as through defecation and urination. b. Terrestrial animals must osmoregulate by drinking and eating, and excreting excess electrolytes in urine. (Fig. 40.2c) 4. Students should be able to explain why maintaining water and electrolyte homeostasis poses similar challenges to marine and terrestrial animals.

E. How do electrolytes and water move across cell membranes?

1. Passive transport is driven by diffusion along an electrochemical gradient and does not require ATP energy.

2. Active transport uses ATP to transport a solute against its electrochemical gradient.

3. Ions and large molecules typically use membrane proteins for passive and active transport. a. In many cases, passive transport occurs through channels in the membrane. b. Passive transport also occurs via protein carriers, which is called facilitated diffusion. c. Active transport is based on membrane proteins called pumps, which change in conformation when they bind to ATP or are phosphorylated. d. The sodium-potassium pump is a very important pump in animals.

4. Secondary active transport occurs when active transport sets up a gradient. That gradient can be used to transport another solute against its electrochemical gradient. a. A cotransporter that moves solutes in the same direction is called a symporter.

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b. A cotransporter that moves solutes in the opposite direction is called an antiporter.

5. Water moves through membranes via special channels called aquaporins. Since there are no known active water transporters, cells move water by moving solutes.

F. Types of nitrogenous wastes: impact on water balance 1. Animal cells utilize amino acids and nucleic acids that can break down into ammonia.

2. Ammonia is toxic to cells.

3. Because ammonia is water soluble, excretion of ammonia leads to water loss, but the degree of water loss varies based on the method of ammonia excretion.

4. Forms of nitrogenous waste vary among species (Table 40.1) a. Freshwater fishes dilute ammonia to low concentrations and excrete through watery urine. b. Freshwater and marine fishes use diffusion of ammonia across the gills into the surrounding water. c. Mammals and adult amphibians convert ammonia to urea, which is excreted in urine. d. Terrestrial arthropods, birds, and reptiles convert ammonia to uric acid, which is not water soluble and thus results in little water loss during excretion.

5. Why do nitrogenous wastes vary among species? a. Evolutionary lineages: similar groups excrete in similar ways, such as mammals excreting urea and birds excreting uric acid. b. Habitats occupied by the species, such as whether organisms are terrestrial or aquatic. c. Fitness tradeoffs between being nitrogen excretions and water loss.

II. Water and Electrolyte Balance in Marine Fishes A. Osmoconformation versus osmoregulation in marine fishes 1. Marine bony fishes are osmoregulators, whereas sharks are osmoconformers.

2. Osmoconformers, such as sharks, have blood osmolarity that nearly matches that of seawater (although the ionic content is different), and therefore they lose less water through osmosis.

3. Osmoregulators and osmoconformers each have their own costs and benefits and represent tradeoffs—for example, the need to secrete salt. a. Sharks need to secrete salt (NaCl) because sodium and chloride ions diffuse into their gill cells from seawater down their concentration gradients. b. The same mechanisms are crucial to the function of the kidney in humans and other terrestrial animals. c. Cells do not transport ions against their electrochemical gradient directly. Instead, cells move key ions indirectly.

(1) They use active transport to set up a strong electrochemical gradient for a different ion—typically Na+ in animals and H+ in plants.

(2) The sodium or proton gradient is then used to transport an array of other substances without further expenditure of energy.

B. How do sharks excrete salt?

1. The shark rectal gland secretes a concentrated salt solution.

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2. The role of Na+/K+-ATPase a. Researchers determined that the rectal gland concentrates salt via Na+/K+-ATPases. b. Rectal gland function was inhibited by ouabain, a sodium-potassium pump inhibitor. c. The rectal gland cells pump Na+ on the opposite side of the cell and in the direction opposite the direction where sodium ions are actually secreted.

3. A molecular model for salt excretion (Fig. 40.3) a. Na+/K+-ATPase, located in the basolateral membrane, pumps Na+ out of epithelial cells into extracellular fluid, creating a low intracellular Na+ concentration. b. Na+ moves across the basolateral membrane from the extracellular to the intracellular fluid down its concentration gradient by using a Na+/2Cl–/K+ cotransport mechanism, bringing Cl– and K+ into the cell. c. Cl– leaves the cell through apical chloride channels, creating an electrical charge gradient for Na+ transport. d. Na+ moves down its concentration and charge gradients into the lumen via the spaces between the epithelial cells.

4. A common molecular mechanism underlies many instances of salt excretion. a. Marine birds and reptiles drink salt water and must excrete NaCl. They have salt-excreting glands in their nostrils that function much like the shark rectal gland. b. The gills of marine bony fishes contain specialized cells called chloride cells, which are configured much like the cells lining the shark rectal gland. These chloride cells excrete excess salt and maintain electrolyte balance. c. Mammals have similar pumps in their kidneys.

III. Water and Electrolyte Balance in Freshwater Fishes A. How do freshwater fishes osmoregulate?

1. Migratory fish, like salmon and sea bass, which move from freshwater to seawater back to freshwater during their life span, make physiological adjustments in order to maintain water and electrolyte balance during these transitions.

2. Research supports the hypothesis that there is a freshwater version of the classical chloride cell. (Fig. 40.4) a. Osmoregulatory cells may be in different locations depending on the migratory stage. b. Different forms of Na+/K+-ATPase may be activated at different times. c. The orientation of key transport proteins can “flip.”

IV. Water and Electrolyte Balance in Terrestrial Insects A. How do insects minimize water loss from the body surface?

1. The large surface-area-to-volume ratios and the use of air as a respiratory medium present challenges to the maintenance of water balance in insects.

2. A water-impermeable waxy cuticle on the exoskeleton minimizes evaporation from the body surface. (Fig. 40.5a) 3. The openings to the insect tracheal system, used for gas exchange, are called spiracles; they can close to prevent excessive water loss from respiration. (Fig. 40.5b)

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4. The malpighian tubules allow insects to make concentrated urine a. The Malpighian tubule system of insects allows insects to produce concentrated urine, thereby conserving water loss and maintaining the water and electrolyte balance of the hemolymph. b. The Malpighian tubule system forms a pre-urine filtrate from the hemolymph, which is transferred to the hindgut for modifications before excretions. (Fig. 40.6) c. Cells in the membranes of the Malpighian tubules actively transport K+ into the tubule lumen; water (via osmosis) and solutes, other electrolytes, and wastes follow along their concentration gradients via diffusion. d. The filtrate flows into the hindgut, where it joins digested food. e. Epithelial cells in the hindgut transport specific ions out of the filtrate and into the hemolymph—water follows by osmosis, forming concentrated urine. f. The Malpighian tubule system allows well-hydrated insects to produce urine that is hypotonic to their hemolymph and dehydrated insects to produce hypertonic urine.

C. Retaining water and electrolyte balance: an overview 1. There are no mechanisms for actively pumping water.

2. The formation of pre-urine is not particularly selective.

3. Reabsorption is highly selective.

4. Reabsorption is tightly regulated.

5. Reabsorption of water is based on osmotic gradients created by ion pumps.

V. Water and Electrolyte Balance in Terrestrial Vertebrates A. The structure of the mammalian kidney 1. Mammalian kidneys occur in pairs of bean-shaped organs that receive blood containing nitrogenous wastes from the renal artery and send the clean blood away from the kidney through the renal vein. The kidney produces urine, which is transported to the bladder via the ureters. The bladder is emptied via the urethra. (Fig. 40.7a) 2. The kidney is divided into an inner region known as the medulla and an outer region known as the cortex. (Fig. 40.7b) The functional unit of the kidney (the nephron) is typically located in the cortex but can extend through the cortex into the medulla.

B. The function of the mammalian kidney: an overview 1. The nephron has four major parts and is associated with a collecting duct. (Fig. 40.8a) a. The renal corpuscle filters blood to create pre-urine. b. The proximal tube reabsorbs nutrients, ions, and water. c. The loop of Henle creates an osmotic gradient. d. The distal tube reabsorbs ions and water. e. The collecting duct reabsorbs more water.

2. Each nephron is served by blood vessels. (Fig. 40.8b) C. Filtration: the renal corpuscle 1. The nephron is a tube that is closed at one end and open at the other. a. The closed end forms a capsule (Bowman’s capsule) around capillaries (glomerulus). (Fig. 40.9a) b. Glomerular capillaries have large pores, used for filtration. (Fig. 40.9b)

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c. Blood is forced out of the glomerulus by high pressure and is filtered into the Bowman’s capsule.

2. The high blood pressures of closed circulatory systems allow for the formation of pre-urine by filtration of the blood.

3. A filtrate of blood is formed in the renal corpuscle of the nephron as water and smaller solutes, but not large proteins and cells, are forced through glomerular capillary slits into Bowman’s space.

4. The renal corpuscles of the human kidneys can produce 180 liters of filtrate a day, but 99% is reabsorbed. Only a small amount is excreted as urine.

D. Reabsorption: the proximal tubule 1. The filtrate inside the proximal tubule contains water, urea, glucose, amino acids and electrolytes.

2. Active transport occurs in epithelial cells a. The apical membranes of the proximal tubule epithelial cells have microvilli facing the lumen to increase the surface area for reabsorption. (Fig. 40.10a) 3. Ion and water movement is driven by a concentration gradient in the interstitial fluid (Fig. 40.10b) a. Na+/K+-ATPase removes Na+ from the interior of the cell to facilitate entry of Na+ from the lumen. b. The movement of Na+ into the cell along its gradient facilitates movements of other solutes against their gradients. c. Solutes flow into the cell via diffusion across the basolateral membrane into the interstitial fluid and then into blood vessels. d. Water follows by osmosis through aquaporins in the cell membrane.

4. Almost all of the nutrients and two-thirds of the sodium chloride and water that is originally filtered by the renal corpuscle is reabsorbed in the proximal tubule.

5. Students should be able to assign one of the following transport mechanisms to each of the four steps above: diffusion, facilitated diffusion, primary active transport, secondary active transport.

6. Summary: The cells that line the proximal tubule act as a recycling center. a. The filtration step in the renal corpuscle is based on size; the reabsorption step in the proximal tubule selectively retrieves small substances that are valuable. b. The pumps and cotransporters in the proximal tubule recover water, nutrients, and electrolytes but leave wastes. c. The filtrate that leaves the proximal tubule to enter the loop of Henle has a relatively high concentration of waste and relatively low concentration of nutrients.

E. Creating an osmotic gradient: the loop of Henle 1. Werner Kuhn hypothesized that the loop of Henle acts as a countercurrent multiplier, a model supported by other researchers.

2. Testing Kuhn’s hypothesis a. There is a strong gradient of osmolarity inside the loop of Henle from the cortex to the medulla. (Fig. 40.11a) b. Outside the loop of Henle, in the interstitial fluid, concentrations of Na+, Cl–, and urea increase sharply from the cortex to the medulla. (Fig. 40.11b) 412

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3. How is the osmotic gradient established? a. The loop of Henle contains three distinct regions: (Fig. 40.12a) (1) In the thick ascending limb, sodium ions are actively transported out of the solution contained within the thick ascending limb, but chloride ions flow along a concentration gradient.

(2) The descending limb is water permeable but salt impermeable.

(3) The thin ascending limb is permeable to Na+ and Cl– but water impermeable.

4. A comprehensive view of the loop of Henle a. An osmotic gradient is maintained because water moves out of the descending limb into the surrounding tissues, concentrating the salts that remain behind, and salt leaves the ascending limbs. (Fig. 40.12b) b. Salts (Na+, Cl–, and K+) move passively down their concentration gradients into the surrounding tissue in the thin ascending limb and are actively pumped out in the thick ascending limb. (Fig. 40.12b) c. The key point in this model is that the countercurrent flow of fluid, combined with changes in permeability to water and in the types of channels and pumps that are active in the epithelium of the nephron, is self-reinforcing. In this case, the presence of an osmotic gradient stimulates water and ion flows that in turn maintain an osmotic gradient.

5. Students should be able to predict what happens to the osmotic gradient when the drug furosemide inhibits membrane proteins that pump sodium and chloride ions out of the thick ascending limb. Specifically, they should understand how furosemide affects (1) water reabsorbtion in the descending limb, (2) the osmolarity of the filtrate inside the bottom of the loop of Henle, and (3) the reabsorbtion of salt in the thin ascending limb.

6. The vasa recta removes solutes that leave the loop of Henle a. The salt and water that move out of the loop of Henle enter the blood vessels of the vasa recta and are returned to the body. (Fig. 40.13) 7. The collecting duct leaks urea a. The innermost section of the collecting duct is permeable to urea. b. This allows urea to leak out of the collecting duct and form a gradient, with the highest concentration being toward the inner medulla.

F. Regulating water and electrolyte balance: the distal tubule and collecting duct 1. Urine formation is under hormonal control. a. The distal tubule and the collecting duct are the final sites for adjustments to the chemical composition of urine. Much of their activity is under hormonal regulation. b. The amount of Na+ lost in the urine is regulated by the hormone aldosterone. If blood Na+ levels are low, aldosterone stimulates the reabsorption of Na+. Cl– and water follow as Na+ is reabsorbed in this region.

2. How does ADH work? a. If blood osmolarity is high, indicating dehydration, ADH is released and increases the water permeability of the collecting duct through the insertion of aquaporins.

Water moves by osmosis into the medullary tissue and is reabsorbed. Hyperosmotic urine is formed, thus conserving water. (Fig. 40.14a)

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b. If blood osmolarity indicates a well-hydrated state, the absence of ADH keeps the water permeability of the collecting duct low. Hyposmotic urine is produced.

(Fig. 40.14b) 3. Students should be able to predict how urine formation is affected by ethanol, which inhibits ADH release, and by nicotine, which stimulates ADH release.

4. Table 40.2 reviews the structures and functions of the four major regions of the nephron and collecting duct.

G. Urine formation in nonmammalian vertebrates 1. Fishes, amphibians, and nonavian reptiles lack a loop of Henle and thus cannot produce concentrated urine.

2. Reptiles in particular need to worry about water loss and desiccation; as such, they produce uric acid, which is excreted with very little water loss.

3. Reptiles convert the isosmotic urine from the kidneys to hyperosmotic uric acid by absorbing water across the wall of the cloaca and into the bloodstream. The uric acid paste is then excreted along with feces. (Fig. 40.15) 4. Some reptiles also have a bladder that collects isomotic urine from the ureters and stores it before emptying into the cloaca. a. Gila monsters can reabsorb water from the bladder (1) Experimental evidence using radioactive water demonstrates that water is taken up from the bladder and into the bloodstream. (Fig. 40.16a) (2) Pure water injected to either the stomach or the bladder reduced blood osmolarity at a similar rate in dehydrated lizards. (Fig. 40.16b)

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text.

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active transport

chloride cells

filtrate

aldosterone ammonia ammonium ion anadromous antidiuretic hormone (ADH) antiporter apical membrane aquaporins basolateral membrane bladder Bowman’s capsule carrier proteins channels chitin

chloride channel cloaca collecting duct concentration gradient cortex cotransporter countercurrent multiplier descending limb diffusion distal tubule electrochemical gradient electrolyte balance electrolytes equilibrium facilitated diffusion

filtration gill filaments gill lamellae glomerulus hemolymph hindgut homeostasis hyperosmotic hyposmotic hypothesis isosmotic kidney loop of Henle lumen Malpighian tubules

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medulla microvilli Na+/K+-ATPase (sodiumpotassium pump) nephron nitrogenous wastes osmoconformers osmolarity osmoregulators osmosis osmotic gradient osmotic stress

pH pre-urine primary active transport proximal tubule reabsorption renal artery renal corpuscle secondary active transport selectively permeable membrane shark rectal gland solute

spiracles symporter thick descending limb thin ascending limb tracheal system urea ureter urethra uric acid vasa recta water balance wax

Lecture Activities Designing a Filter Estimated duration of activity: 20–30 minutes Some students struggle to understand the details of kidney function because the structure is new and unusual to them. However, the concept of filtering a fluid is very familiar to students, and they may benefit from participating in this activity before you teach them about kidney anatomy and physiology.

After providing a short review of diffusion, osmosis, and tonicity (see Chapter 6), ask students to design a filter that will remove excess salts and toxins from a water supply while leaving some salts and minimizing water loss. Give them a hint: Manipulating the diffusion of salts and osmosis by creating solute gradients and semipermeable membranes within their filter will be useful. Give each group 15 to 20 minutes to design their filter, and have them present it to the class. After the presentations, have the class compile a list of the best features of each of the filters. This list should end up describing the features of the kidney and will be a springboard from which you can teach students how the kidney works. The feature list may include: • Fluid entering the filter at high pressure • Permeability to salts and other substances at one point in the filter, but lack of permeability at other points • A way to draw water back into the system, out of the filtrate

Death by Drinking Water?

Estimated duration of activity: 10 minutes Many of your students may be surprised to learn that drinking too much water too fast can be deadly. The following links are articles about people who either became comatose or died after drinking excessive amounts of water in short periods of time. These articles are short and could be either read to students during class or posted on a course website for them to read. After they have read one or both articles, ask students to hypothesize why consuming too much water is deadly. Alternatively, you may ask students to research water intoxication and come to class prepared to give a full, detailed explanation.

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http://news.bbc.co.uk/2/hi/uk_news/magazine/6263029.stm http://sportsmedicine.about.com/cs/nutrition/a/aa070703a.htm

Discussion Ideas Reviewing the Behavior of Aqueous Solutions Before beginning this unit, ask students to recall the definitions and mechanisms of osmosis and diffusion. How are these processes similar? How do they differ? What factors influence the rates at which these two processes occur?

Comparative Animal Physiology Ask students to develop a rationale for the absence of glomeruli in the nephrons of marine fish kidneys. In the absence of glomeruli, how would urine be formed?

Answer: Marine fish lose water to their hypertonic environment. Aglomerular nephrons reduce the risk of water loss that accompanies the filtration process. Like the Malpighian tubule system of insects, urine formation depends on the process of secretion (and reabsorption).

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Animal Nutrition

Learning Objectives: Students should be able to…

• Identify the major groups of nutrients that animals require to complete their lifecycle.

• Explain how the structure of an animal’s mouthparts corresponds to its feeding and dietary needs.

• Discuss the structure and function of each digestive tract organ in the digestion and • absorption of nutrients.

Describe the causes, symptoms, and consequences of Type 2 diabetes.

Lecture Outline • The process of obtaining energy has four steps: ingestion, digestion, absorption, and elimination. (Fig. 41.1)

I. Nutritional Requirements A. Meeting basic needs in humans 1. A nutrient is any substance that an organism needs to remain alive.

2. Food is any material that contains nutrients.

3. The amount of energy provided by foods is measured in kilocalories (calories). a. Fats and lipids contain more C–H bonds, and therefore more energy, than carbohydrates. b. To maintain health, it is important to understand what nutrients are needed and in what amounts.

4. Recommended daily allowances (RDAs) were devised to identify the daily needs of healthy people.

5. Humans require four classes of essential nutrients: a. Humans must obtain nine (out of 20) essential amino acids that we cannot synthesize. b. Humans can synthesize all fatty acids except for two. c. Vitamins are compounds that often function as coenzymes in metabolic reactions and are needed in only minute amounts. (Table 41.1) d. Minerals are inorganic substances that are used as components of enzyme cofactors or structural materials. (Table 41.2) 1) Some minerals are needed in large amounts (e.g., calcium, phosphorus), whereas others are needed as trace amounts (e.g., iron, magnesium).

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2) Electrolytes (such as Na+, K+, and Cl–) are mineral ions that influence the osmotic balance and are essential for normal membrane activity.

II. Capturing Food: The Structure and Function of Mouthparts A. Four strategies in obtaining food: 1. Suspension feeders feed by filtering.

2. Deposit feeders feed on sediments.

3. Fluid feeders suck or lap up fluids.

4. Mass feeders seize and manipulate chunks of food.

B. Mouthparts as adaptations 1. Across animal groups, a strong correlation exists between the size and shape of mouthparts and the size and shape of food sources. a. Mammals chew their food and swallow in distinct boluses, which has resulted in specialization of tooth forms to utilize a wide range of foods. (Fig. 41.2a) b. Complex multipart skull and jaw bones in snakes allow them to swallow large prey whole. (Fig. 41.2b) 2. The correlation between structure and function is strong because of the direct tie to fitness: Obtaining nutrients is fundamental for survival and reproduction.

C. A case study: the cichlid throat jaw 1. As a group, cichlids consume all types of food in lakes, from plants to animals.

2. They diversified by adaptive radiation; 300 species appear to have evolved from a single common ancestor in less than 10,000 years.

3. Cichlids diversified because they have an extra set of jaws. (Fig. 41.3) a. Pharyngeal (throat) jaws are located well behind the normal oral jaws. b. Pharyngeal jaws are capable of biting, crushing, tearing, and compacting food, unlike the pharyngeal jaws of other fish, which function only in transporting food.

4. Cichlids have toothlike protuberances on their pharyngeal jaws. (Fig. 41.4) a. The protuberances vary in different species and correlate with the function that aids in consuming a food source. b. Cichlids that graze on algae have flattened surfaces suitable for grinding. c. Cichlids that feed by crushing snail shells have a mixture of sharp and flattened protuberances.

III. How Are Nutrients Digested and Absorbed?

A. An introduction to the digestive tract 1. The alimentary canal is the structure in which digestion takes place.

2. Incomplete digestive tracts have a single opening that serves for both ingestion and elimination of waste. (Fig. 41.5) 3. Complete digestive tracts have two openings. They start at the mouth and end at the anus. (Fig. 41.6) 4. A tubelike digestive tract has three advantages: a. It allows animals to feed on large pieces of food. b. Different chemical and physical processes can be separated within the canal. c. Material can be ingested and digested continuously.

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5. The digestive tract is only one part of the digestive system and several systems contribute digestive enzymes or other products to the tract. (Fig. 41.6) B. An overview of digestive processes 1. Distinct chemical changes occur as food moves through each compartment of the digestive tract. (Fig. 41.7) 2. The teeth begin digestion by tearing and chewing. a. In humans, the use of knives and cooking is implicated in the decline in average tooth size over the last several million years. b. The chemical breakdown of carbohydrates by enzymes begins in the mouth.

3. Food passes through the esophagus to the stomach. a. Mechanical and chemical processing occurs in the stomach.

4. Chemical processing of three major types of macromolecules (carbohydrates, proteins, and lipids) is completed in the small intestine. Small molecules resulting from the digestion of macromolecules are absorbed in the small intestine along with water, vitamins, and ions.

5. Water absorption and feces formation occur in the large intestine.

C. The mouth and esophagus 1. Digestion starts in the mouth. a. Starch breakdown was the first enzyme-catalyzed reaction discovered. b. Salivary amylase cleaves glycosidic bonds in starch, glycogen, and other polymers. c. Cells in the tongue release lingual lipase, which begins the digestion of lipids. d. Saliva also contains water and glycoproteins called mucins.

(1) Mucins plus water produce mucus.

(2) Mucus makes food slippery, facilitating its passage to the stomach.

2. Peristalsis moves material down the esophagus a. Peristalsis, a wave of reflexive contractions, is not a conscious choice but is an automatic reaction to a stimulus (the act of swallowing). (Fig. 41.8) b. Smooth muscles contract and relax in a coordinated fashion to propel material to the stomach.

3. A modified esophagus: the bird crop a. In an array of bird species, the esophagus has a prominent, widened segment called the crop where food can be stored and, in some cases, processed. b. This allows birds to eat a large amount of food in a short amount of time.

D. The stomach 1. The stomach is a tough, muscular pouch bracketed on either end by sphincters.

(Fig. 41.9) a. When food enters the stomach, muscular contractions cause churning that mixes the contents. b. Some mechanical breakdown of food occurs.

2. The stomach as a site of protein digestion a. The stomach contains hydrochloric acid and is thus highly acidic. b. William Beaumont established that digestion of meats occurs in the stomach by studying a man who had a shotgun injury and thus a permanent hole in his stomach. .

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c. Later, Theodor Schwann purified pepsin, the stomach enzyme that digests protein. d. Biologists hypothesized that because pepsin digests protein, it must be produced in an inactive form or else it would destroy the cells that produce it.

(1) Microscopic studies identified granules in the chief cells of the stomach.

(2) Further work showed that the granules contained pepsinogen, an inactive precursor of pepsin.

(3) Contact with stomach acid converts pepsinogen to pepsin.

3. Which cells produce stomach acid? a. Parietal cells are located along canals in the stomach lining that lead to the stomach lumen. (Fig. 41.10a) b. The shape and activity of the cells vary as digestion proceeds. c. Based in part on these observations, researchers inferred that parietal cells are the source of HCl in gastric juice. d. Microscopic studies identified goblet cells as the source of mucus in gastric juice.

The mucus lines the gastric epithelium and protects the stomach from damage due to HCl. 4. How do parietal cells secrete HCl? a. In the 1930s, a researcher found a high concentration of carbonic anhydrase in parietal cells.

(1) Carbonic anhydrase catalyzes the formation of carbonic acid (H2CO3) from CO2 and H2O. (2) Carbonic acid immediately dissociates in solution to bicarbonate ions (HCO–3) and protons (H+). b. Later, transmission electron microscopy showed that parietal cells are packed with mitochondria. (BioSkills 9) (1) Numerous mitochondria suggested that active transport was occurring. c. Later work confirmed that H+ is pumped into the stomach lumen. d. Chloride ions (Cl–) from blood enter the parietal cells in exchange for bicarbonate ions and then move into the lumen through a Cl– channel to create HCl. (Fig. 41.10b) 5. Students should be able to explain why many heartburn (acid reflux) drugs contain proton pump inhibitors.

6. Ulcers as an infectious disease a. An ulcer is a hole in an epithelium that damages the underlying basement membrane and tissues. b. Ulcers are generally associated with infections from a bacterium called Helicobacter pylori.

7. The ruminant stomach a. Ruminants are grazing animals (cows, sheep, antelope, etc.) that have specialized stomachs for digesting plant matter (cellulose) rather than proteins. b. Ruminants can obtain glucose from cellulose because they have a symbiotic relationship with bacteria and protists living in their digestive tract that can ferment cellulose.

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c. Ruminants have a four-chambered stomach. (Fig. 41.11) (1) The rumen holds the cellulase-containing bacteria that ferment glucose and release fatty acids.

(2) The partially digested plant material (cud) is regurgitated, re-chewed, and re-swallowed.

(3) This time, the food bypasses the rumen and enters the reticulum, where the bacteria continue to digest the cellulose.

(4) From the reticulum, the material enters the omasum, where water is removed.

(5) Last, the food passes through the abomasum, where the ruminant’s own enzymes complete the digestion process. d. A key point here is that most of a ruminant’s food consists of fatty acids (and other compounds that are released as waste products of fermentation reactions in symbiotic organisms) and the symbiotic cells themselves.

8. The avian gizzard a. The avian gizzard is a modified muscular stomach that contains swallowed sand and stones used to crush food.

E. The small intestine 1. Peristalsis in the stomach wall sends food through a sphincter into the small intestine.

2. Folding and projections increase surface area. a. Partially digested food mixes with secretions from the pancreas and liver. b. After food passes through 20 feet of small intestine, digestion is complete and most nutrients, and large quantities of water, have been absorbed. c. The epithelial tissue of the small intestine is highly folded. (Fig. 41.12) (1) The epithelium is folded into fingerlike projections called villi.

(2) Each cell on the epithelium surface has projections on its apical surface called microvilli.

(3) With folds, villi, and microvilli, the absorptive surface area is about 2 million cm2—about the size of a tennis court.

(4) This enormous area increases the efficiency of nutrient absorption. And because each villus contains blood vessels and a lymphatic vessel called a lacteal, nutrients pass quickly from epithelial cells into the body’s transport systems.

3. Protein processing by pancreatic enzymes a. Proteins leaving the stomach are denatured and reduced by pepsin to small polypeptides. b. In the small intestine, these small polypeptides are further broken down into amino acids that can be readily absorbed and transported to cells throughout the body. c. By the late 1800s, a series of digestive enzymes (proteases) had been isolated from the pancreas that reduce polypetides to monomers.

(1) All were synthesized in an inactive form.

(2) In 1900, a researcher demonstrated that pancreatic enzymes were activated by contact with juice in the upper part of the small intestine.

(3) Heating the intestinal juice did not activate the enzymes; thus, denaturation was not the activator.

(4) It was proposed that activation of the enzymes is caused by another enzyme. .

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(5) Researchers purified the pancreatic enzyme trypsinogen and found that it was activated by enterokinase.

(6) Enterokinase phosphorylates trypsinogen, converting it into active trypsin.

(Fig. 41.13) (7) Trypsin then activates other pancreatic enzymes.

4. What regulates the release of pancreatic enzymes? a. The initial hypothesis was that nerves control secretion from the pancreas. b. Bayliss and Starling refuted the hypothesis by showing that weak HCl could stimulate pancreatic secretions even when the nerves to the pancreas and small intestine were cut. c. It was then proposed that a chemical messenger from the small intestine activates secretion from the pancreas.

(1) A piece of intestinal wall was extracted, and the solution from it was injected into a dog’s neck.

(2) As a result, pancreatic secretions were stimulated. d. Conclusion: The hormone, secretin, is produced by the small intestine when food arrives and activates pancreatic secretions. e. Therefore, digestion is under both neural and hormonal control. f. Later work found that secretin induces a flow of bicarbonate ions from the pancreas to the small intestine, which neutralizes stomach acid. g. Another hormone, cholecystokinin, induces the secretion of digestive enzymes from the pancreas and molecules from the liver, and gall bladder to break down lipids. h. Hormones are also involved in stomach function. After nerve stimulation from the arrival of food, certain stomach cells produce gastrin, which stimulates parietal cells to secrete HCl. 5. How are carbohydrates digested and transported? a. Carbohydrates that begin to break down in the mouth are further broken down by compounds produced by the pancreas.

(1) Nucleases that digest DNA and RNA (2) Pancreatic amylase, which breaks down carbohydrates to monosaccharides, such as glucose b. Two general principles about nutrient absorption: (1) It is highly selective: Proteins in the plasma membranes of microvilli are responsible for bringing specific nutrients into the cell.

(2) It is active: It requires an expenditure of ATP to bring nutrients into the epithelium across a concentration gradient. c. In the 1980s, researchers found that nutrient absorption depends on an electrochemical gradient that favors sodium entry into the epithelium. d. They proposed that the apical membrane of epithelial cells contains cotransporters. e. Investigators searched for a hypothesized Na+-glucose cotransporter.

(1) Research Question: How is glucose transported into epithelial cells of the small intestine?

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(2) Hypothesis: Glucose enters epithelial cells along with sodium ions via a Na+-glucose cotransporter protein. a. Null hypothesis: Glucose transport does not depend on Na+ transport.

(3) Experiment: a. From rabbits, they isolated mRNA from cells that were transcribing the putative cotransporter. (Fig. 41.14) (BioSkills 6) b. They injected the mRNAs one by one into frog eggs, which do not normally take up glucose.

(4) Results: They found one mRNA that, when translated by the frog egg, caused both glucose and sodium to enter the egg.

(5) Conclusion: The egg that absorbs the Na+ and glucose received the mRNA from the Na+-glucose cotransporter protein.

(6) Follow-up: They reverse transcribed the mRNA into cDNA. (7) They sequenced the cDNA and then inferred the protein sequence from the gene. f. The current model for glucose absorption (1) The Na+/K+-ATPase (sodium potassium pump) establishes the gradient that favors the entry of sodium into the epithelial cells.

(2) Sodium and glucose enter through the cotransporter.

(3) Glucose enters the blood through a glucose channel in the basolateral membrane of the epithelial cell.

6. Digesting lipids: bile and transport a. Pancreatic enzymes digest fats and carbohydrates as well as proteins. b. Fats are insoluble in water, so they tend to concentrate in globules as the stomach contents are churned. c. Bile salts in the small intestine emulsify the fat globules, breaking them up.

(1) Bile salts function like detergents.

(2) Bile salts are synthesized in the liver and secreted in bile, which is stored in the gall bladder.

(3) Bile entering the small intestine raises the pH and emulsifies fats.

(4) Bile salts act like detergents to emulsify fats. (Fig. 41.15) d. Emulsified fats are digested to fatty acids and small lipids by lipase enzymes secreted by the pancreas (pancreatic lipase). e. Fatty acids and monoglycerides enter the small intestine epithelial cells via diffusion. f. After fatty acids enter these cells, they are packaged into protein-coated complexes, chylomicrons, and transported to fat storage cells and other tissues. g. Table 41.3 summarizes the major digestive enzymes.

7. How is water absorbed? a. When solutes from digested material are brought into the epithelium of the small intestine via active transport, water follow passively by osmosis. b. This is an important mechanism for absorbing water that has been ingested and reclaiming liquid that was secreted into the digestive tract.

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c. Students should be able to predict at least two digestive effects of a molecule that selectively blocks the sodium-glucose cotransporter.

F. The large intestine 1. The primary function of the large intestine is to compact the wastes that remain and absorb enough water to form feces and keep the body well hydrated.

2. These processes occur mainly in the colon, with feces being held in the rectum until excretion.

3. Variations in structure and function a. Many plant-eating, nonruminant animals have a cecum that harbors microorganisms that ferment cellulose. b. The cecum is an outpocketing of the digestive tract located at the start of the large intestine. c. In humans, the cecum is reduced in size to form the appendix. d. Rabbits and some other species ferment food in the cecum and excrete a pellet that is then re-ingested. This process is called coprophagy. e. Others still can form a cloaca, an orifice where both urine and feces collect.

IV. Nutritional Homestasis—Glucose as a Case Study • Too much or too little of a nutrient can be problematic or even fatal.

A. The discovery of insulin 1. Individuals with diabetes mellitus have abnormally high levels of glucose in their bloodstream.

2. Searching for the cause of diabetes a. In 1879, researchers removed the pancreas from a dog and observed that the dog developed diabetes. b. Scientists hypothesized that a compound secreted by the pancreas is involved in removing glucose from the bloodstream. c. Efforts to find an extract of the pancreas that prevented diabetes were unsuccessful due to damage to the active agent by pancreatic digestive enzymes during extraction. d. In 1921, Banting and Best eventually isolated the active agent by blocking the pancreatic duct until the tissue atrophied.

(1) The tissue was then frozen and extracted.

(2) The extract was injected into a diabetic dog, and it stabilized the dog’s blood-sugar levels.

(3) The active agent was named insulin.

B. Insulin’s role in homeostasis 1. Insulin is a hormone produced in the pancreas when blood-glucose levels are high.

2. Insulin binds to receptors on cells throughout the body and stimulates them to increase their uptake and processing of glucose. a. Liver cells and skeletal muscle produce more glycogen.

3. As a result, glucose levels in the blood decline.

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4. When blood-glucose levels drop, such as after exercise, the pancreas secretes glucagon. a. In response to glucagon, liver cells break down glycogen and release glucose.

5. Then glucose levels in the blood rise.

6. Homeostasis of glucose in the blood is controlled by insulin and glucagon.

(Fig. 41.16) C. Diabetes mellitus has two forms.

1. Type 1 diabetes mellitus occurs in people who do not synthesize enough insulin (insulin-dependent diabetes).

2. Type 2 diabetes mellitus occurs in people who are resistant to insulin (non-insulindependent diabetes).

3. In both cases, effector cells do not receive the signals that results in a drop of blood glucose levels.

4. Both types of diabetes lead to the production of large volumes of urine.

5. High glucose concentrations in the pre-urine attract water by osmosis, leading to the high volume of urine in diabetics. a. The body attempts to clear the glucose from the bloodstream. b. The word diabetes means “to run through,” and mellitus means “honeyed” or “sweet.” c. The urine contains sugar in diabetes mellitus.

6. The levels of glucose are high also because glucose cannot be imported into cells that require glucose to function—brain neurons can therefore be starved unless the disease is treated.

7. Students should understand the difference between type 1 and type 2 diabetes mellitus, and be able to explain why insulin injections are more effective in controlling blood glucose levels in individuals with type 1 diabetes mellitus.

D. The Type 2 diabetes mellitus epidemic 1. An epidemic of Type 2 diabetes is currently under way in some human populations. a. In the United States, 11% of people over age 20 and 27% of those over 65 are diabetic. b. African Americans, Hispanics, and Native Americans have higher rates of new cases of diabetes. c. The strong association between the prevalence of diabetes mellitus in parents and children has suggested a genetic component to the disease. d. Strong environmental evidence also exists.

2. Diabetes is correlated with obesity. (Fig. 41.17) a. Researchers found a strong relationship between the incidence of type 2 diabetes mellitus and body mass index (BMI). b. The incidence of obesity is increasing due to the higher fat content of foods and labor-saving devices that reduce physical activity.

3. Type 2 diabetes use to be called “adult onset” diabetes; however, this name has been changed to reflect the increase of occurrence among younger individuals. a. One of every three children born in the U.S. will develop type 2 diabetes mellitus in their lifetimes. .

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Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text.

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abomasum

essential nutrients

omasum

absorption adaptive radiation alimentary canal anus appendix aquaporins bicarbonate ion bile bile salts body mass index (BMI) calorie carbohydrate carbonic acid carbonic anhydrase cecum cellulase cellulose chief cells cholecystokinin chylomicrons cichlid fish colonoral jaws complete digestive tract coprophagy cotransporter crop cud deposit feeders diabetes mellitus digestion digestive tract electrolytes emulsified enterokinase esophagus essential amino acids essential elements

fatty-acid gastric juice feces fluid feeders food gallbladder gastrin gastrointestinal tract (GI tract) gizzard glucagon gluconeogenesis glycogen glycoproteintrypsinogen H. pylori HCl heterotroph hormone incomplete digestive tract ingestion insulin kilocalorie lacteals large intestine lingual lipase lipid liver mass feeders microvilli minerals mouth mucin mucous cell mucus nucleases nutrient nutritional balance obese

pancreas pancreatic amylase pancreatic enzymes pancreatic lipase parietal cells pepsin pepsinogen peristalsis pharyngeal jaws protease protein denaturation recommended daily

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allowance (RDA) rectum reflex reticulum rumen ruminant saliva salivary amylase salivary glands secretin skeletal muscle small intestine smooth muscle sphincter stomach suspension feeders symbiosis trypsin Type 1 diabetes Type 2 diabetes ulcer villi vitamins

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Lecture Activities The Digestion Game Estimated duration of activity: 15–20 minutes This activity adds an element of fun to the large amounts of information that students must “digest” in this chapter. This game may be used as a review of the material after you have covered it in class. Divide the class into two or three teams, and give each team blank sheets of paper on which to write their answers. After each question, the teams will write and reveal their responses. Correct answers earn one point, and incorrect answers result in one point lost.

1. Where are proteins initially digested? stomach 2. Where is the digestion of proteins completed? small intestine 3. From where is secretin released? duodenum 4. An increase in solute concentration in the colon will result in what condition? diarrhea 5. A decrease in solute concentration in the colon will result in what condition? constipation 6. Bile is made in the _____ and released into the _____. liver, duodenum 7. Bile is important in the digestion of _____. fats 8. Salivary amylase in the mouth starts the digestion of _____. carbohydrates 9. Name three organs in which peristalsis occurs. esophagus, stomach, small intestine, large intestine 10. Bicarbonate released into the duodenum neutralizes stomach chyme and prevents what condition? ulcers 11. What do parietal cells release? HCl 12. What do chief cells release? pepsinogen 13. Sodium cotransporters are important for what process that occurs in the small intestine? nutrient absorption 14. Which type of diabetes is an autoimmune disorder? Type 1 15. Which type of diabetes is treatable with insulin injections? Type 1

Intestinal Distress of All Kinds Estimated duration of activity: 5–10 minutes While you are discussing the function of the digestive tract, students may find it interesting and fun to use what they are learning to figure out why the following foods, medications, or disorders cause intestinal distress.

Beans: Beans cause intestinal gas because their high oligosaccharide content makes them digestible by the bacteria in the colon. These bacteria give off methane—intestinal gas.

Olestra: Olestra can cause intestinal gas, bloating, and diarrhea because this artificially made molecule (a monosaccharide with eight fatty acids attached) is too bulky to be digested as it passes through the small intestine. Therefore, it increases the solute concentration in the colon, causing less water reabsorption, which leads to diarrhea. The bacteria in the colon can partially digest this molecule, thus generating methane.

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Too much fruit: Fiber, which is cellulose, cannot be digested by humans; instead, it passes into the colon, increasing the solute concentration and thus decreasing water reabsorption and causing loose stool or diarrhea.

Antibiotics: Not only do antibiotics kill pathogenic bacteria, they kill the good bacteria in the colon. If they are not digesting your leftovers, then the solute concentration in the colon increases, thus decreasing water reabsorption and causing loose stool or diarrhea.

Dehydration: Lack of water intake leads to lack of water in stool and in turn leads to constipation.

Lactose intolerance: Some people lack the enzyme lactase. They do not break down the disaccharide lactose. Lactose passes undigested into the colon, where the bacteria break it down, causing flatulence, bloating, and intestinal distress.

Irritable bowel syndrome: IBS is thought to be caused by an increased number of bacteria in the colon or a supersensitive colon muscle that overreacts to food entering the colon and gas released from the bacteria. People with IBS experience cramping, bloating, diarrhea, or constipation.

Gallstones: Gallstones are caused by the precipitation of cholesterol from the gall when stored in the gallbladder. If a stone is blocking the bile duct, then pain occurs after fatty foods are eaten, since that triggers contraction of the gallbladder.

Discussion Ideas for All Class Sizes Vitamins, Surface Area, and Lactose Intolerance Here are three critical-thinking questions to stimulate group discussion.

• Different organisms vary in their nutritional requirements for vitamins. Some organisms do not need to obtain certain items in their diet that other organisms must have. What reason(s) can you propose for why the nutritional requirement for vitamins varies from one organism to another?

Answer: Some organisms synthesize their own version of certain enzymes and do not need to obtain them from their diet. Also, metabolic pathways can vary in different organisms, and some organisms may not require a particular vitamin because they lack the metabolic pathway in which the vitamin is used.

• Each individual has about 25 feet of intestines. Why is it necessary for the intestines to be so long?

Answer: A very large absorptive surface area is necessary if all the valuable molecules in food are to be absorbed after digestion is complete.

• Bacteria that inhabit the colon are responsible for producing some of the gases that build up in the intestines. They live off of undigested materials that pass into the colon. Some people have the syndrome known as lactose intolerance, which is characterized by discomfort due to bloating and gas. Lactose is a disaccharide sugar found in milk and other dairy products. What has gone wrong in people who develop lactose intolerance?

Answer: A lactose-intolerant person has ceased to synthesize enough lactase enzyme to break down the lactose in the diet. Undigested lactose enters the colon, where it is absorbed by bacteria that grow rapidly and produce gas as a by-product of their metabolism.

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CHAPTER

42

Gas Exchange and Circulation

Learning Objectives: Students should be able to…

• Explain the relationship between the respiratory and circulatory systems.

• Describe the factors that affect oxygen and carbon dioxide levels on land and in the water.

• Explain how gas-exchange organs function to maximize the rate of diffusion of gases.

• Explain how gases are transported in the blood.

• Explain how the circulatory system functions to supply tissues with oxygen and nutrients and removes wastes.

Lecture Outline I. The Respiratory and Circulatory Systems A. Gas exchange in all animals involves a four-part sequence: (Fig. 42.1) 1. Ventilation: Air or water moves through the gas-exchange organ.

2. Diffusion at the respiratory surface: Carbon dioxide and oxygen are exchanged between the air or water and the blood at the respiratory surface.

3. Circulation: Dissolved gases (e.g., oxygen, carbon dioxide) are transported throughout the body along with nutrients, wastes, and other molecules, via the circulatory system.

4. Cellular respiration: the cells’ use of oxygen and production of carbon dioxide. In tissues where cellular respiration has lead to low O2 and high CO2, gas exchange occurs between blood and cells.

B. Steps one and two are accomplished by the respiratory system, consisting of structures where gas exchange occurs.

1. In most species, this is the lungs, tracheae, or gills but gas exchange can also occur through the skin.

C. Step three typically involves the circulatory system.

II. Air and Water as Respiratory Media A. How do oxygen and carbon dioxide behave in air?

1. The oxygen content of the atmosphere is 21% no matter what the altitude.

2. However, the air at higher altitudes is less dense, so less total O2 is available.

3. The partial pressure of oxygen indicates how much oxygen is present, taking into account the density of the air at any given altitude. (Fig. 42.2) a. The partial pressure of O2 at sea level is 160 mm Hg. b. The partial pressure of O2 on Mt. Everest is 53 mm Hg. .

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4. In both air and water, gases move from a region of high partial pressure to a region of lower partial pressure.

B. How do oxygen and carbon dioxide behave in water?

1. Water contains far less oxygen compared to air. a. To extract a given amount of oxygen, an organism has to process 30 times more water than the amount a terrestrial animal breathes.

2. Water is far more dense than air, so it requires more energy to ventilate a gas-exchange surface with water than with air.

3. What affects the amount of gas in a solution? a. The solubility of the gas in water b. Water temperature c. The presence of other solutes in the water d. The partial pressure of the gas in contact with the water e. The surface area of the water 4. What affects the amount of oxygen available in an aquatic habitat? a. Habitats with large amounts of photosynthetic organisms tend to be oxygen-rich. b. Habitats with organisms that feed off of organic matter tend to be oxygen-poor. c. Surface area has a great effect on oxygen’s ability to diffuse into water.

(1) Shallow ponds and streams tend to be more oxygenated than deep bodies of water.

(2) Unless currents mix water, water near the surface has a much higher oxygen content than deeper water.

(3) Rapids and waterfalls have the highest oxygen content.

(4) Bogs and other stagnant habitats have very little oxygen.

III. Organs of Gas Exchange A. Respiratory organs provide a large surface area for gas exchange.

1. The surface area must be large enough to meet the demands of a large body filled with cells.

B. Physical parameters: the law of diffusion 1. Fick’s law states that the rate of diffusion depends on five parameters: (Fig. 42.3) a. Solubility of the gas b. Temperature c. Surface area available for gas exchange d. Partial-pressure gradient across that surface e. Thickness of the gas-exchange surface 2. Fick’s law states that oxygen and carbon dioxide diffuse in the largest amounts when three conditions are met: a. The surface area of the gas-exchange surface is large. b. The respiratory surface is very thin. c. The partial-pressure gradient is large.

C. How do gills work?

1. The gill structure allows for a large surface area for gas exchange.

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2. Invertebrate gills vary in structure. a. Some invertebrates have gills that project from the body surface and contact the surrounding water directly (external gills). (Fig. 42.4a) b. In other invertebrates, the gills are internal, so water must be driven over them.

(Fig. 42.4b) 3. The gills of all bony fishes are similar. (Fig. 42.5) a. They are located on both sides of the head. b. A flap of skin, the operculum, is moved to bring water over the gills. c. Fast-swimming fish also move water over their gills by swimming with their mouths open using a process called ram ventilation.

4. Gills use a countercurrent system, which maintains a partial-pressure gradient for gas exchange across the entire surface. (Fig. 42.6) a. A slight gradient in partial pressure exists along the entire length of the lamella. b. A large difference in oxygen partial pressure exists between the start and the end of the system.

5. In a concurrent system: a. A large gradient in partial pressure of oxygen would exist at the start of the system but would rapidly decline. b. A relatively small difference in partial pressure would exist between the start and end of the system.

6. The countercurrent flow makes gills extremely efficient at extracting oxygen from the water.

D. How do insect tracheae work?

1. In aquatic habitats, ventilation tends to disrupt water and electrolyte balance, and homeostasis must be maintained by an active osmoregulatory system.

2. In contrast, breathing leads to a loss of water by evaporation in terrestrial environments.

3. Openings, called spiracles, regulate the rate of gas exchange to minimize water loss.

(Fig. 42.7) 4. Ventilation of the tracheal system is understood based on experiments that measured the PO2 of oxygen during insect flight. (Fig. 42.8) a. Research Question: Does physical activity affect air movement through the tracheal system? b. Hypothesis: Air moves through the tracheal system faster during physical activity. c. Null hypothesis: Physical activity does not affect the rate of air movement. d. Experiment: Partial pressure of oxygen was measured in the hawkmoth flight muscles both at rest and during activity. e. Prediction: Flying will increase ventilation in the tracheal system, causing an increase in PO2 in wing muscle. (Null prediction: Flying will not increase ventilation in the tracheal system and PO2 will decline in the wing muscle during flight.) f. Result: initial drop in oxygen, followed by recovery of oxygen levels as flying continued g. Conclusion: In flying insects, contraction of the flight muscles helps to ventilate the tracheae. .

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5. The muscle relaxations cause the tracheae to dilate and expand. (Fig. 42.9) 6. Muscle contractions cause the trachae to compress.

7. As volume occupied by a fixed amount of gas expands, the gas pressure decreases; as the volume occupied by a fixed amount of gas decrease, the pressure increases.

8. In larger insects, larger tracheal diameters further promote movement of gases. a. Disproportionately large tracheae allow for effective ventilation and allows for extremely large insects. b. Giant insects flourished in the Paleozoic era. c. The Paleozoic era had higher oxygen concentrations than today. (Fig. 42.10) d. During the late Paleozoic era, the difference in the partial pressures of oxygen was high, which may have increased the rate of movement of gases through the tracheal system to permit ventilation in extremely large insects. e. At the start of the Mesozoic era, oxygen concentrations dropped and many large insects went extinct, likely in part because they could not supply their tissues with sufficient oxygen.

E. How do vertebrate lungs work?

1. Most mammals draw air in through both the mouth and nose. a. The air enters the lungs via the trachea, through bronchi into bronchioles.

(Fig. 42.11a) b. Bronchioles end in millions of tiny sacs called alveoli. (Fig. 42.11b) (1) Alveoli increase the respiratory surface area.

(2) Alveoli are very thin, which maximizes gas exchange. (Fig. 42.11c) 2. Mammalian lungs are ventilated through contraction and relaxation cycles of the muscles of respiration. a. In contrast to frogs, which push air into their lungs via positive pressure ventilation, mammals draw air into their lungs via negative pressure ventilation. b. As the diaphragm contracts with inhalation, the lung volume increases, creating negative pressure inside the lung and drawing air in. (Fig. 42.12a) c. Exhalation is driven by the elastic recoil of the lung when the diaphragm relaxes. d. Ventillary forces, such as negative pressure ventilation, can be modeled by a balloon in a jar. (Fig. 42.12b) 3. Ventilation of bird lungs is constant, allowing for birds’ vigorous exercise at high altitudes. (Fig. 42.13) a. During inhalation, air flows into the trachea and enters two large air sacs posterior to the lungs. b. During exhalation, air flows out of the air sacs into branching airways in the posterior of the lung (parabronchi). c. During the next inhalation, air enters the parabronchi in the anterior part of the lungs and onto a series of air sacs anterior to the lungs. d. During the next exhalation, air moves out of the anterior sacs, through the trachea and is expelled from the body, when air from the second inhalation is flowing through the lungs. e. With this unidirectional ventilation system, oxygenated air is moved over the respiratory surface during both inhalation and exhalation.

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f. Efficiency of the system is due to gas exchange occurring during both inhalation and exhalation, reduced dead space, and blood circulation occurs in a cross-current pattern (more efficient than web pattern found in mammals).

F. Homeostatic control of ventilation 1. The resting breathing rate is established by the medullary respiratory center (MRC) in the brain.

2. During exercise, active muscles take up more oxygen and give off more carbon dioxide. a. This decreases the partial pressure of oxygen in the blood and increases the partial pressure of carbon dioxide in the blood. b. This carbon dioxide diffuses into the cerebrospinal fluid (CSF) and combines with water to make carbonic acid, which then dissociates to form a bicarbonate ion and a hydrogen ion.

CO2+HOHCO 2 2

H+ +HCO−3 3

c. This reduces the pH of the blood and the CSF. The reduced pH is sensed by a population of nerve cells that signal the MRC to increase the breathing rate. d. The increase in breathing rate increases the rate of oxygen delivery to tissues at the rate at which carbon dioxide is eliminated from the body, restoring POP CO2 to their 2 resting levels.

IV. How Are Oxygen and Carbon Dioxide Transported in Blood?

A. The composition of blood 1. Blood is composed of cells in a watery extracellular matrix.

2. Formed elements include platelets, white blood cells, and red blood cells. a. Platelets are cell fragments that are essential in blood clotting. b. White blood cells are part of the immune system and fight infections. c. Red blood cells transport oxygen from the lungs to the tissues.

B. Structure and function of hemoglobin 1. Hemoglobin consists of four polypeptide chains, each with an iron-containing heme group that can carry one oxygen molecule. (Fig. 42.14) 2. What is cooperative binding? a. Dissociation curves show that there is only a 25% change in saturation as hemoglobin delivers one molecule of oxygen to the tissue. (Fig. 42.15) b. The binding of one oxygen molecule to hemoglobin makes it easier to bind successive oxygen molecules; this is known as cooperative binding. c. Cooperative binding also means that the loss of an oxygen molecule makes the loss of an additional oxygen molecule more likely.

3. Why is cooperative binding important? a. Cooperative binding means that hemoglobin will become saturated with oxygen at even a small partial-pressure gradient. b. Cooperative binding makes hemoglobin extremely sensitive to changes in the tissue PO. 2 (1) With cooperative binding, large amounts of oxygen are delivered to resting and exercising tissues. (Fig. 42.16a) .

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(2) Without cooperative binding, smaller amounts of oxygen are able to be delivered to resting and exercising tissues. (Fig. 42.16b) 4. How do pH and temperature affect oxygen unloading from hemoglobin? a. During exercise, temperature and partial pressure of CO2 rise in the active muscle. c. The CO2 produced by exercising muscle reacts with the water in blood to form carbonic acid, which dissociates and releases a hydrogen ion. As a result, the pH of the blood in exercising muscle drops. d. Decreases in pH and increases in temperature alter hemoglobin’s conformation. e. As the graph in Figure 42.17 shows, these shape changes make hemoglobin more likely to release O2 at any given value of tissue PO2. This phenomenon is known as the Bohr shift. f. The Bohr shift is important because it makes hemoglobin more likely to release oxygen during exercise or other conditions in which PO2 is high, pH is low, and tissues are under oxygen stress.

5. Oxygen delivery by hemoglobin is extremely efficient. a. As an example of the Bohr shift, biologists had rainbow trout swim in a tunnel b. against the current.

The current was increased, requiring the fish to swim faster, and periodically, the c. biologists sampled the O2 content of the arterial and venous blood.

Oxygen levels in the arteries remained constant, meaning that the gills worked to d. keep the hemoglobin saturated with oxygen. e. In contrast, venous oxygen levels dropped with increasing swimming speed.

A combination of increased temperature, lower pH, and lower PO2 caused hemoglobin levels to become deoxygenated.

6. Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin does to ensure that the fetus gets adequate oxygen during development. (Fig. 42.18) C. CO 2transport and the buffering of blood pH 1. Red blood cells contain carbonic anhydrase. It catalyzes the formation of carbonic acid, which dissociates into bicarbonate and a hydrogen ion. a. The protons produced help to induce the Bohr’s shift, which facilitates the release of oxygen from hemoglobin. b. The partial pressure of carbon dioxide in blood drops as CO2 is converted to soluble

bicarbonate ions, maintaining a gradient that favors the entry of CO2 into red blood cells.

c. Carbonic anhydrase makes CO2 uptake from tissues more efficient. (Fig. 42.19) d. Most of the CO2 transported in the blood is in the form of bicarbonate ions. e. The hydrogen ions bind to hemoglobin, which acts as a buffer, preventing a change in pH. f. When bicarbonate is transferred to the lung, the reaction is reversed and CO2 is

released to the environment.

V. Circulation A. The function of a circulatory system is to carry blood or hemolymph into close contact with every cell in the body.

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B. What is an open circulatory system?

1. In open circulatory systems, fluid connective tissue (hemolymph) is actively pumped through vessels, but the hemolymph is not confined to the vessels.

2.

The hemolymph comes into direct contact with the tissues, molecules exchanged between the hemolymph and tissues do not have to diffuse across the wall of a blood vessel.

3. Spiders have an open circulatory system (Fig. 42.20) where hemolymph moves as a result of contractions of a heart as well as through general body movements moving the hemolymph to and from the heart.

4. The low pressure of this system means that the rate of flow may be low, and there is no mechanism for redirecting flow to specific tissues.

C. What is a closed circulatory system?

1. In closed circulatory systems, blood flows in a continuous circuit throughout the body 2. under pressure generated from the heart.

Because blood is confined to vessels, the system can generate pressure and maintain 3. high flow rates.

There are three types of blood vessels, and their structure correlates with their function: a. Arteries are thick-walled vessels that take blood away from the heart.

(1) In closed systems, blood is pumped from the heart into a large artery called the (2) aorta.

The walls of the aorta have elastic fibers that allow it to stretch when blood is (3) pumped from the heart at high pressure.

When pumping ends, the diameter of the aorta returns to its original size, (4) propelling the blood away from the heart.

Small arteries are called arterioles. b. Capillaries are vessels that are just one cell thick allowing for gas and nutrient c. exchange between blood and tissues. (Fig. 42.21a) Veins are thinner-walled vessels (compared to arteries) that carry deoxygenated blood back to the heart. (Fig. 42.21b) (1) Blood pressure drops as the blood passes through the capillaries, and it is very low by the time the blood reaches the veins.

(2) Veins have valves that keep the blood moving back toward the heart.

(3) Veins are embedded in skeletal muscle, and contractions in those muscles help propel the blood to the heart.

4. Exchange between blood plasma and interstitial fluid a. The thin walls of capillaries, combined with increased pressure of a closed circulab. tory system results in a constant leakage of fluid into the spaces between cells.

Two forces affect this flow: (Fig. 42.22) (1) The force created by the pressure of the blood in the capillary forces fluid out of (2) the capillary.

There is more solute (due to large proteins such as albumin) in the blood than in the interstitial fluid, causing water to return to the capillary.

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c. This results in fluid and ions exiting the capillary at the arteriole end and fluid returning at the venous end.

5. The lymphatic system returns excess interstitial fluid to circulation.

6. The fluid inside the lymphatic system is called “lymph.”

D. How does the heart work?

1. Atria receive blood returning from circulation; ventricles force blood out of the heart 2. and back into circulation.

The number of heart chambers differs among vertebrate groups. (Fig. 42.23) The a. number of atria and ventricles increased as vertebrates diversified.

In fish, the circulatory system forms a single circuit in which one loop services the b. gills and the body. In other groups, there are separate circuits to the lungs and to the body.

Increased numbers of chambers allow for two separate circuits—one supplying the c. organ(s) of gas exchange and the other supplying the remaining tissues. d. a terrestrial environment, blood must be pumped at higher pressures to account In for the pressures of gravity. Having two separate circuits can also help to overcome this issue: (1) Pulmonary circulation is a low-pressure circuit to and from the lungs.

(2) Systemic circulation is a higher-pressure circuit to and from the rest of the body.

(3) The pulmonary and systemic circulation systems are separate in the fourchambered hearts of crocodiles, birds, and mammals but are only partially separated in the three-chambered hearts of amphibians, turtles, lizards, and snakes.

3. The human circulatory system consists of one four-chambered heart and separate pulmonary and systemic circuits. (Fig. 42.24) a. Half of the heart pumps blood to the lungs to pick up oxygen; the other half pumps blood to the body to give away oxygen. b. This process ensures that blood is delivered to both the lungs and the body tissues at high pressure.

4. The route of blood through the heart (Fig. 42.25) a. Deoxygenated blood from the body returns to the right atrium via the venae cavae. b. When the atria contract, blood is pumped from the right atrium to the right ventricle.

(1) Blood flows through the heart in only one direction due to valves between the atria and ventricles.

(2) When these valves are not effective, blood can flow back into the atria from the ventricles, causing a heart murmur. c. After ventricular contraction, the deoxygenated blood leaves the right ventricle and travels through the pulmonary artery to the lungs. d. Oxygenated blood from the lungs returns to the left atrium of the heart via the pulmonary vein. e.

Oxygenated blood is then pumped to the left ventricle and ultimately through the f. aorta to the body.

Partial pressures of gases vary throughout the human circulatory system.

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(1) In healthy individuals, gas exchange in the lungs and tissues is rapid compared to the rate of blood flow through capillaries, which ensures that the partial pressures for oxygen and carbon dioxide in the tissues and systemic veins are equal.

5. Electrical activation of the heart a. Pacemaker cells located in the sinoatrial (SA) node initiate contractions of the mammalian heart. (Fig. 42.26) (1) Cells in the heart are branched and contact several other cardiac muscle cells, (2) each joined by intercalated disks.

These cells are electrically continuous because they are connected via gap (3) junctions.

(4) Electrical activation starts with the SA node generating an electric signal.

(5) The electric signal moves quickly through the atrial muscle cells and the atria contracts and ejects blood into the ventricles.

(6) Once the electrical signal has swept through the atria, it arrives at the atrioven(7) tricular node, a set of specialized atrial cells.

The atrioventricular node delays the signal before passing it on to the ventricles.

The delay ensures that the ventricles fill completely with blood from the atria before contracting. After the delay, the impulse causes both ventricles to contract as the atria relaxes. The ventricles empty.

(8) The ventricles relax and their cells recover, restoring their original electrical state. b. An electrocardiogram is a recording of these events. (Fig. 42.26) c. Students should be able to predict how the amount of blood ejected from the ventricles would change if there were no delay at the AV node.

6. The cardiac cycle a. One cardiac cycle consists of a contraction phase, called systole, and a relaxation phase, called diastole.(Fig. 42.27) b. During diastole, the atria and ventricles fill with blood. c. Systole creates the pressures needed to eject blood into the pulmonary and systemic d. circuits.

Blood pressure is reported as systolic/diastolic.

(1) High blood pressure, hypertension, can be a dangerous health condition.

E. Patterns in blood pressure and blood flow 1. Blood pressure drops as blood progresses from the arteries to the capillaries and on to the veins. (Fig. 42.28) a. Capillaries branch and rebranch to form networks. The increased total crosssectional area of the capillaries causes a large drop in blood pressure.

(1) Mechanical resistance to flow is a function of the total cross-sectional area; friction losses are high in capillary beds.

(2) Velocity of blood flow also decreases as the same amount of blood flows through a larger area (network).

2. Maintaining blood pressure and flow within a normal range is critical to the health of an individual.

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a. Blood flow is carefully regulated at several points throughout the circulatory system.

(1) Contracting or relaxing smooth muscle that surrounds arterioles can affect blood flow (e.g., dilating during heat or exercise). b. Changes in blood pressure are detected by baroreceptors (pressure receptors).

(1) A drop in blood pressure triggers an increase in the heart rate, stroke volume, and constriction of blood vessels—changes that help return blood pressure to normal.

(2) A rise in blood pressure triggers a decrease in the heart rate, stroke volume, and dilation of blood vessels—changes that help return blood pressure to normal.

3. Cardiac disease is a group of ailments collectively affecting the heart and blood vessels. a. It is the number one cause of human death worldwide. b. Many factors contribute to cardiac disease including age, diet, inactivity, obesity, c. and genetics.

A decrease in the diameter of blood vessels through a loss of elasticity (atherosclerosis), constriction (e.g., tobacco use), or blockage (plaque formation) can increase blood pressure and weaken the walls of arteries. d. Blocked arteries can cause myocardial infarctions (heart attacks).

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text. air sacs alveoli aorta arteries arterioles atherosclerosis atmospheric pressure atrioventricular (AV) node atrium baroreceptors bicarbonate blood-pressure gradient Bohr shift bronchi bronchioles buffer capillaries capillary beds carbonic acid 438

carbonic anhydrase cardiac cycle cardiac output cardiovascular disease cellular respiration circulatory system closed circulatory system clots conformational change connective tissue constriction cooperative binding countercurrent exchange diaphragm diastole diastolic blood pressure diffusion dilation dissociation curve EKG

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elastic exhalation extracellular matrix Fick’s law of diffusion formed elements gas exchange gill gill filaments gill lamellae heart heart murmur heart rate heart valves heme hemoglobin hemolymph hypertension inhalation insect tracheal system intercalated discs

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interstitial fluid interstitial space lung lymph lymphatic ducts lymphatic system medullary respiratory center mm Hg myocardial infarction negative pressure open circulatory system operculum oxygen-hemoglobin equilibrium curve pacemaker cells parabronchi

partial pressure partial-pressure gradient plasma platelets positive pressure ventilation pulmonary artery pulmonary circulation pulmonary vein red blood cells respiratory epithelium respiratory organ respiratory system saturation shunt sigmoidal sinoatrial (SA) node solubility

spiracle stem cells stroke volume surface area systemic circulation systole systolic blood pressure trachea tracheae valves veins venae cavae ventilation ventricle venules white blood cells

Lecture Activities A Murder Mystery Estimated duration of activity: 15–20 minutes This activity has students take a real-life situation and explain it based on what they have learned about how the heart works. The following news article describes a murder committed by a nurse who injected patients with KCl. http://www.cmaj.ca/cgi/reprint/157/6/757.pdf You may either post this article on your class website for students to read before class or read it in class. After reading and discussing the general information provided about the murder, have your students work together as a team of expert witnesses to explain (in detail) why KCl would kill someone.

Answer: Potassium is normally concentrated inside heart cells and must remain concentrated inside the cells to set up the appropriate concentration gradients for action potential firing. If it becomes more concentrated outside the cells (as it does after a massive KCl injection into the bloodstream), then the electrochemical gradient that drives sodium into the cells during the action potential will be disrupted, cardiac cells will not fire, and the heart will stop (cardiac arrest).

Using a Concept Map to Discover the Link between Smoking and Cardiac Health Estimated duration of activity: 5–10 minutes This short activity may be used as a tool to summarize the material in Chapter 42. Many students have been told that they should not smoke, but they do not really understand the dangers of smoking to overall respiratory and cardiac health. You can highlight these dangers while reviewing the important link between the cardiovascular and respiratory systems. Begin the concept map by writing the question “Why is smoking bad for you?” in the center of the board. Circle it and wait for students to reply. They will offer several deleterious effects of smoking on the lungs and respiratory system (carbon monoxide decreases the oxygencarrying capacity of red blood cells, tar is carcinogenic and is deposited in the lungs, etc.). .

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Then you can connect the concepts by mapping how the respiratory system is connected to the cardiovascular system, demonstrating the far-reaching consequences of cigarette smoke (whether smoked directly or inhaled secondhand).

Discussion Ideas for All Class Sizes Fick’s Law • After introducing Fick’s law, ask students to predict how the answers to the following questions will affect the rate of gas exchange across the gill epithelium of a marine fish.

1) How will the following affect the rate of O2 uptake from the environment?

• decrease in water temperature • increase in the O2 affinity of hemoglobin • increase in the surface area of the gill • decrease in the PCO levels of the blood 2

• decrease in the PCO levels of the environment 2

2) How will the following affect the rate of CO2 elimination?

• increase in water temperature • increase in the NaCl content of the water • increase in the PCO2 of the water • increase in PCO blood levels 2

• decrease in the thickness of the gill epithelium • Ask students to predict whether the affinity for O2 of fetal hemoglobin is greater than, less than, or equal to that of maternal hemoglobin.

• Air breathing evolved in aquatic vertebrates many times. Air-breathing fish remain in the water but rise to the surface to take in air to supplement the oxygen supply generated by their gills.

1) Ask students to predict why the number of air-breathing fish species is much higher in the tropics than in temperate environments.

2) Ask students whether they would expect the circulatory systems of air-breathing fish to be identical to those of strictly water-breathing fish. If not, what adaptations might be present to support air breathing?

Answers: The gills are not adapted for air as a respiratory medium; they collapse when not in water. Some air-breathing fish store air in the gut; others in the swim bladder or mouth. Some absorb air across the skin surface. Wherever gas exchange with air takes place, that organ is heavily vascularized.

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Learning Objectives: Students should be able to…

• Describe the mechanisms in neurons that establish and maintain membrane potentials.

• Explain the steps in an action potential.

• Explain the events that occur at a chemical synapse when a neuron communicates with another tissue.

• Describe the structure and function of the components of the vertebrate nervous system.

Lecture Outline I. Principles of Electrical Signaling A. Neurons are organized into two major types of nervous systems: 1. Nerve nets are a diffuse arrangement of cells and are found in cnidarians (jellyfish, hydra, anemones).

2. Central nervous systems (CNS) are large numbers of neurons aggregated into clusters (ganglia). a. Often, organisms with a CNS have a large cerebral ganglion (brain) located at the anterior end. b. Sensory appendages take information in and send it to the brain for processing, which interprets the information and sends electrical signals to muscles and tissues to respond to the sensory stimuli.

B. Types of neurons 1. Sensory receptors receive information about the external environment and body interior. a. Sensory cells on the periphery of the body (skin, eyes, ears, mouth, and nose) transmit information about the external environment. b. Sensory cells inside the body monitor conditions that are important in homeostasis (e.g., blood pH and temperature).

2. The pathway of information flow in the central nervous system (CNS) in vertebrates a. The brain integrates sensory information and sends signals to effector cells in glands or muscles. b. The CNS consists of the brain and spinal cord. c. Cells within the CNS, called interneurons, pass signals from one neuron to another and integrate the information from many sensory neurons. d. Interneurons also make connections between sensory neurons and motor neurons. .

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e. Motor neurons and sensory neurons are bundled together to form strands of nervous tissue known as nerves, which send signals to effector cells in glands or muscles.

3. The peripheral nervous system (PNS) consists of all the components of the nervous system that are outside the CNS. a. Typically, sensory information from the PNS is sent to the CNS, where it is processed.

4. When you prick your finger and withdraw your hand, this reaction is a reflex—an involuntary response to an environmental stimulus. (Fig. 43.1) C. The anatomy of a neuron 1. Neurons are small, transparent, and morphologically complex.

2. In 1898, Camillo Golgi discovered that silver nitrate solution makes some neurons visible.

3. Then Golgi and Ramon y Cajal found that most neurons have dendrites, a cell body, and one or more axons. (Fig. 43.2a) a. Dendrites collect chemical signals.

(1) They are highly branched but are generally no more than 2 mm long.

(2) The number and arrangement of dendrites vary in different neurons.

(3) Many brain neurons have only dendrites and lack axons. b. The cell body, or soma, integrates incoming signals and generates outgoing electrical signals to the axon(s). c. The cell body is where incoming signals are integrated and an outgoing signal is sent to the axon. d. Axons can be more than a meter long and pass along chemical signals to dendrites of another cell or to an effector cell. e. Cell body can also receive signals from axons of other cells.

4. Most neurons receive inputs from many different neurons and send projections to several different neurons. (Fig. 43.2b) D. An introduction to membrane potentials 1. Ions carry an electric charge, and the cytoplasm and extracellular fluids contain unequal distributions of ions.

2. Cells are inherently electrical in nature.

3. A difference of electrical charge between any two points creates an electric potential.

4. A membrane potential occurs when the positive and negative ions on either side of the membrane is unequal. a. The membrane potential is large if the charge difference across the membrane is large. b. Membrane potentials refer specifically to a separation of charge immediately adjacent to the plasma membrane. c. Even if large membrane potentials exist, there may be no charge separation farther away from the membrane. d. Membrane potentials are measured in millivolts (1/1000th of a volt) and are expressed in terms of inside relative to outside (often negative in charge). e. Resting neurons have a membrane potential of about -65 mV.

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5. If a membrane potential exists, the ions on either side of the membrane have potential energy. a. Ions move across membranes in response to charge and concentration gradients⎯that is, an electrochemical gradient. b. A flow of charge is called an electric current.

E. How is the resting potential maintained?

1. When a neuron is not communicating with other cells or tissues, the difference in charge across its membrane is called the resting potential. (Fig. 43.3) a. The interior side of the membrane has low concentrations of Na+ and Cl– ions, high concentrations of K+ ions, and some amino acids that have lost a proton (negatively charged). b. The extracellular fluid has high concentrations of Na+ and Cl– compared to the intracellular fluid. c. Ions cannot cross a phospholipid bilayer, but ions can move across plasma membranes efficiently in three ways: (1) Flowing along the electrochemical gradient through ion channels (2) Carried via a membrane cotransporter proteins along with ions that are diffusing along their electrochemical gradient (3) Actively pumped against an electrochemical gradient by a membrane protein that hydrolyzes ATP 2. The role of Na+/K+-ATPase a. Na+ and Cl– cross the membrane more slowly than K+, but some exchange occurs. b. The sodium-potassium pump actively moves 3 Na+ ions out of the cell for every 2 K+ ions brought into the cell. c. In this way, active transport via Na+/K+-ATPase ensures that eventually the concentration of K+ is much higher inside the cell than outside, while the concentration of Na+ is lower inside than outside.

3. The role of the K+ leak channel a. When a neuron is at rest, the plasma membrane is relatively impermeable; however, many neurons contain potassium leak channels that allow for K+ to leak out of the cells.

(1) K+ ions can move through leak channels down their concentration gradient and exit the neuron.

(2) As K+ leaves the cell, the interior of the cell becomes more negatively charged.

(3) The buildup of negative charge inside the cell eventually begins to attract K+.

(4) The membrane reaches a voltage at which there is an equilibrium between the concentration gradient that moves K+ out and the electrical gradient that moves K+ in.

(5) This equilibrium is the equilibrium potential for K+.

4. Students should be able to predict what would happen to the membrane potential if Na+ or K+ were allowed to diffuse freely across the membrane.

F. Using microelectrodes to measure membrane potentials 1. In the 1940s, A. L. Hodgkin and Andrew Huxley pioneered the use of giant axons in squid for electrical signaling studies. .

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a. The giant axon conducts signals to contract muscles that expel water from a cavity to give the squid jet-propulsion motion. b. The squid giant axon is particularly large: about 500 μm in diameter. c. A wire could be inserted down the giant axon; later, glass microelectrodes were used.

2. Hodgkin and Huxley recorded resting potentials in the giant axon and found it could be disturbed through an action potential when the axon was stimulated.

G. What is an action potential?

1. An action potential is a rapid, temporary change in the resting potential.

2. It consists of a three-phase signal: (Fig. 43.4) a. During depolarization the membrane becomes less polarized.

(1) The membrane potential moves from highly negative toward 0 mV, and then is briefly positive. b. A rapid repolarization phase changes the membrane potential back to negative. c. A hyperpolarization phase occurs where the membrane is slightly more negative than the resting potential. d. All three phases of an action potential take only a few milliseconds.

3. Action potentials will not occur unless the depolarization phase reaches a minimum change from the resting potential. This minimum amount is the threshold potential.

4. An action potential occurs because specific channels in the plasma membrane open or close in response to changes in voltage.

5. Action potentials always have the same three phases, but the threshold potential and the peak depolarization may be different among species or among different neurons within a species.

6. Hodgkin and Huxley made other important observations about action potentials. a. An action potential is an all-or-none event; partial action potentials do not exist. b. All action potentials for a given neuron have the identical shape and magnitude. c. Action potentials are propagated down the length of the axon, with no change in shape or magnitude.

7. These observations suggest a mechanism for electrical signaling. a. Information is in the form of action potentials that travel down neurons. b. The frequency of action potentials, not their magnitude, is meaningful.

II. Dissecting the Action Potential A. Distinct ion currents are responsible for depolarization and repolarization 1. Hodgkin and Huxley tested the ion currents that cause an action potential. a. They recorded electrical activity in axons bathed in seawater containing different concentrations of ions. b. Results (1) Removing Na+ from the bathing solution abolished the action potential.

(2) When [Na+] in the external solution was varied, the peak of the action potential tracked the Eion of Na+, based on [Na]+outside the cell. c. The data supported the hypothesis that an action potential begins when Na+ flows into the neuron.

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2. What ion flow is responsible for repolarization? a. If radioactive K+ is used, a strong outward flow of K+ is detected during repolarization. b. An action potential consists of a strong inward flow of Na+ followed by a strong outward flow of K+.

3. Students should be able to add the labels “Sodium channels open, Na+ enters” and “Potassium channels open, K+ leaves” to Figure 43.4.

B. How do voltage-gated channels work?

1. Voltage-gated channels open and close in response to changes in membrane voltage. a. The conformation of the channel protein changes in response to the charges present at the inside of the membrane. b. The shape of the voltage-gated channel changes depending on the membrane potential. (Fig. 43.5) c. Voltage clamping makes it possible to hold an axon at any voltage and record the electrical currents that occur at that voltage.

2. Patch-clamping studies of single channels a. Erwin Neher and Bert Sakmann perfected a technique called patch clamping. b. This technique involves using extremely fine-tipped microelectrodes to isolate a single channel and record from it. (Fig. 43.6) c. Using patch-clamping, researchers demonstrated that currents flowed through individual channels and that different channels behaved differently.

(1) Voltage-gated channels are either open or closed.

(2) Sodium channels open quickly after depolarization.

(3) Potassium channels open with a delay after depolarization.

3. Positive feedback occurs during depolarization a. Sodium channels are more likely to open as the membrane depolarizes. b. This begins a positive feedback loop that results in more and more sodium channels opening. c. This explains why the action potential is an all-or-none phenomenon.

4. Using neurotoxins to identify channels and dissect currents a. Neurotoxins are poisons that affect nerve function, often resulting in convulsions, paralysis, or unconsciousness. b. Researchers tested the tetrodotoxin from puffer-fish on giant axons from lobsters. c. After tetrodotoxin was applied, the resting potential was normal, the action potential was abolished, outward-directed K+ flow was normal, and inward-directed Na+ flow was abolished. d. It was concluded that puffer-fish toxin specifically blocks the voltage-gated Na+ channels of neurons.

C. How is the action potential propagated?

1. Hodgkin and Huxley proposed a three-step model to explain how an action potential propagates down an axon. (Fig. 43.7a) a. Step 1—An influx of Na+ at the start of an action potential repels cations, pushing them away from the sodium channels.

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b. Step 2—As positive charges are pushed away from the sodium channels, they depolarize adjacent portions of the membrane. c. Step 3—Nearby voltage-gated sodium channels open when the membrane reaches the threshold point and an action potential is generated. d. The action potential is continuously regenerated at adjacent areas of the membrane as it travels down the axon. (Fig. 43.7b) 2. Why don’t action potentials propagate back up the axon? a. After Na+ channels have opened and closed, they inactivate and will not open again for a short time (the refractory state). b. Due to the hyperpolarization phase, a larger than normal stimulus would also be required to achieve the threshold potential to trigger an action potential (due to the membrane being more negative than the resting potential).

3. Axon diameter affects speed a. The squid’s axon is large because a charge spreads faster in an axon with a larger diameter. b. Cations moving down axons with larger diameters meet less resistance to flow compared to those moving through more narrow axons. c. Giant axons propagate action potentials more quickly than smaller axons.

4. Myelination affects speed. a. Most vertebrates do not have giant axons; instead, they have specialized accessory cells whose membranes increase the efficacy of action potentials. b. In the CNS, these accessory cells are oligodendrocytes. c. In the PNS, the accessory cells are Schwann cells. d. Collectively, accessory cells are called glia. e. When oligodendrocytes or Schwann cells wrap around an axon, they form a myelin sheath that acts as an electrical insulation. (Fig. 43.8a) f. The myelin sheath prevents ions from leaking back across the plasma membrane of the neuron. g. Cations moving down the axon are able to spread until they hit an unmyelinated section of the axon. h. Unmyelinated sections are called nodes of Ranvier (Fig. 43.8b), which contain voltage-gated sodium and potassium channels. i. Electrical signals jump rapidly from node to node much faster than down an unmyelinated axon of the same diameter. j. Myelination is an adaptation that facilitates rapid transmission of electrical signals in axons with small diameters. k. In people who have multiple sclerosis, the oligodendrocytes are targeted by the immune system, which damages the myelin sheath and results in impaired electrical signaling, weakened muscles, and loss of coordination.

III. The Synapse A. Neurotransmitters 1. Most neurons are not directly connected yet still communicate with one another.

2. Otto Loewi showed that chemical neurotransmitters are the indirect mechanism that transmits electrical signals from cell to cell.

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a. Hypothesis: The signal from nerve to muscle is delivered by a chemical. b. Experiment: Loewi investigated the stimulus to the vagus nerve and heart of a frog.

(Fig. 43.9) (1) Hypothesis: Molecules called neurotransmitters carry information from one neuron to the next.

(2) Null hypothesis: Information is not transferred between neurons in the form of molecules.

(3) Experiment: After stimulating the vagus nerve of the frog heart (to slow the heartbeat), collect the solution that surrounds the nerve end and add it to a second heart that did not have the vegus nerve stimulated.

(4) Prediction: The heart will slow.

(5) Null prediction: There will be no change in heart rate.

(6) Result: The heartbeat slowed after the solution was added. c. Conclusion: The vagus nerve releases molecules that slow the heartbeat.

Neurotransmitters contain information.

B. Synapse structure and neurotransmitter release 1. Transmission electron microscopy revealed the nature of the synapse. (Fig. 43.10) a. Membranes of axons are separated by synaptic clefts. b. The ends of axons contain sac-like structures called synaptic vesicles. c. Synaptic vesicles were hypothesized to contain stored neurotransmitters.

2. Model of synaptic transmission is based on anatomical and chemical studies (Fig. 43.11) and contains five steps: a. The action potential arrives at the end of the axon. b. Depolarization created from the action potential opens voltage-gated calcium channels and results in an influx of calcium into the cell. c. Calcium triggers synaptic vesicles to fuse with the plasma membrane of the presynaptic neuron and release neurotransmitters into the presynaptic cleft. d. Neurotransmitters function as a ligand and bind to receptors in the postsynaptic cell, which changes the membrane potential of the postsynaptic cell. The change in membrane potential may trigger an action potential in the postsynaptic cell. e. The response ends and the neurotransmitter is broken down and taken back up by the presynaptic cell.

C. What do neurotransmitters do? (Table 43.1) 1. Patch-clamping studies using radioactive neurotransmitters showed that many receptors for neurotransmitters are ligand-gated channels. a. Ligand-gated channels open and close in response to the binding of a specific ligand to a receptor molecule. b. After the binding of a neurotransmitter to a ligand-gated ion channel in the membrane of the postsynaptic neuron, the channel opens and ions flow along an electrochemical gradient. c. The neurotransmitter's chemical signal is transduced to an electrical signal (a change in the membrane potential of the postsynaptic neuron).

2. Not all neurotransmitters bind to ion channels.

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a. Some bind to receptors that activate enzymes whose action produces second messengers in the postsynaptic neuron. b. Second messengers are chemicals that travel through the cell and induce changes in enzyme activity, gene transcription, or membrane potential.

D. Postsynaptic potentials 1. Ligand-gated sodium channels on the membranes of dendrites are in high concentrations near synapses.

2. When neurotransmitters bind, ligand-gated sodium channels open and let sodium into the cell, causing depolarization. (Fig. 43.12a) a. Depolarization increases the likelihood that an action potential will fire in the postsynaptic neuron. b. Changes that increase the likelihood of an action potential are called excitatory postsynaptic potentials (EPSPs).

3. When the receptor activity at the synapse induces an efflux of K+ or an influx of Cl–, the postsynaptic membrane hyperpolarizes. (Fig. 43.12b) a. Hyperpolarization decreases the likelihood that an action potential will fire in the postsynaptic neuron. b. Changes that decrease the likelihood of an action potential are called inhibitory postsynaptic potentials (IPSPs).

4. When an EPSP and IPSP occur at the same time, they cancel each other out.

(Fig. 43.12c) 5. Postsynaptic potentials are graded. a. Unlike action potentials, EPSPs and IPSPs are not all-or-none events. b. They are graded in size and short-lived. c. The size of an EPSP or IPSP depends on the amount of neurotransmitter released. d. Both types of signals are short lived because neurotransmitters do not bind irreversibly to channels in the postsynaptic cell. e. The length of the EPSP or IPSP depends on how quickly the neurotransmitter is inactivated by enzymes or taken up by the presynaptic cell and recycled. f. The street drugs cocaine and amphetamine alter neuron activity by inhibiting the uptake and recycling of neurotransmitters. (Table 43.1) 6. Summation and threshold a. The dendrites and cell body of a neuron typically makes hundreds or thousands of synapses with other neurons. (Fig. 43.13a) b. EPSPs and IPSPs that are occurring at any of the synapses lead to short-lived surges in charge in the dendrites and cell body. c. The changes in membrane potential are additive. (Fig. 43.13b) (1) If an EPSP and an IPSP occur close to each other in space and time, they may cancel each other out.

(2) If a series of EPSPs are close together, they sum and make an action potential more likely. d. An action potential begins at the axon hillock.

(1) Na+ channels that trigger the start of an action potential are located at the beginning of the axon in the axon hillock.

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(2) Charge spreads to the axon hillock as EPSPs and IPSPs interact in the dendrites and cell body.

(3) If the membrane at the axon hillock depolarizes past a threshold, enough Na+ channels open to trigger positive feedback and an action potential, which propagates down the axon.

(4) Neurons receive input from many synapses, and because IPSPs and EPSPs sum, information in the form of electrical signals is modified at the synapse before being passed along.

IV. The Vertebrate Nervous System A. What does the peripheral nervous system do?

1. The PNS is divided into two systems: a. The afferent system carries information to the CNS. b. The efferent system carries information away from the CNS. c. Neurons in the afferent system monitor conditions inside and outside the body.

Neurons in the efferent system carry signals that allow the body to respond to changed conditions.

2. The efferent system of the PNS is further divided into two systems: (Fig. 43.14) a. The somatic nervous system responds to external stimuli and controls voluntary movement. b. The autonomic nervous system responds to internal stimuli and controls internal processes such as digestion and heart rate. c. In the somatic nervous system, skeletal muscles serve as the effectors. d. In the autonomic nervous system, smooth muscle, cardiac muscle, and glands serve as the effectors.

3. Two distinct types of autonomic nerves serve many glands and organs: (Fig. 43.15) a. Parasympathetic nerve signals promote relaxation and digestion by slowing down the heart and stimulating the digestive tract. b. Sympathetic nerve signals prepare organs for stressful situations by speeding up the heart, stimulating the release of glucose from the liver, and inhibiting the digestive tract.

4. The enteric nervous system (possibly the third division of the PNS) consists of neurons embedded in the wall of the GI tract. It interacts with autonomic nerves but can also function independently. It plays a major role in regulating digestion.

B. Functional anatomy of the CNS 1. The spinal cord collects and transmits information from throughout the body. a. Parasympathetic nerves originate at the base of the brain or spinal cord. b. Most sympathetic nerves originate in the spinal cord. c. Most sensory and motor neurons project to or from the spinal cord. d. Other than reflexes, all the information that travels to or from the spinal cord is sent to the brain for processing.

2. General anatomy of the human brain a. The brain is the most complex organ in animals and is divided into four structures: cerebrum, cerebellum, diencephalon, and brain stem. (Fig. 43.16a)

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b. The cerebrum is the largest portion of the brain and is the seat of conscious thought and memory.

(1) The cerebrum is divided into the left and right hemispheres.

(2) Each cerebral hemisphere has four major lobes: parietal, frontal, temporal, and occipital. (Fig. 43.16b) (3) The left and right hemispheres are connected by the corpus callosum. c. The cerebellum coordinates motor patterns. d. The diencephalon relays sensory information to the cerebellum and controls homeostasis. e. The brain stem connects the brain to the spinal cord and regulates the heart, lung, and digestive system. f. The relative size of the entire brain, and its component structures, is variable among vertebrates. (Fig. 43.17) 3. Mapping the brain I: lesion studies a. Early brain research focused on studying people who had specific mental defects caused by brain damage. b. Paul Broca claimed that specific regions of the brain coordinate specific functions.

(1) He studied a patient who could understand language but couldn’t speak.

(2) Autopsy revealed that the patient’s brain had a lesion in the left frontal lobe of the cerebrum.

(3) Broca concluded that the left frontal lobe is responsible for speech. c. Broca hypothesized that functions are localized to specific brain areas. Studies of patients who have had part of the brain removed helped confirm Broca’s hypothesis.

(1) Phineas Gage recovered from a brain injury where a railroad spike was blasted through the frontal lobe of the cerebrum. (a) After the incident, his personality changed, providing evidence that the frontal lobe of the cerebrum is implicated in personality and emotion.

(2) Henry Molaison had a portion of the temporal lobe removed in 1953 to prevent life-threatening seizures. (a) The man retained normal intelligence and could remember his childhood, but he lost his short-term memory. (b) This example and additional experiments in mice provide support that aspects of memory are governed by the hippocampus and interior sections of the temporal lobe.

4. Mapping the brain II: electrical stimulation of conscious patients a. Wilder Penfield pioneered the study of brain function in conscious patients.

(1) All the patients had severe brain seizures. b. Electrical stimulation of portions of the cerebrum caused patients to report sensations of movement, temperature, touch, and so on. c. Penfield mapped the sensory and motor regions of the cerebrum. (Fig. 43.18) d. Some patients reported that electrical stimulation triggered vivid memories.

(1) Could some neurons store specific memories?

(2) The hypothesis that memories are stored in specialized cells is controversial.

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(3) A recent study supports this hypothesis: (a) Researchers attached electrodes that recorded electrical activity of neurons to the temporal lobes and hippocampi of the study subjects. (b) The subjects were shown pictures of celebrities, places, or objects. (c) In one subject, a specific neuron fired when the subject viewed images of Jennifer Aniston and did not fire when other images were viewed.

(Fig. 43.19) (d) The formation or retrieval of memories associated with specific concepts may involve specific neurons.

4. Mapping the brain III: optogenetics a. Optogenetics is a technique that uses light to control the activity of targeted neurons. b. By inserting algal gene that codes for light-activated ion channels, researchers were able to study the behavior of mice while stimulating action potentials in the neurons that expressed the algal ion channel. (Fig. 43.20) C. How do learning and memory work?

1. Memory is the retention of learned information. a. Learning and memory are closely related. b. Studies of memory generally involve tandem studies of learning.

2. Recording from single neurons during memory tasks a. Researchers recorded individual neurons in the temporal lobes of seizure-prone humans. b. Words were projected onto a screen and patients were asked to read them silently, or aloud, and/or to remember and repeat them later. c. Neurons in the cerebrum’s temporal lobe were active when the patient was remembering the objects and repeats their names out loud but quiet when the patient was identifying objects.

3. Documenting changes in synapses a. Research on the molecular basis of memory is based on two ideas: (1) Some type of short-term or long-term change in neurons must occur during learning and memory. (a) The change could be structural, such as modifications in the number of synaptic connections the neuron makes. (b) The change could be chemical, such as alterations in the amount of neurotransmitter released.

(2) Understanding the changes is easier if an extremely simple system of neurons is studied. b. Eric Kandel and colleagues studied the sea slug Aplysia californica. (Fig. 43.21a) (1) Touching the siphon of an Aplysia causes it to withdraw its gill. (a) The reflex involves a sensory neuron that is activated by touch and a motor neuron that projects to a gill muscle. (Fig. 43.21b) (b) Retracting the gill protects it from predators.

(2) The Aplysia reflex is modified by learning. (a) Aplysia also withdraws the gill if its tail is touched. (b) Aplysia does not withdraw its gill if the siphon is lightly touched. .

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(c) Researchers performed repeated tail touching accompanied by light touching of the siphon. (d) They found that a light touch of the siphon caused Aplysia to withdraw its gill. c. Follow-up studies showed that the neurons involved in learning in Aplysia release serotonin, which causes an EPSP in the motor neuron to the gill.

(1) More serotonin is released after learning has taken place.

(2) Increased serotonin results in more EPSPs. (Fig. 43.22b) (3) The motor neuron is more likely to generate action potentials.

(4) Conclusion: In Aplysia, changes in the nature of the synapse correlate with learning and memory.

(5) The change in responsiveness of the synapse is known as synaptic plasticity. d. Aplysia sensory and motor neurons in culture also exhibit learning and memory responses.

(1) Serotonin was applied repeatedly to synapses between cultured sensory and motor neurons. (Fig. 43.22a) (2) Stimulation of the sensory neuron one day later produced a huge increase in EPSPs. (Fig. 43.22c) (3) Additional synapses were established and neurons that were not stimulated by serotonin had normal numbers of synapses and normal EPSPs. (4) The structure and behavior of the postsynaptic motor neuron had changed based on its experiences.

4. Documenting changes in neurons a. Based on the observation that neurons do not show signs of mitosis, neurogenesis (the formation of new neurons) is not thought to occur in adults.

(1) Goldman and Nottebohm revisited this hypothesis by experimenting in songbirds.

(2) Researchers injected canaries with radiolabeled thiamine, which is incorporated into newly synthesized DNA. (Fig. 43.23a) (3) Using autoradiography (BioSkills 6), cells that had taken up the radiolabeled thiamine (had undergone new cellular growth) would be exposed on the film.

(4) Researchers found that adult songbirds are capable of producing new neurons.

(5) Follow-up studies have confirmed that new neurons are incorporated into the song control region of the brain each spring, causing them to grow in size.

(Fig. 43.23b) (6) The new neurons promote learning and memory in the song-control system.

(7) Studies in mammals found evidence of neurogenesis; however, the number of cells was lower in adult mammals compared to adult birds.

(8) New techniques that specifically label neurons definitively showed that neurogenesis occurs in all vertebrates, including humans. b. Learning, memory, and control of complex behaviors involve molecular and structural changes in synapses as well as changes in the number of neurons.

(1) Most researchers now agree that long-term memory also involves changes in gene expression.

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Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text. absolute temperature (T) action potential afferent division Aplysia californica autonomic nervous system axon axon hillock brain stem cell body/soma central nervous system (CNS) cerebellum cerebrum corpus callosum dendrite depolarization diencephalon dopamine effector efferent division electric current electrical potential electrochemical gradient equilibrium potential excitable membranes excitatory postsynaptic potentials (EPSPs) extracellular fluid Faraday constant (F) frontal lobe gas constant (R) glass microelectrode

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glia hyperpolarization inhibitory postsynaptic potentials (IPSPs) interneuron ion channels ions leak channels learning left hemisphere ligand ligand-gated ion channels membrane potential memory millivolt (mV) motor neuron multiple sclerosis myelin sheath myelination Na+/K+-ATPase natural logarithm Nernst equation nerve nets nerves neuron neurotoxins neurotransmitters node of Ranvier occipital lobe oligodendrocytes parasympathetic nerve parasympathetic nervous system parietal lobe patch clamping

peripheral nervous system (PNS) phencyclidine (PCP) polarization positive feedback postsynaptic neuron potential energy presynaptic neuron reflex refractory repolarization resting potential right hemisphere Schwann cells second messenger selectively permeable sensory neuron serotonin siphon somatic system squid giant axon summation sympathetic nerve sympathetic nervous system synapse synaptic cleft synaptic plasticity synaptic vesicles temporal lobe thermodynamic potential threshold voltage voltage clamping voltage-gated channels

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Lecture Activities Trivial Pursuit Estimated duration of activity: 5–10 minutes This chapter is rich with information that will enable your students to understand the biology behind a wealth of interesting trivia. Although it is not necessarily our goal as educators to explain the oddities of life, these little facts and tidbits can capture the attention of even the most apathetic student and can be used as a means to cement the concepts that you have so diligently taught.

Toxic cosmetics: Botulinum toxin (BOTOX) injection is a popular cosmetic treatment to eliminate wrinkles and lines on the face. However, few people realize that BOTOX is actually a neurotoxin!

• When discussing synaptic transmission, use the neuromuscular junction as a model synapse. Tell students that BOTOX inhibits the release of acetylcholine, and then ask them to conclude how this is useful in flattening wrinkles.

Answer: Wrinkles are formed when facial muscles contract, so BOTOX injections inhibit ACh release, blocking synaptic transmission at the neuromuscular junction and thus “relaxing” the muscle.

• Alternatively, in the class period before you discuss synaptic transmission, instruct students to research botulinum toxin on the Internet before coming to class. Then ask them to explain what medical treatments it is used for and how it works at the synaptic level to treat those conditions.

Answer: Besides being a popular cosmetic treatment, BOTOX has been used to treat excessive sweating, migraine headaches, and tension headaches.

• Twenty questions: When discussing synaptic transmission, bring up BOTOX, but do not tell students how it works. Simply tell them that it is used to treat wrinkles. Then give them the opportunity to play “20 questions” with you to try to figure out what botulinum toxin does at the synapse and how that ultimately relaxes wrinkles.

Deadly sushi: In Japan, puffer-fish sushi is considered a delicacy—a deadly delicacy, if your chef is not an expert. Fugu is a raw puffer-fish delicacy that can be prepared only by licensed chefs. If the fugu is not cut carefully, the meat can become tainted with the deadly neurotoxin tetrodotoxin. Tetrodotoxin blocks voltage-gated sodium channels and prevents the generation of action potentials. Sixty percent of people who eat tainted fugu die.

• When explaining action potentials, use the fugu story to highlight the importance of voltage-gated sodium channels in the generation of action potentials.

• Alternatively, when discussing action potentials, ask your students what would happen if the voltage-gated sodium channels were blocked. Then you can move into the fugu story.

• Twenty questions: When discussing action potentials, bring up fugu, but do not tell students how it works. Simply tell them that it is a Japanese delicacy that, if not prepared correctly, can kill you by stopping all neurotransmission in the body. Then give them the opportunity to play “20 questions” with you to try to figure out how tetrodotoxin blocks the generation of action potentials.

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This Is Your Brain on Drugs Estimated duration of activity: 30 to 60 minutes Whether your students are nonmajors, biology majors headed for research careers, or premed students headed for medical careers, they will all be interested in understanding how popular psychoactive drugs work, why they are initially rewarding, and why they can be so dangerous. You can simply present these ideas to students, but students will learn better if they explain the ideas to one another. You may assign mini-presentations. Or you may divide the class into small groups; provide each group with printed information about a specific drug type; give the groups 15 minutes to learn and discuss the physiological mechanism of action, why that mechanism leads to a particular sensation, and why it can be dangerous; and then have each group present its drug to the class. If you are short on class time, you can use each mini-presentation as a springboard for a particular topic in this chapter (see the * under each drug).

Methamphetamine Physiological mechanism of action: Triggers massive release of dopamine, one of the major neurotransmitters in the limbic system Sensory effects: Euphoria, heightened mental and physical performance, weight loss Health dangers: Desensitization and down regulation of dopamine receptors occur, which then requires that more of the drug be taken for the same high. Because the drug is not metabolized or eliminated from the brain tissue, it begins to accumulate and become toxic to the cells in the limbic system. Once these brain regions are damaged, the ability to control emotions is lost and rage, panic, and hallucinations result. In addition, because dopamine is an important regulatory neurotransmitter in the basal ganglia, insensitivity to dopamine results in a Parkinson’s disease-like disorder that includes involuntary twitching. *Related lecture topics: Dopamine, dopamine receptors, limbic system, basal ganglia Ecstasy (MDMA) Physiological mechanism of action: Triggers the release of serotonin, blocks the reuptake of serotonin, and eventually depletes serotonin by damaging serotonergic cell axons Sensory effects: Creates euphoria, heightens senses, makes users feel less inhibited, may facilitate weight loss Health dangers: Complicating effects of MDMA include nausea, paranoia, confusion, severe anxiety, inability to regulate body temperature, kidney failure, and others. *Related lecture topics: Serotonin, neurotransmitter reuptake via transporters, cell anatomy (axons) Cocaine Physiological mechanism of action: Blocks the removal of dopamine from synapses in the nucleus accumbens, part of the limbic system (the area of the brain that mediates emotional responses) Sensory effects: Euphoria, increased energy, reduced appetite, heightened mental awareness Health dangers: Tolerance and addiction, cardiovascular problems, depression *Related lecture topics: Neurotransmitter removal (reuptake/enzymatic degradation), brain anatomy (limbic system) .

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Marijuana (THC) Physiological mechanism of action: Binds to THC receptors (natural function currently unknown) that are concentrated in the frontal cortex. These receptors are inhibitory and seem to “calm” the frontal cortex via inhibition during times of heightened neuronal firing.

Sensory effects: Euphoria, slowed reaction time, relaxation, loss of inhibitions, impaired balance and judgment Health dangers: Impaired learning and memory, cardiovascular problems, anxiety, panic attacks, tolerance, addiction *Related lecture topics: Summation, inhibition Nicotine Physiological mechanism of action: Nicotine is an agonist for the nicotinic acetylcholine receptor (a ligand-gated sodium channel). The neuromuscular junction has nicotinic acetylcholine receptors.

Sensory effects: Initially causes a sense of alertness, then later causes feelings of calmness and serenity Health dangers: Lung dysfunction, cancer (lung, mouth), impaired circulation, tolerance, addiction *Related lecture topics: Ligand-gated neurotransmitter receptors, neuromuscular transmission, acetylcholine, excitatory neurotransmitter receptors Dextramethorphan Physiological mechanism of action: A noncompetitive NMDA receptor antagonist (like ketamine and PCP) Sensory effects: Hallucinations, out-of-body sensations Health dangers: Increased heart rate, increased blood pressure, sleepiness, decreased reaction time, blurred vision, confusion, paranoia, seizures *Related lecture topics: Neurotransmitter receptor antagonists, NMDA receptors, glutamate/glutamate receptors Ephedra (ephedrine, pseudoephedrine) Physiological mechanism of action: Agonists of adrenergic receptors (that normally bind epinephrine and norepinephrine) Sensory effects: Increased alertness, decreased appetite, weight loss, energy, increased heart rate, increased physical performance Health dangers: Sleeplessness, restlessness, irritability, vomiting, urinary disorders, headache *Related lecture topics: Sympathetic nervous system, norepinephrine/epinephrine Acid (LSD) Physiological mechanism of action: Inhibits the firing of serotonergic neurons Sensory effects: Excitement, hallucinations, changed thought patterns, feelings of insight Health dangers: Confusion, paranoia, quickly changing emotions *Related lecture topics: Neuronal inhibition, serotonin

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Discussion Ideas for All Class Sizes Potential Energy Students often have difficulty with the concept of potential energy. Several different terms that are used for different types of potential energy are introduced in this chapter. The following challenge questions can be presented to the class to generate discussion about what each of the terms means, how they are similar, how they are different, and the circumstances under which each should be used.

• Compare and contrast the following types of potentials: 1) membrane potential 2) electrical potential 3) equilibrium potential 4) thermodynamic potential What is the definition of each? Give an example illustrating the nature of each potential.

Answers: 1) A membrane potential is a separation of charge across a cell membrane. All membranes in living organisms have a membrane potential because pumps and channels in the membrane produce gradients of ion concentration and charge across the membrane. For example, a membrane potential develops when more Na+ is pumped outside the cell than K+ is pumped inside. This creates a net positive charge on the outside of the membrane relative to the inside.

2) An electrical potential, or voltage, results from positive and negative charges not canceling each other out. The magnitude of the voltage is determined by how large the difference is between positive and negative charges. For example, in living cells, the resting potential is often—70 millivolts, due to the presence of many large organic anions in the cell that cannot cross the membrane, plus the activity of the sodium-potassium pump, plus the establishment of equilibrium potentials by sodium, potassium, and chlorine ions. The inside of the cell is 70 millivolts more negative than the outside of the cell.

3) An equilibrium potential is the voltage at which a particular ion shows no net movement across the membrane because equilibrium has been reached between its concentration gradient and the electrical gradient across the membrane. An example is the resting potential, which results from sodium, potassium, and chlorine each reaching its equilibrium potential.

4) The thermodynamic potential is RT/F, which is the gas constant multiplied by the temperature in degrees Kelvin, divided by the Faraday constant. The Faraday constant specifies the amount of charge carried by 1 mole of a univalent ion. At room temperature (25°C), RT/F is 25 mV. The thermodynamic potential is used in the Nernst equation to calculate the equilibrium potential for a particular ion. Because the number for converting from natural log to base 10 log is 2.3, the Nernst equation can be simplified to the following for a univalent cation such as potassium: + + E ion = 58 log ( [K]o/[K]i)

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• What is the difference between voltage clamping and patch clamping? What types of information does each technique provide?

Answers: In voltage clamping, the researcher is able to set the membrane at a specific, selected voltage. The effect of that voltage on current flow across the membrane can then be determined. The movement of specific ions can be followed as the membrane potential is changed. As voltage-gated ion channels in the membrane open or close, the negative feedback system of the voltage-clamping device prevents the changes in the membrane current from altering the membrane potential. The patch-clamping technique isolates a single ion channel in order to study the function of that channel. A micropipette with a diameter of about 1 micrometer is placed on the membrane, and suction is applied. The micropipette is filled with a salt solution. The composition of the salt solution can be altered to determine the effect on the isolated channel. The patch-clamping technique provides information about how individual channels work—what causes them to open or close (voltage or ligands), and how slowly or quickly they open or close.

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CHAPTER

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Animal Sensory Systems

Learning Objectives: Students should be able to...

• Explain the process whereby sensory stimuli are transduced into action potentials.

• Explain how hearing and the lateral line system utilize mechanoreceptors to transduce sound waves into action potentials.

• Explain how photoreception is used in vision.

• Explain how certain molecules in the environment are detected by chemosensation.

• Describe how animals use thermoreception, electroreception, and magnetoreception to sense their environment.

Lecture Outline • Sensing and interpreting changes in the environment are fundamental to how organisms work.

• Male moths fly toward female pheromones, bats pursue the moths using ultrasonic cries, and the moths hear the bats coming and perform evasive maneuvers.

• Humans cannot perceive any of this because we lack the appropriate sensory capabilities.

I. How Do Sensory Organs Convey Information to the Brain?

A. Sensory information is detected by sensory neurons or a specialized sensory receptor that makes a synapse with a sensory neuron. (Fig. 44.1) 1. The central nervous system (CNS) integrates information from sensory systems.

2. The brain responds with impulses that are sent to specific muscle groups (effectors) that respond to the stimulus.

B. Two processes are involved in sensing a change in the environment: 1. Transduction of the external signal into an internal signal in the form of an action potential 2. Transmission of the signal to the central nervous system (CNS) C. Sensory receptors are categorized by the type of stimulus they respond to.

1. Mechanoreceptors respond to touch or pressure.

2. Photoreceptors respond to wavelengths of light.

3. Chemoreceptors perceive specific molecules.

4. Thermoreceptors sense changes in temperature.

5. Nociceptors sense harmful stimuli such as tissue damage.

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6. Electroreceptors detect electric fields.

7. Magnetoreceptors detect magnetic fields.

D. Sensory transduction 1. Sensory receptors and neurons transduce the sensory information into changes in membrane polarization.

2. Large changes in a sensory receptor’s membrane potential changes the firing rate of action potentials sent to the brain.

3. Sensory stimuli cause ions to flow across the membrane, depolarizing or hyperpolarizing the neuron. (Fig. 44.2a) 4. The rate of action potentials can provide information of the intensity of the stimulus. a. For example, louder sounds induce a higher frequency of action potentials in the sensory cells of the inner ear. (Fig. 44.2b) E. Transmitting information to the brain 1. Receptor cells tend to be highly specialized. a. For example, each sound receptor in the human ear responds best to a certain sound frequency.

2. The pattern of action potentials from a receptor cell contains information about the frequency, intensity, and duration of the stimulus.

3. Each type of sensory neuron sends its information to a specific part of the brain.

II. Mechanoreception: Sensing pressure changes A. Animals have a variety of mechanisms for identifying changes in pressure.

1. Crabs have a fluid-filled organ with pressure-receptor cells (statocyst) that detects whether the animal is upside down.

2. Animals can detect physical pressure on the skin.

3. Animals detect sound waves, which is pressure waves in air.

4. Animals detect pressure waves in water via their lateral line.

B. How do sensory cells respond to sound waves and other forms of pressure?

1. Direct physical pressure, or distortion of the plasma membrane by bending, causes conformational changes of ion channels causing them to open or close. a. The membrane depolarizes or hyperpolarizes. b. The result is a new pattern of action potentials.

2. In vertebrates, ion channels that respond to pressure are located in hair cells.

(Fig. 44.3a) a. Stiff outgrowths called stereocilia, arranged in order of height, are located at one end of the cell. b. Many hair cells also have a single kinocilium, a true cilium with a 9 + 2 arrangement of microtubules.

3. Signal transduction in hair cells a. The extracellular fluid around stereocilia has a very high K+ concentration. b. If the stereocilia bend in the direction toward the kinocilium, K+ channels open and depolarize the cell. (Fig. 44.3b) c. If the stereocilia bend in the opposite direction, K+ channels close and the cell hyperpolarizes.

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d. Tiny threads connect the tips of stereocilia. e. One hypothesis is that when stereocilia bend, the threads pull open ion channels in the membrane of the next taller stereocilium, like tiny trapdoors.

C. Hearing: The mammalian ear 1. Hearing is the sensation produced from wave-like changes in pressure (sound) in air or water.

2. The number of waves in one second is the frequency, and different frequencies are perceived as different pitches.

3. The vertebrate ear has three parts—outer ear, middle ear, and inner ear—separated by membranes. (Fig. 44.4) a. Sound waves reaching the outer ear are funneled into the ear canal and cause the tympanic membrane to vibrate, passing the vibrations to the middle ear. b. The middle ear of mammals contains three ear bones, or ear ossicles. c. The last ossicle, known as the stapes, passes the vibrations to the inner ear via a membrane called the oval window. d. The oval window transmits the vibrations to the fluid-filled cochlea in the inner ear, where the hair cells are.

4. The middle ear amplifies sound. a. The middle ear transfers vibrations from a large surface (the tympanic membrane) to a much smaller surface (the oval window), thereby increasing the force of vibration by a factor of 15. b. In mammals, the three ossicles act as levers that amplify the vibrations.

(1) Overall, sound passing through the mammalian middle ear is amplified by a factor of 22. c. Biologists interpret the mammalian middle ear as an adaptation for increasing sensitivity to sound.

5. The inner ear contains the fluid-filled cochlea, which detects the frequency of sounds. a. The cochlea is divided by internal membranes into three chambers. (Fig. 44.5a) b. Hair cells are in the middle chamber, sandwiched between two other membranes: the basilar and tectorial membranes. (Fig. 44.5b) c. When the basilar membrane vibrates, the stereocilia on the hair cells are bent one way and then the other by the tectorial membrane. d. Each segment of the basilar membrane vibrates in response to a different frequency of sound.

(1) This is because the basilar membrane is very stiff near the oval window and very flexible at its end. (Fig. 44.6) (2) High-frequency sounds cause the stiff parts to vibrate; low-frequency sounds cause the more flexible parts to vibrate. e. Therefore, a hair cell in a particular place on the membrane responds only to sounds of a certain frequency. f. The brain, in turn, receives action potentials from the neurons associated with each hair cell and interprets them as a particular pitch—that is, a specific frequency of sound.

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g. Complex sounds contain a variety of frequencies and cause a specific combination of hair cells to fire action potentials.

6. Elephants detect infrasound. a. Infrasounds have a frequency that is too low for humans to hear (below 20 Hz). b. Katherine Payne recorded elephants at the Oregon Zoo with a microphone capable of picking up extremely low-frequency sound and found that the elephants were producing infrasonic calls. c. Recent research shows that elephants have the best infrasonic hearing of any land mammal.

(1) Infrasonic sounds can travel great distances.

(2) Infrasonic calls may enable elephants to find one another even when they are miles apart.

7. Bats detect ultrasound. a. Ultrasounds have a frequency higher than the range of human hearing (above 20,000 Hz, or 20 kHz). b. In the 1930s, Donald Griffin and Robert Galambos demonstrated that bats produce ultrasonic cries and that they need their mouths and ears (but not their eyes) to avoid obstacles when flying. c. They concluded that bats navigate around obstacles by emitting ultrasonic sound waves that bounce off surfaces (echolocation). The returning sound waves are sensed by the inner ear. (Fig. 44.7) d. Other animals, such as dolphins and shrews, also use sonar.

D. The lateral line system in fish and amphibians a. Most fishes and larval amphibians have a line of hair cells in a gel-like domed structure, cupulae, inside canals that run down each side of their body. (Fig. 44.8a) b. This system allows the animals to detect pressure changes in water, such as from waves or another animal swimming by. (Fig. 44.8b) c. Research Question: What cues do predatory fish use to detect prey at night?

(Fig. 44.9) (1) Hypothesis: Predatory fish detect prey using the lateral line system.

(2) Alternative hypothesis: Predatory fish detect prey using smell and taste.

(3) Null hypothesis: Predatory fish use neither sense to detect prey.

(4) Experiment: Researchers chemically ablated the lateral line system (blocked the hair cells) or surgically ablated the ability to smell and taste (removed the lobes of the brain that respond). A third group was kept intact as a control. The researchers added guppies and recorded the success rate.

(5) Prediction: Fish with the lateral line ablated would have decreased success rates compared to control.

(6) Alternative Prediction: Fish with smell/taste ablated would have decreased success rates compared to control.

(7) Null Prediction: There will be no difference among the three groups.

(8) Results: Lateral line ablated fish had a 17% success rate, smell/taste ablated fish had a 60% success rate, and control fish had a 65% success rate.

(9) Conclusion: Nocturnal catfish use the lateral line system to detect prey.

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III. Photoreception: Sensing light A. Most animals are able to sense light.

1. Some species have simple light-sensitive eyespots.

2. Image-forming eyes are found in vertebrates, cephalopod mollusks, and arthropods.

3. In most cases, a species’ sensory abilities correlate with its environment and its mode of life—how it finds food and mates.

B. The insect eye 1. Insects have a compound eye composed of numerous light-sensing columns called ommatidia. (Fig. 44.10a) 2. Each ommatidium has its own lens and receptor cells that send axons to the brain.

(Fig. 44.10b) 3. Each ommatidium contributes information about a small part of the visual field, like a pixel on a computer screen.

C. The vertebrate eye 1. Simple eyes—structures with a single lens that focuses light on a layer of many receptor cells—evolved independently several times. a. Cephalopods independently evolved a camera eye.

2. The structure of the simple eye (Fig. 44.11a) a. The outermost layer is the sclera (white of the eye) and the cornea (a transparent sheet of connective tissue). b. Just inside the cornea is the iris, which contracts or expands to adjust the size of the pupil. c. The pupil is the hole in the center of the iris. d. Light enters the eye through the cornea and passes through the pupil and a curved, clear lens. e. The cornea and lens together focus light onto the retina at the back of the eye. f. The retina has a thin layer of photoreceptors and several layers of neurons. g. The retina is composed of three distinct cell layers. (Fig. 44.11b) (1) Photoreceptors are held in place by pigmented epithelium.

(2) Photoreceptors synapse with an intermediate layer of connecting neurons known as bipolar cells.

(3) Bipolar cells connect with one another and with neurons called ganglion cells, which form the innermost layer of the retina. (a) The axons of the ganglion cells project to the brain via the optic nerve. (b) Blind spots occur because there are no photoreceptors where the optic nerve leaves the retina.

3. Photoreceptors in the eye consist of two types: rods and cones. (Fig. 44.12a) a. Rods are highly sensitive to dim light but not to color; thus, night vision is largely black and white. b. Cones are less sensitive to dim light but are stimulated by different wavelengths (colors). c. Rods dominate the retina of humans. d. One small spot in the center of the retina, the fovea, has only cones. e. The high density of cones in the fovea maximizes image resolution. .

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4. How do rods and cones detect light? a. Rods and cones have outer segments packed with membrane-rich disks. The membranes contain large quantities of a molecule called rhodopsin. (Fig. 44.12a) b. Rhodopsin consists of a protein (opsin) associated with a light-absorbing molecule (retinal). (Fig. 44.12b) (1) Retinal changes shape when it absorbs a photon of light. In response, opsin also changes shape. (Fig. 44.12c) (2) This leads to a series of reactions that closes the membrane channels and decreases the number of action potentials produced. a. When retinal is not activated by light, sodium channels in the rod’s plasma membrane are open. Entry of sodium continually depolarizes the cell. Exposure to light inhibits both processes through a series of events. (Fig. 44.13) b. After the activation of rhodopsin via a conformational change, transducin activates an enzyme called phosphodiesterase (PDE). c. PDE breaks down the cyclic guanosine monophosphate (cGMP) to guanosine monophosphate (GMP). d. As cGMP declines, the cGMP-gated sodium channels close. e. When the sodium channels close, sodium entry decreases and the membrane hyperpolarizes.

5. Students should be able to explain why (1) cGMP acts as both a second messenger and a ligand in this system and how transducin compares to the G proteins (introduced in Chapter 11) and (2) why closing Na+ channels results in hyperpolarization.

6. Color vision: The puzzle of Dalton’s eye a. The 18th-century chemist John Dalton realized that he and his brother had redgreen color blindness. (Fig. 44.14) b. Dalton hypothesized that red wavelengths failed to reach his retinas due to absorption by a bluish fluid in his eyes. However, when he died, autopsy revealed that his eyes had normal fluid, so his hypothesis was incorrect.

7. Color vision: Multiple opsins a. The human retina contains three types of color sensitive photoreceptors, each with a different opsin that can absorb red, green, or blue light (called L, M, and S opsins, for long, medium, and short wavelengths, respectively). (Fig. 44.15) b. The brain distinguishes colors by integrating information from the three types of opsins. c. For example, a wavelength of 560 nm stimulates L cones strongly, M cones to an intermediate degree, and S cones hardly at all. This combination is perceived as yellow. d. We now know that red-green color blindness is often caused by defective M or L opsins. e. In the 1990s, DNA testing of Dalton’s preserved eyes revealed that he had a normal L allele but he lacked a functional M allele. This was the cause of his color blindness.

8. Do other animals see color? a. Animals vary greatly in their color-vision capabilities.

(1) Nocturnal animals have many rods and few cones, giving them great sensitivity to dim light but poor color vision.

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(2) Many vertebrates and invertebrates have four or more opsins; they probably can perceive more colors than we can. b. Sensory capabilities correlate with an animal’s environment and way of life.

(1) Coelacanths have two opsins sensitive to different wavelengths of blue light, probably an adaptation to a deepwater environment.

(2) Fruit-eating primates (including humans) can distinguish wavelengths of around 550 nm, allowing them to differentiate the greens, yellows, and reds of ripe and unripe fruits.

(3) Birds and insects are able to perceive ultraviolet light. Flowers often have ultraviolet patterns that serve as nectar signals for pollination. Some birds have ultraviolet patterns in their plumage that are used as a criterion in mate selection.

IV. Chemoreception: Sensing Chemicals A. Taste (gustation) and smell (olfaction) are due to chemoreceptors that detect specific molecules.

1. A specific molecule causes a change in membrane potential in the chemoreceptor cell.

B. Taste: Detecting molecules in the mouth 1. Chemoreceptor cells are clustered in taste buds, which occur mostly on the tongue. a. Each taste bud contains about 100 spindle-shaped taste cells that synapse to taste neurons. (Fig. 44.16) 2. Salt and sour a. Salty and sour sensations result from the activity of ion channels.

(1) Salty tastes are due to Na+ in food entering Na+ channels in taste cells and depolarizing their membranes.

(2) Sourness is due to H+ in food entering proton channels in taste cells and depolarizing their membranes.

(3) Therefore, foods that have a low pH taste sour.

3. Why do many different foods taste bitter? a. Many molecules with very different structures are perceived as tasting bitter. b. The ability to taste bitterness has a genetic component. Example: Phenylthiocarbamide (PTC) c. Genetic mapping has identified a family of 40 to 80 genes that code for receptor proteins, each of which binds to only one type of bitter molecule.

(1) Each taste cell can have many different bitter receptors.

(2) These results explain why so many different molecules taste bitter. d. Animals use their extensive array of bitter receptors to avoid toxic compounds in plants, many of which have a bitter taste.

4. What is the molecular basis of sweetness and other tastes? a. Three closely related membrane receptors detect sweetness as well as glutamate and other amino acids. b. Glutamate triggers the umami sensation (meaty sensation of monosodium glutamate (MSG)). Glutamate is sensed by one pair of the three receptors. c. Sweetness is sensed by a different pair of these receptors.

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C. Olfaction: Detecting molecules in the air 1. Airborne molecules that convey information about the environment are known as odorants.

2. Odorants diffuse into a mucous layer in the roof of the nose and activate olfactory neurons. (Fig. 44.17) a. The axons of the olfactory neurons project into the olfactory bulb of the brain.

3. Mammals have a gene family that codes for 800 to 1,500 different olfactory receptors, each of which binds to a small set of molecules. a. In humans, about half of these genes are mutated and nonfunctional. b. Each olfactory neuron has only one type of receptor. c. Neurons with the same type of receptor project to the same region in the olfactory bulb. d. Bloodhounds have the strongest sense of smell of any dog. The combination of many functional receptor proteins along with a vast surface area of olfactory cells makes them able to detect even 1 one or two human skin cells. (Fig. 44.18) e. In mice, particular smells are associated with the activation of a certain subset of the 2,000 tiny regions (glomeruli) in the olfactory bulb of the brain.

4. Pheromones provide information about members of the same species. a. Pheromones are chemicals that are secreted into the environment and affect the behaviors or physiology of animals of the same species. b. Thousands of pheromones have been identified that serve functions such as alerting other individuals of intruders, or signaling that a female is ovulating. c. In insects, pheromones typically bind to receptors in the antennae, whereas in tetrapod vertebrates, pheromone receptors are typically found in the vomeronasal organ. d. The vomeronasal organ is distinct from the olfactory bulb. e. It is currently debated whether or not humans have pheromones.

V. Other Sensory Systems A. All animals can sense temperature and painful stimuli, and some can perceive electric or magnetic fields.

B. Thermoregulation: Sensing temperature 1. Animals detect heat energy by thermoregulation and adjust behavioral or physiological responses accordingly.

2. Thermoreceptors can be found in the CNS to help regulate internal body temperature. a. In mammals, the hypothalamus senses departures from homeostasis and sends signals to effectors to restore homeostasis.

3. Thermoreceptors are also commonly found on the skin to sense changes in external temperature. a. Some receptors depolarize in response to heating; others depolarize in response to cooling.

4. Extreme temperatures are sensed by nociceptors, which sense painful stimuli in response to chemicals, excessive pressure, and tissue damage.

5. Pit vipers are named after the two temperature sensitive pits beneath their nostrils.

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b. Pit vipers use the thermoreceptors to detect heat energy given off by prey.

(Fig. 44.19b) 6. Students should be able to predict whether a rattlesnake could strike effectively at its prey with (1) its eyes covered but pits exposed, (2) its eyes and pits covered with cotton cloth, and (3) its eyes and pits covered with an opaque heat-blocking material.

C. Electroreception: Sensing electric fields 1. All animals give off weak electric impulses arising from activity of nerves and muscles.

2. Water is a good conductor of electrical impulses.

3. Many fish use electroreception to locate prey, detect predators, and navigate.

4. In sharks, part of the lateral line system is specialized to detect electric fields rather than pressure. a. Tiny pores on the head of the shark contain ampullae of Lorenzini that are lined with hair cells that depolarize in response to a change in electrical polarity.

(Fig. 44.20) b. Sharks can detect electrical currents as weak as a nanovolt. c. Combined with their excellent olfactory abilities, electroreception makes them finely tuned predators. d. Sharks may also use electroreception, in conjunction with magnetoreception, to navigate.

5. Electrogenic fishes have an organ that generates their own electrical field, which is stronger than that created from muscle and nerve activity. a. The currents generated by moving the electric organ through the water move in an arc. (Fig. 44.21) b. Disruptions of the currents within the arc help to identify if prey or predators are nearby as well as to navigate and for communication. c. Some electrogenic fishes produce currents strong enough to kill prey (or even humans).

D. Magnetoreception: Sensing magnetic fields 1. The earth produces magnetic fields as it rotates on its axis.

2. Magnetoreception has been described in bacteria, fungi, invertebrates, and all vertebrate classes.

3. Terrestrial organisms can sense magnetic fields directly. a. European robins kept in cages with no visual cues to the outdoors were observed to sit at one edge of their cages at the beginning of the migratory season. b. Application of artificial magnetic fields changed the birds’ orientation. c. Disruption of the magnetic field prevents birds from migrating properly.

4. What allows birds to be able to sense a magnetic field? a. Deposits of iron inside the sensory neurons in the beak may play a role in stimulating a response to changes in magnetic fields. b. Severing these neurons prevents birds from responding to the artificial changes in the magnetic field. c. Magnetoreception is also dependent on vision: covering one eye interferes with the ability to navigate using magnetic cues. .

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5. Animals do not use individual senses in isolation but instead combine information from multiple senses when locating prey, avoiding predators, or communicating with other animals.

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text. action potential amplification ampullae of Lorenzini basilar membrane bipolar cell bitter taste blind spot chemoreceptor cochlea compound eye cone cornea depolarization ear canal ear ossicles echolocation effector electrogenic fishes electroreceptor eyespot fovea frequency ganglion cell glomerulus (plural: glomeruli) gustation hair cell hearing hertz (Hz) hyperpolarization infrasound inner ear

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iris kinocilium L opsin/red opsin lateral line system lens M opsin/green opsin magnetoreceptor mechanoreceptor membrane potential/ membrane voltage middle ear monosodium glutamate (MSG) nociceptor olfaction olfactory bulb olfactory receptors ommatidium (plural: ommatidia) opsin optic nerve outer ear oval window phenylthiocarbamide (PTC) pheromone photoreceptor pitch pressure receptor pupil red-green color blindness retina retinal

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rhodopsin rod S opsin/blue opsin salty taste sclera sensory neuron sensory organ sensory receptor simple eye sonar sound sour taste stapes statocyst stereocilium (plural: stereocilia) sweet taste taste buds taste cells tectorial membrane thermoreceptor transduction transmission tympanic membrane ultrasonic hearing ultraviolet light umami/meaty taste vision voltage-gated channel vomeronasal organ

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Lecture Activities Find Your Blind Spot!

Estimated duration of activity: 5 minutes Many students are unaware that they truly have a blind spot in their vision. Use this activity to have students find their own blind spots.

1. Ask students to get out a clean sheet of paper and put two distinctive dark marks on the paper about 3 or 4 inches apart, like this: X

O

Instruct students to make the mark on the right very black and obvious but not too large (a black circle about 1 cm in diameter is perfect).

2. Tell students to try to find the blind spot in their right eye. This means students must close their left eye and keep them closed. Instruct students to focus (with their open right eye) on the left mark—the X on the sheet of paper.

3. Now have students move the piece of paper slowly closer to their face and farther away from their face. Warn students to keep staring at the left mark and keep their left eye completely closed, but to continue paying attention to the right mark in their peripheral vision. They will see the mark on the right appear and disappear as it passes over the blind spot.

4. Students may wish to repeat the exercise for the other eye.

(Students who have difficulty with this exercise are almost always either looking at the wrong mark or have the wrong eye open.)

Discussion Idea: Eyes for Day, Eyes for Night In the fovea, humans have about 160,000 cone cells per square millimeter. Humans are not nocturnal, cannot see very well in the dark, and have poorer visual acuity than many other animals. Here are three questions you can ask your students: • How might the fovea of a predatory hawk, which depends on excellent vision to sight prey during daylight hunting, be different from the fovea of a human?

Answer: Animals with better visual acuity have a denser packing of photoreceptor cells in the fovea. A hawk has about a million cone cells per square millimeter in the fovea.

• What type of cells would you expect to find in the fovea of a nocturnal animal?

Answer: Rod cells. A nocturnal animal depends on the better night vision that rod cells offer, so its fovea is packed with rod cells.

• What type of fovea would you expect in a herbivorous grazer that has to constantly scan the horizon for danger? (Hint: Foveas can come in different shapes.)

Answer: Most grazers have long, skinny foveas that are oriented parallel to the horizon when the animal has its head down in a grazing position. This allows them to focus on the entire horizon at once.

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Student-Led Research: Different Sensory Worlds Estimated duration of activity: 15 minutes (for in-class reports) Many animals have sensory abilities that are quite different from those of humans. The text chapter introduces a few of these examples, such as ultrasonic hearing in elephants, echolocation in bats, and ultraviolet vision in birds. Students are often interested in learning more about these and other cases. You may assign small groups of students to research the sensory abilities of certain animals outside of class, and then have them give a brief report to the whole class. Students could pick animal species that particularly interest them, but you may wish to steer them toward those species that have been well studied for certain sensory capabilities.

Good case studies: Barn owl hearing, bat echolocation, dolphin echolocation, magnetic senses in homing pigeons, “binoculars” in certain snapping shrimp, scallop eyes, raptor vision, ultrasonic hearing (and ultrasonic songs) in mice, ultrasonic vocalizations in elephants, infrared detection in pit vipers, electric field detection in electric eels, ultraviolet vision in honeybees, ultraviolet plumage patches in birds, multiple opsin genes in cichlid fishes.

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CHAPTER

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Animal Movement

Learning Objectives: Students should be able to...

• Describe the molecular changes that occur during muscle contraction and relaxation.

• Compare and contrast smooth muscle, cardiac muscle, and skeletal muscle.

• Compare and contrast the structure and function of hydrostatic skeletons, endoskeletons, and exoskeletons.

• Describe the processes by which animals locomote.

• Compare and contrast animal locomotion on land, in water, and in air.

Lecture Outline • Muscle generated movements were a key innovation in animal evolution.

• Muscles generate movement by exerting force and causing shape changes.

• Movement falls into two categories: • Movement of the entire organism relative to its environment, which includes locomotion such as undulating, jetting, swimming, walking, running, jumping, gliding, and flying.

• Movement of one part of the animal relative to other parts (not involved in locomotion)

I. How Do Muscles Contract?

A. Early muscle experiments 1. In the second century, Galen proposed that spirits flowed from nerves to muscles, inflating them and increasing their diameter.

2. This hypothesis persisted until the 17th century, when Descartes suggested that nerves carry fluid from the pineal gland to the muscles that make them shorten and swell.

3. Jan Swammerdam tested Descartes hypothesis. a. He placed frog muscle tissue in an airtight syringe with the nerve protruding outward from a small hole and a drop of water placed at the tip. b. He stimulated the muscle; if the muscle volume changed during contraction, the drop of water would move. c. The water did not move; thus, the volume of the muscle stayed constant. d. The experiment demonstrated that the contraction mechanism is inherent to the muscle itself and the nerve is not like a water hose filling up a balloon (the muscle).

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e. Luigi Galvani confirmed this conclusion in the 1790s when he severed the nerve to a frog’s leg muscle and connected the two sides of the cut to a metal conductor.

Contraction occurred and he concluded that the nerve and muscle possess “animal electricity” that can induce contraction.

B. The sliding-filament model 1. Muscle tissue is comprised of muscle fibers, which are long, thin muscle cells.

2. Within each fiber are many threadlike filaments (myofibrils) that appear striped or striated due to alternating light-dark units called sarcomeres that repeat along the length of the myofibril. (Fig. 45.1) 3. Sarcomeres shorten as muscles contract and lengthen as muscle relaxes.

4. There are two types of protein found in the sarcomere: actin and myosin 5. Hugh Huxley and Jean Hanson observed that there were two types of filaments within the sarcomere: a. Thick filaments are composed of myosin. b. Thin filaments are composed of actin.

6. Huxley and Hanson noticed that the dark bands did not change during contraction but that the light bands become narrower.

7. Together, they proposed the sliding-filament model (Fig. 45.2) that states that the filaments slide past one another during contraction, allowing the sarcomere to shorten without changing the lengths of the thin or thick filaments. a. Thin filaments are composed of two coiled chains of actin and are anchored to one wall (Z disk) of the sarcomere. b. Thick filaments are composed of multiple strands of myosin and are anchored to the middle of the sarcomere. c. Contraction is caused by actin and myosin sliding past each other. Analogy: A line of weightlifters pulling on ropes to pull two large trucks toward each other 8. Students should be able to explain which elements in the analogy represent the Z disks, which represent the thin filaments, and which represent the thick filaments.

C. How do actin and myosin interact?

1. Each myosin head can bind to actin, and the head region can catalyze the hydrolysis of ATP to ADP (and a phosphate ion). When ATP or actin binds to myosin, there is a conformational change. (Fig. 45.3) (BioSkills 6) 2. Myosin (not actin) is the site of active movement.

3. Rayment and colleagues proposed a four-step model for the actin-myosin interaction: (Fig. 45.4) a. Step 1: ATP binds to the myosin head and causes a conformational change that releases the head from the actin filament. b. Step 2: When ATP is hydrolyzed to ADP (and an inorganic phosphate), the myosin head straightens and pivots and binds to a new actin subunit farther down the filament. The myosin head is not cocked in a high-energy state. c. Step 3: When inorganic phosphate is released, the neck bends back to its original position. The bending, called a power stroke, moves the entire thin filament relative to the thick filament. d. Step 4: After ADP is released, the myosin head is ready to bind another molecule of ATP. 472

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4. The same basic ratcheting mechanism between actin and myosin is responsible for movement of amoebae and slime molds.

5. Actin and myosin have played a critical role in the diversification of eukaryotes because it has allowed for movement in the absence of flagella and cilia.

6. Muscles are able to relax in the presence of ATP due to tropomyosin and troponin working together to block the myosin binding sites on actin. a. Tropomyosin and troponin prohibit the thick and thin filaments from sliding past each other and allow the muscle to relax. (Fig. 45.5a) b. When calcium binds to troponin, the troponin-tropomyosin complex moves to expose the myosin binding sites on actin and contraction can occur. (Fig. 45.5b) D. How do neurons initiate contraction?

1. Action potentials from a motor neuron arrives at a muscle cell and result in muscle contraction. (Fig. 45.6) a. Action potentials trigger the release of the neurotransmitter acetylcholine from the motor neuron into the synaptic cleft between the motor neuron and the muscle cell. b. Acetylcholine diffuses across the synaptic cleft and binds to acetylcholine receptors on the plasma membrane of the muscle cell. c. The membrane depolarizes, and if sufficient acetylcholine is applied, it triggers an action potential of the fiber. d. The action potentials propagate along the muscle fiber into the interior of the fiber via invaginations of the muscle cell membrane called T tubules. e. T tubules intersect with sheets of smooth endoplasmic reticulum called the sarcoplasmic reticulum. When an action potential passing down a T tubule reaches one of these intersections, a protein within the T tubule undergoes a conformational change and opens calcium channels in the sarcoplasmic reticulum. f. Calcium causes the myosin binding sites on the actin filaments to be exposed, enabling the contraction to begin.

II. Muscle Tissues A. Vertebrates have three types of muscle tissues: smooth muscle, cardiac muscle, and skeletal muscle. (Table 45.1) B. Smooth muscle 1. Smooth muscle is unbranched and is often organized in thin sheets.

2. Smooth muscle lacks sarcomeres, so it does not have the striated appearance like skeletal muscle.

3. Smooth muscle cells are relatively small and contain a single nucleus.

4. Smooth muscle is essential to the function of lungs, blood vessels, digestive systems, urinary bladder, and the reproductive system.

5. Smooth muscle is innervated with autonomic motor neurons and is responsible for involuntary movements.

6. Acetylcholine from parasympathetic neurons aids in digestion, whereas sympathetic neurons release norepinephrine and the kidneys release epinephrine. Both epinephrine and norepinephrine inhibit digestion by inhibiting contraction of the smooth muscle.

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C. Cardiac muscle 1. Cardiac muscle contains sarcomeres and are striated.

2. Cardiac muscle contains branched cells connected end-to-end by intercalated discs.

3. Cardiac muscle contraction is involuntary and contracts following spontaneous depolarizations.

4. During rest, parasympathic neurons release acetylcholine, which slows down the rate of depolarization of the cardiac cells and a slower heart rate.

5. During exercise, fright, or stress, the sympathetic neurons release norepinephrine and the adrenal glands release epinephrine. Both epinephrine and norepinephrine increase the heart rate and strengthen the force of the cardiac muscle contractions.

6. Increased heart rate and force of contractions increases the amount of blood that is pumped from the heart.

D. Skeletal muscle 1. Skeletal muscle consists of long, unbranched, multinucleated cells.

2. The large cells result from the fusion of many smaller embryonic cells, which also account for the multiple nuclei.

3. Each muscle fiber is packed with microfibrils, which contain thousands of sarcomeres that give the muscle the striated (striped) appearance.

4. Skeletal muscle usually attaches to the skeleton, which facilitates locomotion.

5. In addition, skeletal muscles also control swallowing, defecation, and urination.

6. A significant amount of animal bodies is skeletal muscle. a. Humans are 40 to 45% skeletal muscle. b. The major skeletal muscles in the human body are summarized in Figure 45.7.

7. Skeletal muscle contraction is voluntary and must be stimulated by somatic motor neurons.

8. The force output of skeletal muscles depends on three factors: a. The relative proportion of different fiber types b. The organization of fibers within the muscle c. How the muscle is used 9. Skeletal muscle fibers are divided into three general types based on their structure and function. (Table 45.2) a. Slow muscle fibers contain a high concentration of myoglobin, which is an ironbearing pigment that carries oxygen and gives slow muscle a red appearance.

(1) The myosin of slow muscle contract slowly and hydrolyzes ATP at a slow rate.

(2) Slow muscle and fatigues slowly because they have many mitochondria that can generate steady quantities of ATP through aerobic respiration due to the plentiful supply of oxygen delivered by myoglobin. b. Fast muscle fibers appear white due to a low myoglobin concentration.

(1) They contract rapidly because the myosin hydrolyzes ATP at a rapid rate but also fatigue rapidly because the source of ATP is glycolysis rather than aerobic respiration. c. Intermediate muscle fibers appear pink or red and their contractile properties vary but are intermediate between slow and fast.

(1) They derive ATP from both glycolysis and aerobic respiration.

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d. Slow muscle fibers are abundant in tissues specialized for endurance. e. Fast muscle fibers contract and relax three times faster than slow muscle fibers, making them well suited for quick bursts of activity.

10. Students should be able to predict the most abundant fiber type in the postural muscles of the human neck.

11. The arrangement of muscle fibers within a muscle influences the contractile properties of the muscle. a. Some muscles are organized with fibers parallel to each other in long bands to maximize length change. (Fig. 45.8, left) b. Other muscles are organized in a diagonal or pennate pattern to maximize force.

(Fig. 45.8, right) c. The interaction between the muscle and the skeleton can also influence contraction.

III. Skeletal Systems A. Muscle forces and shape changes are transmitted to other parts of the body and to the environment via the skeleton.

B. Skeletal systems perform four main functions: 1. Protection from physical and biological assaults.

2. Maintenance of body posture despite gravity, wind, and waves.

3. Re-extension of shortened muscles; without mechanisms for re-extension, muscles would shorten only once.

4. Transfer of muscle forces to other parts of the body and environment, enabling a much greater range of force production and shape change than can be accomplished by muscle alone.

C. Trade-offs exist between protection and mobility.

D. Animals can be considered to have one (or more) of three types of skeletal systems.

1. Hydrostatic skeletons use the pressure of enclosed fluids to support the body.

(Fig. 45.9a) 2. Endoskeletons have rigid structures inside the body. (Fig. 45.9b) 3. Exoskeletons have rigid structures on the outside of the body. (Fig. 45.9c) E. Hydrostatic skeletons 1. Hydrostatic skeletons are constructed of an extendable body wall in tension surrounding a fluid or deformable tissue under compression.

2. Pressurized internal fluid enables soft-bodied animals to maintain posture, re-extend muscles, transfer forces to the environment.

3. Hydrostatic skeletons support whole animals such as sea anemones, jellyfish, and mollusks and can also support parts of animals such as tongues and penises of humans.

4. Hydrostatic skeletons composed mostly of muscle are called muscular hydrostats.

5. Antagonistic muscle groups (longitudinal vs. circumferencial) cause shape changes in hydrostatic skeletons such as shortening and lengthening of segments in earthworms.

(Fig. 45.10) 6. Alternating contractions of longitudinal and circumferential muscles pass down the earthworm in waves (peristalsis) and allow the worm to move.

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F. Endoskeletons 1. Invertebrate endoskeletons: a. Sponges secrete stiff spines of silica or calcium carbonate to provide structural support. b. Echinoderms have calcium carbonate plates beneath the skin suspended in a flexible matrix.

2. Vertebrate endoskeletons: a. Vertebrate endoskeletons are composed of rigid levers (bones) separated by joints. b. Vertebrates change the shape of their body by changing joint angles between the bones in the limbs, and between the limbs and the body, rather than changing the shape of the body segments. c. Vertebrate skeletons are composed of three main elements: (1) Bones, which are made of calcium phosphate, calcium carbonate, and protein fibers. (a) There are 206 bones in the human body. (Fig. 45.11a) (b) Bones meet and interact at joints (articulations). (c) Bones articulate in ways that limit the ranges of motion. (Fig. 45.11b) (2) Cartilage is made of cells within a gelatinous matrix of polysaccharides and protein fibers. (a) Cartilage can be rigid (e.g., jaws in stingrays) or rubbery (e.g., pads that cushion joints in knees or backs of humans) (3) Ligaments are bands of fibrous connective tissue, primarily collagen, that connect bones to other bones and stabilize joints. d. Tendons are bands of fibrous connective tissue, primarily collagen, that connect skeletal muscles to the bones. e. Vertebrate skeletons move by changes in joint angles controlled by antagonistic muscle groups.

(1) Flexor muscles, such as the hamstring, decrease the angle of the joint and pull bones close together.

(2) Extensor muscles, such as quadriceps, increase the angle of the joint and straighten the bones back out. (Fig. 45.12) e. The primary function of bones is mechanical, but they also provide storage of calcium and other minerals, and the interior of long bones contains bone marrow— the source of red and white blood cells.

G. Exoskeletons 1. The origin of exoskeletons allowed for diversification of arthropods.

2. The exoskeletons of arthropods consist of a cuticle formed from a composite of proteins and chitin.

3. Chitinous ingrowths of the skeleton form apodemes where muscles attach.

4. Crustaceans (e.g., lobsters, crabs) have exoskeletons composed of a cuticle mineralized with calcium carbonate, making the shells thick, hard, and heavy. a. The buoyancy of the marine environment helps to support the weight of the thick cuticle.

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5. Arthropods (like vertebrates) have paired flexor-extensor muscles that operated their jointed skeletons, causing movement based on joint angles. (Fig. 45.13) 6. In contrast to vertebrates, the muscles must be packed within the skeletal tubes. a. Muscles of many arthropods have the pennate arrangement of fibers to boost force output by increasing cross-sectional area, but not muscle width, during contraction. b. The pennate arrangement limits the range of motion, which is compensated by the placement of their apodemes, which can transduce a small shortening of muscle into large changes in joint angles.

7. The rigid exoskeleton must be shed (molted) periodically to allow for growth, making individuals vulnerable to predation during the period when their exoskeleton is soft.

IV. Locomotion A. There are many diverse ways that animals move on land, in water, and in the air.

B. How do biologists study locomotion?

1. The field of biomechanics applies the principles of engineering to quantify the mechanics of organisms.

2. Biomechanics studies the physical act of locomotion at different levels including material properties, structures, motions, forces, energetics, ecology, and evolution.

3. How are the material properties of tissues important to locomotion? a. Contractile properties of muscles influence the generation of forces for locomotion. b. Passive material properties of the skeletal elements influence the transmission of forces. c. R. Alexander hypothesized that the large tendons of the lower legs of terrestrial animals work as springs when animals run.

(1) Alexander measured the elastic properties of the tendon by clamping a piece of tendon in a device that measures the pulling force of tissue as it is stretched and released.

(2) The tendon returned 93% of the energy invested (7% lost to heat). d. The high rate of elastic-energy storage explains how tendons reduce the amount of energy necessary to go from step to step.

5. How is musculoskeletal structure adapted for locomotion? a. Biologists tested the hypothesis that an early tetrapod amphibian (Ichthyostega) could walk on all four limbs. (Fig. 45.14) 6. What does locomotion look like in living animals? a. Photographs and high-speed video and digital images provide many insights into locomotion. (Fig. 45.15) b. For most animals, the pattern of movement during locomotion varies with speed.

7. What forces are involved in locomotion? a. For an animal to move forward, it must push something backward (as per Newton’s third law of motion). b. The types of forces important to locomotion vary depending on whether the animal is locomoting on land, in water, in air, or in combination of the three.

(1) On land, gravity is a major influence on locomotion.

(2) In water, the force that resists forward motion through fluid known as drag is a major influence on locomotion. .

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(3) In air, gravitation forces are very important and as a result many animals that fly or glide are lightweight, and must create a force called lift to counteract gravity.

However, they must also resist drag and as a result tend to be more streamline in shape. c. Quantifying forces in fluids require indirect measurements by visualizing the airflow (or waterflow) around the animal such as the flow of air around the wings of a hovering bat. (Fig. 45.16) 8. What is the cost of locomotion? a. Alexander hypothesized that animals locomote using the most energy-efficient gait at each speed. (Fig. 45.17) b. Research Question: Do horses minimize the cost of locomotion? c. Hypothesis: Horses choose gaits that minimize energy use at different speeds. d. Null hypothesis: Horses do not choose gaits based on costs of locomotion. e. Experiment: (1) Measure oxygen consumption of horses trained to walk, trot, and gallop at a range of speeds on a treadmill. Energy used per distance traveled at different speeds was calculated.

(2) Videotape the same horses locomoting freely in the paddock and measure the gaits and speeds they choose. f. Prediction: For each gait, there is a range of speeds where energy use is minimized.

Horses will favor these gaits and speeds. g. Null prediction: There will be no correlation between chosen gaits and energy consumption. h. Results: Speeds and gaits chosen by horses corresponded to lower energy/distance ratios. i. Conclusion: Horses choose gaits that minimize energy use at different speeds and avoid speeds with high-energy consumption. j. Similar studies have been conducted for diverse animals running, swimming, and flying with similar results.

C. Size Matters 1. The ratio of surface volume decreases as the organism gets larger because surface area is proportional to length squared, whereas volume is proportional to length cubed.

2. The weight of an animal is proportional to its volume, and the ability of leg bones to support the weight is proportional to their cross-sectional area. a. Large terrestrial organisms have disproportionately thicker skeletal elements.

(Fig. 45.18) 3. Animals of different sizes maintain similar stresses in their skeletal tissues by altering posture and behaviors. a. Small animals locomote in more crouched postures and make leaps, whereas large animals are more straight-legged so their skeletons rather than their muscles support their body weight.

4. Size affects the costs of locomotion.

5. Schmidt-Nielson compared energy among different types of locomotion, accounting for body size.

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a. Schmidt-Neilson’s data showed that larger organisms use less energy per gram of body tissue per distance traveled than smaller organisms. (Fig. 45.19) b. For any given body size, running is more expensive than flying, and swimming is the least expensive.

6. The laws of physics establish definite constraints on the realm of possible sizes, shapes, and modes of locomotion in animals.

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text. acetylcholine actin actin binding site adrenaline antagonistic muscle group apodemes articulation/joint ATP binding site biomechanics bone cardiac muscle cartilage chitin connective tissue cuticle endoskeleton epinephrine

exoskeleton extensor fast muscle fibers flexor gap junction hydrostatic skeleton intercalated disk intermediate muscle fibers involuntary muscles ligament locomotion molt multinucleated muscle fiber/muscle cell myofibril myosin myosin head norepinephrine

peristalsis rigor mortis sarcomere sarcoplasmic reticulum skeletal muscle sliding-filament theory slow muscle fibers smooth muscle striated T tubules tendon thick filament thin filament tropomyosin troponin uninucleated voluntary muscle Z disk

Lecture Activities Illustrating Physical Constraints on Locomotion Estimated duration of activity: 10–15 minutes, plus time for a write-up.

Materials: At least three objects each that are “light,” “moderate,” and “heavy.”

This exercise can be done in lecture (with materials brought in) or as an out-of-class assignment.

There are physical limitations and trade-offs between body size and locomotion. For example, small animals move in crouched postures and make leaps, whereas large animals are more straight-legged, so their skeletons, rather than their muscles, support their body weight.

This exercise will demonstrate how even within a species, weight can influence posture, movement, and exertion. Have students complete a formal write-up based on their observations. There are questions at each step to guide them. .

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Students should select objects that represent things that are “light,” “moderate,” and “heavy.”

All objects should still be light enough to be able to lift over their head.

1) While standing, take the “light” object and hold it naturally for 1 minute. Record where it is placed relative to your stomach. Do you feel the need to rest?

2) Move the object to the left and extend your arms to the left as far as possible, but still even with your stomach while holding the object. Again, hold this position for 1 minute. Pay close attention to your postural changes. What muscle groups are you using?

3) Repeat the exercise to the right and document your observations.

4) Now lift the object over your head and hold the position for 1 minute.

5) Move the object to the left and extend your arms to the left as far as possible but still over your head. Again, hold this position for 1 minute. Pay close attention to your postural changes. What muscle groups are you using?

6) Repeat the exercise to the right and document your observations.

7) Repeat steps 1–6 with your “moderate” object and record your observations.

8) Repeat steps 1–6 with your “heavy” object and record your observations.

9) Pay attention to how the joints in your back and knees feel during each exercise. Was there a difference in how they felt when you held the light object compared to your heavy object?

Follow-up: If you were to lift a 40-pound bag of dog food, how would you change your stance before picking it up, while lifting it, and after it was settled? Which would you rather do: walk a mile carrying your light object or carrying a 40-pound bag of dog food?

How would you design an experiment based on your observations to test the effects of weight on joint pressures and physical exertion based on weight?

This exercise will also contribute to writing objectives by having students do a formal write-up of their observations.

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Chemical Signals in Animals

Learning Objectives: Students should be able to...

• Describe the major categories of chemical signals in animals, including their source, targets, and functions.

• Give specific examples of how hormones affect development, sexual maturation, and homeostasis in animals.

• Explain how hormone production and secretion in animals are regulated by the nervous •

system, other hormones, and negative feedback.

Compare and contrast how lipid soluble and water soluble hormones act on target tissues.

Lecture Outline I. Cell-to-Cell Signaling: An Overview A. Electrical and chemical signals allow animals to coordinate the activities of cells throughout the body.

B. Major categories of chemical signals (Table 46.1) 1. Autocrine signals act on the same cell that secretes them. a. Cytokines are autocrine signals that amplify the cell’s response to certain stimuli.

Example: interleukin 2 2. Paracrine signals act on neighboring cells. Example: insulin (which also acts as a hormone) 3. Endocrine signals are hormones. a. A hormone is any chemical signal that circulates through body fluids and affects distant target organs.

4. Neural signals are neurotransmitters that are released from neurons that diffuse short distances to other neurons at synapses.

5. Neuroendocrine signals are also released from neurons, but they go into the bloodstream and act on distant cells. Example: ADH C. Hormone-signaling pathways 1. Some hormones are released directly from endocrine cells to effector cells in response to a stimulus—that is, there is no CNS involvement. (Fig. 46.1a) 2. Most hormones are regulated by the CNS. a. Neurons in the CNS may respond by releasing neuroendocrine signals that act on effector cells directly. (Fig. 46.1b) CNS neurons may secrete hormones that

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stimulate endocrine cells to release other hormones, which then affect the effector.

(Fig. 46.1c) 3. The nervous system and endocrine system are tightly integrated.

4. Endocrine secretion is regulated by negative feedback (also called feedback inhibition). a. A hormone inhibits the gland that secreted it, decreasing further production of that hormone. Analogy: a thermostat (Making Models 46.1) 5. In animal cell-cell signaling, feedback inhibition reduces production or secretion of the hormone, or both.

D. What makes up the endocrine system?

1. The endocrine system is a collection of cells, tissues, and organs responsible for hormone production and excretion.

2. Major glands of the human endocrine system and their function (Fig. 46.2) a. Hypothalamus b. Pituitary gland c. Thyroid gland d. Parathyroid glands (four) e. Kidneys (two) f. Adrenal glands (two) g. Pancreas h. Ovaries/testes (two) 3. Many hormone-secreting cells are not located in glands but in other organs. Example: the islet of Langerhans 4. Exocrine glands release their secretions through ducts into a space other than the circulatory system: Example: salivary glands E. How do researchers identify a hormone?

1. In the mid-1800s, Arnold Berthold found that roosters with their testes removed had less aggressive tendancies. a. Researchers hypothesized that the testes release a chemical signal that causes young male roosters to act aggressively. b. Research question: What causes typical male anatomy and behavior in roosters?

(Fig. 46.3) c. Hypothesis: A chemical signal released from the testes is responsible for aggression and comb and wattle growth in roosters. d. Null hypothesis: A chemical signal released from the testes is not responsible for aggression and comb and wattle growth in roosters. e. Experiment: One group of male chicks were castrated, one group of male chicks were castrated and had their testes re-implanted in their abdominal cavity, and one group of male chicks were castrated and had testes implanted from another male chick. f. Prediction: Castrated males will fight less and have smaller wattles and combs than castrated males with testes implanted in the body. g. Null prediction: There will be no difference in fighting behavior and comb and wattle size between groups.

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h. Result: Castrated males fought less and had smaller wattles and combs. i. Conclusion: A chemical signal released from the testes into the bloodstream causes males to fight and stimulates comb and wattle growth.

F. A breakthrough in measuring hormone levels 1. Radioimmunoassays allow for quantification of hormones by adding radioactively. labeled hormones that compete with the unknown amount of hormone to bind to an antibody.

2. To understand hormone function, it is important to consider the effects it has on the entire organism.

II. How Do Hormones Act on Target Cells?

A. Hormone concentrations are low, but their effects are large.

1. Hormones bind to hormone receptors in target tissues. It is through these receptors that the effects are amplified.

2. Example: 0.01 mg of growth hormone injected in rats each day for 9 days widened their leg bones by 50% and they gained an average of 10 g.

B. Three chemical classes of hormones: (Fig. 46.4) 1. Polypeptides: Chains of amino acids linked by peptide bonds. Example: secretin 2. Amino-acid derivatives. Example: epinephrine 3. Steroids: A family of lipids characterized by a four-ring structure. Example: cortisol C. Steroid hormones bind to intracellular receptors.

1. Steroid hormones are small lipids that pass directly through membranes and bind to receptors inside the cell.

2. Identifying the estrogen receptor a. Researchers isolated the estrogen receptor by injecting radioactive estradiol into female rats, separating the molecules in the target cells by size, and looking for a band of radioactivity, which contained estradiol bound to its receptor. (Fig. 46.5) (BioSkills 7) b. Later work showed that the estradiol receptor is in the nucleus (but not in the membrane) of the target tissues of estradiol. c. The structure of the estradiol receptor is similar to the receptors for testosterone, glucocorticoids, and other steroids. d. Hormones are broadcast throughout the body via the bloodstream, but they act only on cells that have the appropriate receptor.

3. Documenting changes in gene expression (Fig. 46.6) a. Steroid hormones bind to receptors inside the cell, altering the shape of the receptor and exposing a zinc finger, a DNA-binding region. b. The hormone-receptor complex then moves to the nucleus, binds to DNA at a site called the hormone-response element, and stimulates transcription. c. Many mRNA transcripts are produced from one gene, and each is translated many times, thus amplifying the effect of the hormone.

4. Students should be able to predict what would happen to the estradiol response if an individual had mutations that changed the DNA sequence of its hormone-response element.

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D. Polypeptide hormones bind to receptors on the plasma membrane.

1. Hormones that are not lipid soluble bind to receptors on the surface of cells.

2. The hormone never enters the target cell; the message is transduced, or changed, into a message in the interior of the cell.

3. Identifying the epinephrine receptor a. Epinephrine has four different receptors, each of which is found in different tissues and causes different responses. b. A tissue’s response to a hormone is determined by the presence or absence of specific receptors for the hormone.

4. What acts as the second messenger? a. How does a signal from epinephrine increase glucose levels in the blood?

(1) Phosphorylase catalyzes a reaction that cleaves glucose molecules off glycogen.

(Fig. 46.7a, b) (2) Phosphorylase exists in active and inactive forms and is activated when an enzyme adds a phosphate group to it.

(3) When epinephrine is added to free-cell extracts from liver tissue, the amount of phosphorylase increases. (Fig. 46.7c) (4) cAMP acts as a second messenger that carries the epinephrine signal inside the cell to activate enzymes such as phosphorylase.

(5) The role of cAMP was confirmed in rat heart tissues. (a) After epinephrine was applied, cAMP increased in the cell, a few seconds later the contractile force of the cells increased; phosphorylase also increased but doesn’t peak until approximately 40 seconds later.

5. A phosphorylation cascade (Fig. 46.8) a. Binding of epinephrine to its receptor initiates a series of reactions in which one enzyme or molecule activates the next one, often by phosphorylation. b. Once ephinephrine binds the receptor, it activates a G protein, which then activates adenylyl cyclase. c. cAMP initiates a signal transduction cascade by binding cAMP-dependent kinase A, which phosphorylates an enzyme called phosphorylase kinase, which phosphorylates phosphorylase. d. Because each molecule in the sequence activates several molecules in the next step, the hormonal signal is amplified.

(1) One molecule of epinephrine can result in the release of millions or billions of glucose molecules.

(2) Other hormone receptors (such as the other epinephrine receptors) may use different second messengers, such as Ca2+ and IP3.

E. Why do different target cells respond in different ways?

1. The same chemical messenger can trigger different responses in cells from different organs or in cells at different developmental stages.

2. The reason is that the cells contain different receptors, second messengers, amplification steps, protein kinases, enzymes, or transcriptionally active genes.

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III. What Do Hormones Do? A. Hormones coordinate the activities of cells in three arenas: 1. Development, growth, and reproduction 2. Response to environmental challenges 3. Maintenance of homeostasis B. How do hormones direct developmental processes?

1. Triiodothyronine (T3) causes most of the major changes that occur during amphibian metamorphosis. (Fig. 46.9) a. Students should be able to suggest a hypothesis explaining why different frog cells can respond to T3 in such different ways.

2. Insect metamorphosis depends on two hormones: juvenile hormone (JH) and ecdysone. (Fig. 46.10) a. If JH levels are high, a surge of ecdysone causes molting. b. If JH levels are low, a surge of ecdysone triggers metamorphosis.

3. Sexual development and activity in vertebrates a. Early embryonic development (1) Fetal testes secrete testosterone, which induces development of the male reproductive tract.

(2) Fetal testes also secrete Müllerian inhibitory substance, which inhibits development of the female reproductive tract.

(3) In females, the ovaries secrete estradiol, which is required for further development of the female reproductive tract. b. Later in life, surges of testosterone (males) or estradiol (females) cause puberty. c. In seasonally breeding animals, environmental cues (e.g., day length) can trigger the release of reproductive hormones.

4. Growth in mammals a. Growth hormone (GH) stimulates growth in the length of the long bones of the limbs and the vertebrae. b. Growth hormone levels increase during puberty.

5. How photoperiod affects sex hormone release a. An increase in daylight during spring can stimulate the release of sex hormones in seasonally reproducing mammals, reptiles, and birds. b. Photoreceptors respond to light and send signals to the hypothalamus, which initiates the production of sex hormones. c. In mammals, photoreceptors in the retina sense light and signal to the pineal gland, which secretes melatonin. d. Stimulation of the photoreceptors reduces melatonin secretion; as a result, animals experience a daily rhythm in melatonin levels that are highest at night. e. As the length of daylight increases in spring, there are fewer hours of darkness, resulting in overall lower levels of melatonin. (Fig. 46.11) f. Lizards have photoreceptors contained in a pineal (or parietal) eye on the top of their skull. (Fig. 46.12)

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6. Some chemicals can disrupt hormone signaling. a. Commercially produced chemicals, even at low concentrations, can adversely affect human and wildlife health because they interfere with normal hormone signaling (endocrine disruptors).

(1) Exposure to the herbicide atrazine causes feminization and sex changes of male frogs, causes reproductive tissue abnormalities, reduced spawning in fishes, and low sperm counts and increased risk of birth defects in humans. b. Xenoestrogens are endocrine disruptors that bind to estrogen receptors and induce estrogen-like effects.

C. How do hormones coordinate responses to environmental change?

1. Short-term responses to stress a. Stress causes a short-term response called the fight-or-flight response. b. The sympathetic nervous system releases epinephrine from the adrenal medulla. c. Research Question: How does epinephrine affect the body? (Fig. 46.13) d. Hypothesis: Epinephrine causes changes involved in the fight-or-flight response. e. Null hypothesis: Epinephrine is not involved in the fight-or-flight response. f. Experiment: Inject human volunteers with epinephrine or saline solution, and then document changes in fatty-acid and glucose concentrations in blood, pulse rate, blood pressure, and oxygen consumption in the brain. g. Prediction: Epinephrine increases fatty-acid and glucose concentrations in the blood, pulse rate, blood pressure, and brain oxygen consumption relative to controls. h. Null Prediction: No difference in physiological state of individuals based on molecule injected. i. Result: Individuals injected with epinephrine had higher fatty-acid and glucose concentrations, higher pulse rates, blood pressure, and brain oxygen consumption compared to controls. j. Conclusion: Epinephrine causes an array of changes associated with the fight-orflight response. k. Epinephrine coordinates activities of cells in many organs and systems in the body to prepare an individual to cope with a life-threatening situation.

2. Long-term responses to stress a. Individuals under prolonged stress have increased levels of cortisol. b. Cortisol prepares an individual for long-term stress by ensuring that fuel will be available, particularly glucose.

(1) Cortisol induces the synthesis of liver enzymes that make glucose.

(2) It makes resting muscle and fat cells resistant to insulin (sparing glucose for the brain and for exercising muscles).

(3) It promotes the release of fatty acids from adipose tissue. c. The long-term stress response has costs if it is continued for too long.

(1) Muscles may become weak due to the breakdown of proteins.

(2) Wound healing is slowed, and immune and inflammatory responses are suppressed.

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D. How are hormones involved in homeostasis?

1. A homeostatic system includes a sensory receptor, integrator, and effector. a. Hormones are often the messengers between integrators and effectors.

2. Insulin, glucagon and blood glucose homeostasis a. When an animal eats, rising blood glucose levels stimulate the release of insulin from the pancreas which stimulates the removal of glucose from the blood to store. b. During times between meals, the need for glucose stimulates the release of glucagon from the pancreas which causes glucose to be released from storage.

3. ADH, aldosterone, and water and electrolyte balance a. ADH (antidiuretic hormone, from the pituitary) conserves water by causing the kidney to reabsorb water in the collecting ducts. Alcohol inhibits the release of ADH, causing diuresis and dehydration. b. Aldosterone (from the adrenal cortex) conserves sodium by causing the kidney to reabsorb Na+ ions in the distal tubules.

4. EPO and oxygen availability a. EPO is released from the kidney when oxygen levels are low. b. EPO stimulates bone marrow to produce more red blood cells. c. Some endurance athletes use EPO injections to increase the oxygen-carrying capacity of their blood, despite the risk of increased blood viscosity, heart attack, and stroke.

IV. How Is the Production of Hormones Regulated?

A. The hypothalamus and pituitary gland 1. Most hormones are controlled by the CNS, usually via the hypothalamus.

2. Controlling the release of glucocorticoids a. The hypothalamus releases CRH (corticotropin-releasing hormone). b. CRH stimulates the pituitary to release ACTH (adrenocorticotropic hormone). c. ACTH stimulates the adrenal gland to release glucocorticoids, such as cortisol.

3. Feedback inhibition by glucocorticoids (Fig. 46.14) a. Cortisol inhibits ACTH release from the pituitary. b. Cortisol also inhibits CRH release from the hypothalamus.

4. Patterns in glucocorticoid release a. In unstressed individuals, the level of cortisol has a daily rhythm, with a natural peak in the morning. b. In stressed individuals, cortisol levels can remain high for the whole day.

5. The hypothalamic-pituitary axis: an overview a. Many hormones are controlled by similar relationships of the hypothalamus, the pituitary, and a target gland or cells.

6. The posterior pituitary (Fig. 46.15a) a. The posterior pituitary is an extension of the hypothalamus and produces ADH and oxytocin. b. ADH triggers water reabsorption in the kidney. c. Oxytocin triggers uterine labor contractions and milk release.

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7. The anterior pituitary (Fig. 46.15b) a. The hypothalamus secretes releasing hormones that stimulate the pituitary to secrete a particular hormone. b. Many of the hormones produced by the anterior pituitary stimulate the production of other hormones.

B. Control of epinephrine by sympathetic nerves 1. During the fight-or-flight response, sympathetic nerves trigger the secretion of epinephrine from the adrenal medulla into the bloodstream.

2. Other sympathetic nerves secrete the closely related molecule norepinephrine directly onto target cells at synapses.

3. Epinephrine and norepinephrine are both catecholamines.

4. Catecholamines function as neurotransmitters and hormones.

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text.

3', 5' cyclic GMP (cGMP) adenylyl cyclase adipocyte adrenal cortex adrenal gland adrenal medulla adrenocorticotropic hormone (ACTH) aldosterone amino acid derivative amplification anterior pituitary antidiuretic hormone (ADH) autocrine signal cAMP-dependent protein kinase A catecholamine cholesterol corticotropinreleasing hormone (CRH) cortisol Cushing’s disease cyclic adenosine monophosphate (cAMP) 488

cytokine diacylglycerol (DAG) DNA-binding domain ecdysone/20-hydroxyecdysone effector cell endocrine disruptor endocrine gland endocrine signal endocrine system epinephrine/adrenaline erythropoietin (EPO) estradiol estrogen exocrine gland fatty acid fight-or-flight response follicle-stimulating hormone (FSH) G protein gene gene expression glucagon glucocorticoid glycogen gonad growth hormone (GH)/somatotropin

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holometabolous metamorphosis homeostasis hormone hormone-response element hypothalamic-pituitary axis hypothalamus insulin integrator islets of Langerhans juvenile hormone (JH) kidney leptin luteinizing hormone (LH) melatonin mineralocorticoid molting Müllerian inhibitory substance (MIS) negative feedback/feedback inhibition neural signaling neuroendocrine signal neurosecretory cells neurotransmitter norepinephrine ovary .

oxytocin pancreas parabiosis paracrine signal parathyroid gland phosphorylase phosphorylase kinase photoperiod photoreceptor pineal gland pituitary gland polypeptide

posterior pituitary primary sex determination puberty second messenger secretin sensory receptor set point signal transduction signal transduction cascade small intestine somatostatin steroid

sympathetic nervous system testis (plural: testes) testosterone thyroid gland thyroid-stimulating hormone (TSH) thyroxine (T4) transcription triglyceride triiodothyronine (T3) xenoestrogen zinc finger

Lecture Activities Pairs of Problems: Hormone Deficiency versus Hormone Resistance Estimated duration of activity: 5 minutes Many endocrine disorders occur in one of two forms: inability to produce the hormone or inability to respond to it. Although these two conditions have very different causes, they result in nearly identical symptoms. These related endocrine diseases are often a useful tool to teach students the importance of hormone receptors. Divide the students into pairs or small groups to discuss the two problems listed below about diabetes and insulin. After they answer the questions, have them choose another hormone and repeat the exercise.

Background information: After a person eats a meal, his or her blood glucose levels may become very high, which causes the pancreas to release insulin. Insulin enables cells to take up and use glucose for fuel. The result is that the cells have fuel and blood glucose levels return to normal.

Insulin problem 1: A person’s pancreas cannot produce insulin.

Insulin problem 2: A person’s cells do not have enough insulin receptors, so cells cannot detect the presence of insulin and respond to it.

Questions: • What would the person’s symptoms be? (blood glucose, in this case) • Would the levels of insulin in the blood be high or low?

• Could this person be treated successfully with injections of insulin?

Have students work on these problems in small groups for a few minutes and then ask for their answers. Students will probably have realized on their own that the “can’t make the hormone” (hormone deficiency) and “can’t respond to the hormone” (hormone resistance) problems result in identical symptoms. In the case of insulin, these symptoms include very high levels of blood glucose and cells that cannot get enough fuel (cells that are “starving in the midst of plenty”).

However, the hormone concentration and the proper course of treatment differ in each case. People with insulin deficiency (also known as type I diabetes) have very low levels of insulin and can be easily treated with insulin injections. People with insulin resistance (type II diabetes) have very high levels of insulin (point out to students that this is because the

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pancreas is correctly sensing the high levels of blood glucose). Unfortunately, insulin resistance cannot be cured with an injection of the hormone.

Once students have gone through the insulin example, have them do the same activity with another hormone of their choice, such as GH, leptin, or testosterone. For example, students should be able to figure out that leptin deficiency and leptin resistance both cause obesity, or that growth hormone deficiency and growth hormone resistance both cause dwarfism. Working through a couple of examples like this will help students gain an understanding of how hormones are released in response to certain signals and why hormone receptors are so critical to the action of hormones. Students will also appreciate that hormone resistance is very difficult to treat. Many of the most intractable health problems in the United States are related to conditions of hormone resistance (diabetes and obesity, for example).

Hormone Puzzles: What If...?

Estimated duration of activity: 3 minutes per question Studying endocrine disorders can help students gain a more intuitive understanding of how hormones work and what they do. Have students consider each of the following examples (all are real conditions): 1. Adrenal glands usually produce a small amount of androgens—hormones related to testosterone—even in women. What if a woman had an adrenal tumor that caused her adrenal gland to produce too much testosterone?

2. Fat tissue normally produces a small amount of estrogen. What if an obese man had so much fat tissue that his estrogen level started getting high?

3. What if a weight-lifter began taking very large amounts of synthetic androgen (chemicals that act like testosterone)? What would happen to that man’s own testosterone level—the testosterone that his own testes are producing? (Answering this question requires an understanding of negative feedback.)

4. What if a male human fetus had defective MIS receptors that could not bind to MIS? 5. What if a man had a mutation in his testosterone receptors?

6. What if a person had a pituitary tumor that caused the pituitary gland to secrete very high levels of growth hormone?

Answers: 1. Adrenal tumors commonly cause women to develop “masculine” traits, such as beards and a lowered voice. (The “bearded women” in 19th-century travelling circuses often turned out to have adrenal gland tumors.)

2. Obese men frequently develop breasts due to a high level of estrogen. Sometimes these men even begin to lactate.

3. Weight-lifters who take large amounts of anabolic steroids usually show a sharp decrease in their own production of testosterone because the anabolic steroids exert negative feedback on the pituitary and hypothalamus. (Often the testes shrink and the men can become temporarily sterile.)

4. Defective MIS receptors in a male mammal result in a condition called MIS insufficiency.

In humans, this results in a complete (though small) female reproductive tract that exists side by side with a complete and fully functional male reproductive tract. (No eggs are produced, because there are no ovaries.) Usually these men have a normal male external appearance, have normal testes, produce sperm, and have normal fertility. They are often 490

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unaware of their condition unless a doctor happens to notice the extra set of ducts during abdominal surgery.

5. Without the ability to respond to testosterone, the male ducts disappear. MIS, however, is produced normally, and the female ducts disappear, too. The embryonic genitalia, in the absence of testosterone, develop into female genitalia. The result is an individual who has apparently normal female genitalia. Internally, there is a pair of isolated testes that are not connected to any duct system at all. (This condition is called AIS, androgen insensitivity syndrome.)

6. Pituitary tumors that cause excess GH to be produced result in a condition called acromegaly. The individual is very tall. GH also causes continued growth of facial bones after puberty, resulting in a thickened brow ridge and a heavy jaw. (Students may be familiar with Andre the Giant, the professional wrestler who appeared in the classic movie The Princess Bride. Andre the Giant had acromegaly and eventually died due to cardiac complications of the disease.)

Learning the Function of the Thyroid Gland through Case Studies Estimated duration of activity: 15–20 minutes The thyroid gland makes an excellent case study for tying together several concepts, including feedback inhibition, the role of receptors in signaling processes, and the control of basal metabolic rate.

Background: The hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the pituitary to release thyroid-stimulating hormone (TSH). TSH, in turn, stimulates the thyroid to produce and secrete thyroxine (T4). T4 is an inactive form of hormone that contains four iodine atoms. It is converted by target tissues into an active form, triiodothyronine (T3, containing three iodine atoms), which binds to receptors on the membrane of the cells and triggers an increased metabolic rate (increased oxygen consumption, heat production, etc.). As occurs in many homeostatic systems in the body, negative feedback controls thyroxine release. T4 binds to receptors on the cells of the pituitary and inhibits the release of TSH. Normally, this chain of events establishes homeostasis of thyroxine and TSH in the blood.

Problems arise in some individuals who have defective receptors for thyroxine in the pituitary, have defective receptors on body cells, or lack iodine in their diet. Each of these situations leads to either hypothyroidism or hyperthyroidism.

If T3 receptors are defective,T3 cannot stimulate normal levels of metabolism in these cells. The symptoms will be those of hypothyroidism—lethargy and poor tolerance of cold temperatures. The individual will have abnormally high circulating levels of all hormones: TRH, TSH, T4, and T3. This condition can lead to goiter (an overgrowth of the thyroid gland) because prolonged stimulation by TSH causes the thyroid to grow.

If iodine is lacking in the diet, the thyroid gland cannot make enough T4. The individual will have abnormally low levels of T4 and T3, and he or she will exhibit the symptoms of hypothyroidism. However, because the levels of circulating T4 are so low, feedback inhibition of the pituitary is ineffective. The pituitary continues to secrete TSH, which continues to stimulate the thyroid to try to produce more thyroxine. The result is goiter because prolonged stimulation by TSH causes the thyroid to grow.

If T4 receptors are defective, feedback inhibition of the pituitary is blocked. The blood will have normal to high levels of thyroxine and high levels of TSH. The individual will exhibit symptoms of nervousness, excessive thinness, poor tolerance of heat, and a tendency to tire .

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easily. These are symptoms of hyperthyroidism. Hyperthyroidism also leads to goiter because prolonged stimulation by TSH causes the thyroid to grow.

Cases: Present these three scenarios, and ask students to determine what is wrong with the patient in each case.

• Patient X presents with lethargy, poor tolerance to cold, high circulating levels of thyroxine, high circulating levels of TSH, and goiter. What would explain these symptoms?

Answer: Defective T3 receptors on body cells.

• Patient Y presents with lethargy, poor tolerance to cold, low circulating levels of thyroxine, high circulating levels of TSH, and goiter. What would explain these symptoms?

Answer: Not enough iodine in the diet.

• Patient Z presents with nervousness, poor tolerance to heat, mood swings, and a tendency to tire easily. Also, the individual has high circulating levels of thyroxine, high levels of TSH, and goiter. What would explain these symptoms?

Answer: Defective T4 receptors on the pituitary.

Student-Directed Research: Bovine Growth Hormone in Milk Estimated duration of activity: 10–15 minutes in class, after students have done research on their own.

Milk production in mammals is stimulated by several hormones, one of which is growth hormone. Dairy farmers have discovered that giving growth hormone to dairy cows makes them produce more milk. The cow form of growth hormone is called bGH (bovine growth hormone) or bST (bovine somatotropin). Could this hormone get into the milk and affect the people who drink it? Have students research the following questions outside of class. (You may wish to assign different questions to different teams of students.)

1. Is bGH (or bST) a polypeptide, a modified amino acid, or a steroid?

2. Which of the three forms of hormones (polypeptides, modified amino acids, or steroids) is most likely to differ among species?

3. Is bGH different from human growth hormone?

4. Can bGH bind to the human growth hormone receptor?

5. Suppose you were given a shot of pure bGH. Could it affect you?

6. Does bGH milk differ in any way from milk from nontreated cows?

7. Is it legal to sell bGH milk in the United States? In other countries?

8. Is it legal to label milk as “hormone-free” in the United States?

9. Have students bring milk cartons to class to see whether they have bGH-related labeling.

10. Wrap up by asking students whether or not they drink bGH-treated milk.

Answers: 1. bGH is a polypeptide.

2. Generally, polypeptides most often differ among species. Steroid hormones are often completely identical among species, but polypeptides often differ due to genetic mutations that change their amino acid sequence.

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3. bGH has a single amino-acid difference from human GH. 4. The single amino acid difference prevents bGH from binding to the human growth hormone receptor (or indeed to any primate’s growth hormone receptor).

5. bGH cannot affect humans.

6. Although the inability of bGH to affect humans directly is reassuring, there is some evidence that cows that receive bGH produce milk that is slightly different in some other ways. For instance, milk from bGH-treated cows tends to have higher levels of a different hormone called insulin-like growth factor, and some evidence indicates that IGF can affect humans. (This field of research is in flux, so you may wish to research this yourself.)

7. bGH milk is banned in several countries, including Canada. In the United States, however, not only is bGH milk not banned, but there are several laws that protect its sale (see question 8).

8. In the United States, the manufacturer of bGH has repeatedly lobbied against the labeling of non-bGH milk on the grounds that this would inaccurately imply that something is wrong with bGH milk. As of this writing, in most states, bGH-free milk can be labeled as “bGH-free” only if a disclaimer is also included that says that the FDA has determined that there is no difference between bGH and non-bGH milk. Milk in the United States may not be labeled as “hormone free” because all milk includes some natural hormones. As of this writing, there are proposed bills pending in several states that would completely ban the labeling of bGH-free milk.

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CHAPTER

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Animal Reproduction and Development

Learning Objectives: Students should be able to...

• Contrast asexual and sexual reproduction, and describe the structure of sperm and eggs.

• Contrast external and internal fertilization and oviparity and viviparity.

• Describe how the structure of male and female reproductive organs relate to their functions.

• Describe the hormonal signaling that regulates puberty and the human menstrual cycle.

• Identify the major events that occur during pregnancy and childbirth in mammals.

• Describe the events of fertilization in animals, including the mechanisms to prevent fertili• zation by more than one sperm.

• Describe the events of cleavage during animal development.

• Describe the collective and coordinated cell movements that lead to each distinct cell layer and the body axes during gastrulation.

Describe the processes that form somites and the neural tube.

Lecture Outline I. Asexual and Sexual Reproduction A. The differences between asexual and sexual reproduction typically translate to differences in the amount of genetic variation in the offspring.

B. Asexual reproduction is usually based on mitosis and results in offspring that are genetically identical to the parents.

C. Sexual reproduction is based on meiosis and the fusion of gametes and results in offspring that are genetically different from the parents.

D. How does asexual reproduction occur?

1. When budding occurs, an offspring forms on or in a parent and eventually breaks free.

(Fig. 47.1a) 2. Fission is the process by which two new individuals are formed when an animal splits apart. (Fig. 47.1b) 3. During parthenogenesis, female offspring are produced from unfertilized eggs.

(Fig. 47.1c)

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E. Switching reproductive modes: a case history 1. Many animal species regularly switch between asexual and sexual reproduction.

2. Daphnia, crustaceans that live in freshwater habitats, switch to sexual reproduction when environmental cues indicate that conditions are becoming adverse. a. They reproduce asexually in the spring and summer via parthenogenesis. (Fig. 47.2) b. In late summer, they switch to sexual reproduction.

3. What environmental cues (proximate cause) trigger the switch? a. In addition to low food availability and short day lengths, exposure to poor quality water resulting from crowding is also required for the switch to sexual reproduction.

(1) The percentage of females that reproduced sexually increased with increasing population densities. (Fig. 47.3a) (2) By varying water quality (crowded vs. clean), food concentration (low vs. high), and day length (short vs. long), the only combination that resulted in sexually reproducing individuals was crowded water, low food, and short day lengths.

(Fig. 47.3b) 4. Why (ultimate cause) do Daphnia switch between asexual and sexual reproduction? a. Sexually produced offspring are genetically diverse. b. Genetically variable offspring have higher fitness in environments with rapidly evolving parasites, deteriorating physical conditions, or other types of rapid environmental change.

5. Students should be able to predict the conditions under which asexual reproduction would be found as the favored method of producing offspring.

F. Mechanisms of sexual reproduction: gametogenesis 1. Gametogenesis is the formation of gametes (eggs or sperm). a. Spermatogenesis takes place in the male gonads, the testes. b. Oogenesis takes place in the female gonads, the ovaries.

2. Spermatogenesis in mammals (Fig. 47.4a) a. In males, a diploid spermatogonium divides via mitosis to generate primary spermatocytes. b. Then one primary spermatocyte divides via meiosis to give rise to two secondary spermatocytes at the end of meiosis I and ultimately into to four spermatids (haploid) at the end of meiosis II. c. Each spermatid matures into a sperm. d. Men produce sperm continuously throughout their adult life.

3. Structure and function of sperm a. Mammalian sperm consists of four major compartments: head, neck, midpiece, and tail. (Fig 47.5) b. The head contains the nucleus and an acrosome. The acrosome contains enzymes that allow the sperm to penetrate the barrier surrounding the egg. c. The neck contains a centriole that will combine with the centriole in the egg to form a centrosome, which is required for spindle formation during mitosis.

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d. The midpiece contains mitochondria, which produce ATP required to power movement. e. The tail consists of a flagellum that makes swimming possible.

4. Oogenesis in mammals (Fig. 47.4b) a. In females, a diploid oogonium divides via mitosis to generate a primary oocyte. b. Then one primary oocyte undergoes meiosis, but the meiotic divisions are unequal, producing only one large egg (and three tiny polar bodies). c. Women produce primary oocytes only before birth, and then the oocytes stop development for decades.

5. Structure and function of eggs a. Eggs tend to be larger than sperm because they contain nutrients required for the embryo’s early development. b. It consists of a haploid nucleus, a full complement of organelles, and a supply of nutrients supplied by the yolk. c. Exterior to the plasma membrane, a sheet of glycoproteins called the vitelline envelope surrounds the eggs and provides protection.

(1) In aquatic species, there is an additional jelly layer. (Fig. 47.6a) (2) In humans and other mammals, the vitelline envelope is particularly thick and called the zona pellucida. The zona pellucida is then surrounded by a thick layer of cells known as the corona radiata. (Fig. 47.6b)

II. Reproductive Structures and Their Functions A. The male reproductive system 1. The external anatomy of the male genitalia consists of the scrotum, which holds the testes, and penis, which functions as the copulatory organ.

2. The human male reproductive system (Fig. 47.7) a. Spermatogenesis occurs in the testes and sperm is stored in the epididymis. b. Additional fluids are added by the prostate gland, the bulbourethral glands, and the seminal vesicles. (Table 47.1) c. Semen (sperm plus fluids) is transported through the vas deferens and ejaculated through the urethra.

3. In some species, the accessory fluids form a plug that can seal the female’s reproductive tract.

4. In some species, the penis has a bone called the baculum that helps stiffen the penis during copulation.

B. The female reproductive system 1. The ovary produces eggs (ova).

2. The reproductive tract of female birds produces a hard-shelled egg (a single cell) that contains all the nutrients needed for embryonic development. The egg is “laid” from the cloaca—a chamber that the reproductive, digestive, and excretory systems flow into. (Fig. 47.8) 3. The human female reproductive system (Fig. 47.9) a. The external anatomy includes the labia minora, labia majora, clitoris, vaginal opening, and urethral opening.

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b. The internal anatomy is dominated by two functions: (1) Production and transport of eggs: An ovary produces eggs, which then move through an oviduct (Fallopian tube) to the uterus.

(2) Development of offspring: The embryo develops in the uterus and eventually passes through the cervix and vagina to be born.

III. Fertilization and Egg Development A. External fertilization 1. Most animals that rely on external fertilization live in aquatic environments.

2. These animals also produce extremely large numbers of gametes. For example, a female sea star releases 100 million eggs. (Fig. 47.10) 3. Gamete release is synchronized with environmental cues (day length, etc.) along with cues from other individuals (courtship rituals, pheromones).

B. Internal fertilization 1. Internal fertilization occurs in the vast majority of terrestrial animals as well as in many aquatic animals.

2. Internal fertilization can occur two ways: a. The male deposits sperm into the female during copulation. b. The male packages his sperm in a spermatophore, which is then picked up and placed in the female’s reproductive tract by either individual.

3. Sperm competition and second male advantage a. When females mate with multiple males, sperm from the males compete to fertilize the eggs. b. In insects and some other groups, the last male to mate has an advantage (“second-male advantage”). c. Research Question: How does the “second-male advantage” occur in sperm competition? (Fig. 47.11) d. Hypothesis: In sperm storage areas, sperm from the second male displaces sperm from the first male. e. Null hypothesis: The mechanism does not involve sperm displacement from the sperm-storage areas. f. Experiment: (1) Introduce a gene in some male fruit flies that marks their sperm with green fluorescent protein.

(2) Allow females to mate with one male or with two males that have different sperm colors. Observe the sperm in the sperm storage areas after each mating. g. Prediction: When females mate twice, little sperm from the first male will remain in storage. h. Null prediction: When females mate twice, most or all of the sperm deposited by the first male will still be present. i. Results: In females that mate twice, there is a reduction in the amount of sperm remaining from the first male. j. Conclusion: If a female mates a second time, most sperm from the first male she mated with disappears.

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k. In species in which multiple mating by females is common, males have large testes and produce large numbers of sperm. l. Females can play a role in sperm selection by determining which male mates last before fertilization, ejecting undesirable male’s sperm, or other mechanisms of “cryptic female choice.”

4. Why is testis size variable among species? a. Species in which females mate with multiple males, the males tend to have larger testes and produce larger numbers of sperm. b. The lottery hypothesis states that the more sperm, the more likely it will fertilize an egg.

5. How does external anatomy affect sperm competition? a. Variations in the anatomy of the scrotum and penis may be important for sexual display or sperm competition in some species. b. In insects, elaborate genitalia may assist in sperm competition. c. In the seed beetle, there are long genital spines on the end of the genitalia.

(Fig. 47.12a) d. The long spines can stick in the female reproductive tract and prolong copulation.

As a result, males with longer spines sire more offspring. (Fig. 47.12b) C. The cell biology of fertilization 1. For fertilization to take place, the following events must occur: (Fig. 47.13) a. The sperm and egg must be in the same place at the same time. b. The sperm and egg must recognize each other. c. The sperm and egg must fuse. d. Only one sperm must fertilize each egg.

2. How do gametes from the same species recognize each other? a. Sea urchin eggs are exposed to many different species of sperm. How do sea urchin eggs and sperm recognize each other? b. Victor Vacquier (1970s) identified a molecule on sea urchin sperm that recognizes and binds to sea urchin eggs. c. The researchers named this protein bindin. Bindin is species specific. d. Species-specific bindin molecules on sperm bind to species-specific bindin receptors on the surface of the egg, allowing the membranes to fuse, thus preventing cross-species fertilization. e. Students should be able to explain the importance of protein-protein interactions for fertilization. Students should also be able to explain why a molecule that bound to bindin-like protein on the human sperm head would function as an effective contraceptive.

3. What prevents more than one sperm from entering the egg? a. Multiple sperm fertilizing one egg, polyspermy, results in a zygote with more than two copies of each chromosome. b. Some mechanisms block polyspermy. c. In almost all species, fertilization results in the formation of a physical barrier to sperm entry. (Fig. 23.3) (1) The entry of sperm triggers the release of calcium ions inside the egg.

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(2) In response to calcium, cortical granules fuse with the plasma membrane and release their contents to the exterior of the cell.

(3) The substances released from cortical granules include proteases that digest the binding sites for bindin on sperm.

(4) The contents of the cortical granules accumulate between the plasma membrane and the vitelline envelope, drawing water into that space and forming a fertilization envelope.

(5) All of these events form a barrier to polyspermy. d. In mammals, fertilization triggers the release of cortical granule contents, and proteases from the cortical granules destroy the egg-cell receptor for sperm.

(1) These proteases actually cause the release of any additional sperm that are already attached to the zona pellucida.

(2) The destruction of the egg-cell receptor also prevents the binding of additional sperm.

D. Why do some females lay eggs, while others give birth?

1. Oviparous (egg-laying) species lay eggs that must contain all the nutrients required for the developmental period.

2. In viviparous (live-bearing) species, the embryos are retained within the mother’s body. The embryos receive nourishment directly from the mother.

3. Ovoviviparity is retention of an egg within the mother’s body, with the offspring nourished by the yolk of the egg.

4. Viviparity may be an adaptation that improves the survival of offspring in cold climates. a. In Sceloporus lizards, ovoviviparity has evolved several times, usually in highaltitude lizards. (Fig. 47.15) (BioSkills 13)

IV. Embryonic Development

Human development proceeds in ordered phases. (Fig. 47.16) A. Cleavage 1. Cleavage is the stage of rapid divisions that take place immediately after fertilization. a. Cleavage is the first stage of embryogenesis, the generation of an embryo from a zygote. b. Cleavage divides the egg cytoplasm without additional cell growth. c. The cells created by cleavage are called blastomeres. d. The mass of blastomeres is called a blastula.

2. Cleavage in mammals a. In humans and other mammals, cleavage occurs in the fallopian tube or oviduct.

(Fig. 47.16) b. Cleavage results in a specialized type of blastula called a blastocyst, which has two major populations of cells: (1) The exterior is a thin-walled, hollow structure called the trophoblast.

(2) The interior is a cluster called the inner cell mass (ICM). c. After arriving at the uterus, the blastocyst implants into the uterine wall and the placenta begins to form.

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B. Gastrulation 1. During gastrulation, cells move to form a highly organized gastrula. a. Early experiments tracked cell movements during gastrulation.

(1) Researchers used tiny blocks of agar with a nontoxic dye that were applied to blastula-stage embryos. (Fig. 47.18) (2) The dye diffused into cells of the blastula, and these marked cells could be traced as they moved.

2. Formation of germ layers a. Ectoderm-derived: gives rise to skin and the nervous system b. Mesoderm-derived: gives rise to muscle, most internal organs, and connective tissue c. Endoderm-derived: gives rise to the lining of the digestive tract and associated organs d. Example: Cell movements in frog gastrulation (Fig. 47.18) (1) The blastula is hollow with a fluid-filled blastocoel in the center.

(2) At the beginning of gastrulation, cells begin moving into this space through an invagination that forms the blastopore.

(3) Cells move to the interior of the embryo through the blastopore to become the endoderm that will eventually line the digestive tract.

(4) When gastrulation is complete, three tissue layers are formed. (Fig. 47.19) e. Students should be able to describe how the ectoderm comes to completely cover the embryo.

3. Creating body axes a. The axes begin to be established during cleavage when cytoplasmic determinants are divided into different cell populations. b. By the end of gastrulation, the axes are easily identifiable. c. In frogs, the blastopore becomes the anus, and the region just above the blastopore becomes the dorsal side.

C. Organogenesis 1. Organogenesis is the process of tissue and organ formation that follows gastrulation. a. During organogenesis, cells proliferate and become differentiated. b. Differentiation is a progressive process.

(1) Cells become specified to a particular fate.

(2) A cell’s fate is determined by a complex cascade of interacting cell-cell signals and regulatory transcription factors.

2. Formation of the notochord, neural tube, and somites (Fig. 47.20) a. During gastrulation, three tissue layers are organized and a notochord begins to form.

(1) In some species, the notochord becomes a part of the internal skeleton.

(2) In other species, it functions as a key organizing element during organogenesis. b. Cells in or near the notochord send signals that induce the ectoderm on the dorsal surface to fold upward.

3. Somites are precursors to skin, bone, and muscle. (Fig. 47.21) a. The fate of the somite cell depends on its position within the somite. (Fig. 47.22) 500

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4. Formation of the neural tube and central nervous system a. As this folding continues, the neural tube forms.

(1) The neural tube is the precursor to the brain and spinal cord.

(2) The notochord provides physical support as the neural tube forms. b. As the neural tube forms, mesodermal cells become organized into blocks of tissue called somites.

(1) As organogenesis continues, cells near the neural tube and notochord become organized into blocks of mesodermal tissue.

(2) Somite formation is a response to changes in cell adhesion molecules that keep mesodermal cells attached to each other. c. The anterior potion of the neural tubes contributes to the brain, and the posterior portion contributes to the spinal cord. (Fig. 47.23)

V. The Role of Sex Hormones in Mammalian Reproduction A. Testosterone and estradiol (an estrogen) are steroid hormones.

1. Testosterone is made by specialized cells in the testes.

2. Estradiol is made by specialized cells in the follicle surrounding the developing egg in the ovaries.

3. These hormones are involved in the development of the reproductive tract in embryos, puberty, and gametogenesis in adults.

B. Which hormones control puberty?

1. Puberty is the process that leads to sexual maturation in humans.

2. Puberty is triggered by increased levels of testosterone (in males) or estradiol (in females). a. A 2-year-old boy accidentally exposed to testosterone cream experienced early puberty.

3. GnRH from the hypothalamus stimulates pulses of FSH and LH from the anterior pituitary, which stimulate testosterone or estradiol production. (Fig. 47.24) 4. What regulates the regulatory hormones? a. The trigger for the release of GnRH is not fully understood. b. Nutritional state is probably involved, since well-nourished girls tend to enter puberty earlier.

C. Which hormones control the menstrual cycle in humans?

1. The female cycle consists of two distinct phases: follicular and luteal. (Fig. 47.25) a. The follicular phase lasts about 14 days, during which the follicle matures. Primary oocytes complete meiosis I, and ovulation occurs when the follicle is mature and the secondary oocyte is released into the oviduct. b. The luteal phase averages 14 days and begins with ovulation and continues through the degeneration of the corpus luteum from the ruptured follicle. c. Humans are unusual in that they have a recurring cycle that continues all year long, and they are one of the few mammals to menstruate. d. The majority of mammals reabsorb the uterus lining and have estrous cycles rather than menstrual cycles.

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2. How do pituitary and ovarian hormones interact during a menstrual cycle?

(Fig. 47.26) a. Days 1–7 (1) Uterus sheds lining; follicle begins to develop under FSH. (2) Follicle secretes estradiol and a little progesterone.

(3) Estradiol (low) suppresses LH secretion. b. Days 8–14 (1) Secretion of estradiol increases; stimulates uterine lining to grow.

(2) Enlarged follicle produces more estradiol; now positive feedback on LH. (3) LH level spikes just after estradiol level spikes.

(4) LH spike triggers ovulation and ends follicular phase. c. Days 15–21 (1) Corpus luteum develops from ruptured follicle; it secretes progesterone in response to LH. (2) Rise in progesterone inhibits LH and FSH and vascularizes uterine lining. d. Days 22–29 (1) If fertilization does not occur, corpus luteum degenerates.

(2) Progesterone level falls as corpus luteum shrinks.

(3) Drop in progesterone causes uterine lining to degenerate; menstrual bleeding.

(4) GnRH, LH, and FSH are released from inhibitory control of progesterone.

(5) FSH level rises, and a new menstrual cycle begins.

5. Manipulating hormone levels to prevent pregnancy a. Pills that contain progesterone (and often a small amount of estrogen) exert negative feedback on the pituitary gland. Because there is no LH spike, ovulation does not occur. b. Hormone-based methods are just one of many methods of contraception.

(Table 47.2)

VI. Pregnancy and Birth in Mammals A. Gestation and early development in marsupials 1. Young are born at the end of the estrous cycle, in a very undeveloped state.

(Fig. 47.27a) 2. They crawl into the mother’s pouch and latch onto a nipple, where they remain for the rest of their development. (Fig. 47.27b, c) B. Major events during human pregnancy 1. Fertilization occurs in the oviduct. a. The resulting zygote moves through the oviduct into the uterus.

2. Implantation a. Once in the uterus, the embryo (now a ball of cells) implants in the uterine lining and begins to make human chorionic gonadotropin (hCG). b. hCG keeps the corpus luteum alive so that it can keep producing progesterone, which maintains the pregnancy.

3. The first trimester a. The three embryonic tissues form ectoderm, mesoderm, and endoderm.

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b. The major organs form, including the heart, which begins pumping blood through a circulatory system. The embryo is now called a fetus—at 8 weeks of age in humans.

(Fig. 47.28a) c. The embryonic ectoderm contributes to the formation of membranes such as the amnion, which eventually fills with fluid to provide a protective cushion for the fetus. d. The placenta permits the efficient transfer of nutrients, gases, and wastes between mother and fetus. e. The placenta also produces progesterone and estrogens, which prevent the maturation and ovulation of additional follicles through negative feedback. f. Students should be able to explain why women who produce low levels of progesterone from the corpus luteum are prone to miscarriage.

4. The second and third trimesters are characterized by rapid growth. (Fig. 47.28b, c) C. How does the mother nourish the fetus?

1. Adaptations to enhance oxygen exchange between mother and fetus a. The mother’s respiratory and cardiovascular system changes to enhance the efficiency of nutrient transfer and gas exchange with the fetus. Examples: blood volume expansion, dilation of blood vessels, increased heart rate, increased breathing rate and volume b. Some species have countercurrent exchange between maternal and fetal blood vessels. Example: sheep c. Other species have an extremely large surface area for maternal and fetal circulation. Example: fetal villi in humans d. Fetal hemoglobin has a higher affinity for oxygen than maternal hemoglobin does.

2. Toxic chemicals can be transferred between mother and fetus. a. For example, ethanol (drinking alcohol) destroys developing neurons in fetal brains, and as a result, infants with Fetal Alcohol Syndrome have reduced brain sizes.

D. Birth (Fig. 47.29) 1. Oxytocin stimulates uterine (smooth muscle) contractions during birth.

2. As labor progresses, the cervix softens and becomes dilated.

3. Uterine contractions become forceful, longer, and more frequent, eventually expelling the fetus.

4. After the baby is born, the placenta is expelled.

5. Although the process seems straightforward, many complications are possible.

6. As a result of sterile technique, antibiotics, and blood transfusion, the mortality rate of women during childbirth has decreased dramatically during the last century.

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text.

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accessory fluids acrosomal process acrosomal reaction acrosome amnion asexual reproduction baculum bindin birth blastocoel blastocyst blastomeres blastopore blastula budding bulbourethral gland cervix chordates cleavage clitoris cloaca cloning contraception copulatory plug corona radiata corpus luteum countercurrent flow cryptic female choice ectoderm ejaculation ejaculatory duct embryo embryogenesis endoderm epididymis estradiol estrogen eutherian external fertilization fallopian tube false penis femme fatale fertilization fertilization envelope

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fetal alcohol syndrome (FAS) fetal hemoglobin fetus fission follicle follicle-stimulating hormone (FSH) follicular phase gametes gametogenesis gastrulation genitalia germ layers gestation giant sperm gonad gonadotropin-releasing hormone (GnRH) hermaphroditic human chorionic gonadotropin (HCG) hypodermic insemination hypothalamus implantation inner cell mass (ICM) internal fertilization labia majora labia minora labor lactation luteal phase luteinizing hormone (LH) marsupial menstrual cycle menstruation mesoderm monotreme myoblasts negative feedback neural tube notochord oogenesis oogonium

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organogenesis ovaries ovary oviduct oviduct/Fallopian tube oviparous ovoviviparous ovulation ovum/egg oxygen-hemoglobin dissociation curve oxytocin parthenogenesis penis pheromone pituitary gland placenta polar body polyspermy positive feedback pregnancy primary oocyte primary spermatocyte progesterone prostate gland protease proximate causation puberty scrotum secondary oocyte secondary spermatocyte second-male advantage semen seminal vesicles sexual reproduction somites sperm sperm competition spermatid spermatogenesis spermatogonium spermatophore steroid hormone testes

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testosterone tissue trimester trophoblast ultimate causation

umbilical cord urethra uterus vagina vas deferens

vitelline envelope viviparous yolk zona pellucida zygote

Lecture Activities Contraceptives as a Teaching Tool Estimated duration of activity: 0–50 minutes There is no doubt that college students are interested in learning more about contraceptives.

By taking advantage of this natural interest, you may use the subject of contraception to reinforce what you are teaching about the mammalian reproductive system. This activity may take any of these forms: • Writing assignments that students complete individually (or in small groups) outside of class. For example, have students build a website or design a pamphlet that explains the various contraceptive devices.

• In-class presentations given by individual students or small groups. For example, have students come up with an 8- to 10-minute marketing campaign that would convince physicians to recommend a particular type of contraceptive to their patients. Or have students write a public service announcement for their college community.

• An in-class discussion or game in which the students play “20 questions” to figure out how each contraceptive works biologically, with you as the information source. They can ask you yes/no questions, and the first student (or team) to fully explain the mechanism of action “wins” (candy, a pat on the back, an extra-credit point, etc.).

Contraceptives Barrier contraceptives: Male condoms Female condoms Diaphragms Spermicides Intrauterine devices Hormonal contraceptives: Estrogen/progestin pills/injectables/patches Progesterone-only pills/injectables/patches Norplant Depo-Provera Extended-release pills

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Natural methods: Lactational amenorrhea method (LAM) Rhythm method/standard days method Coitus interruptus Emergency contraception Designing an Oral Contraceptive for Men Estimated duration of activity: 20 minutes Currently, no oral contraceptives are available for men, although a few are under development. This activity asks students to design such a drug. It will require them to closely examine the male reproductive system to identify the best target for the drug. Divide class into groups of two to five students, and have each group develop an oral contraceptive for men.

Give the groups about 10 minutes to discuss their ideas and come up with a proposal. Then have each group explain how its oral contraceptive would work. As each group presents, ask the other groups to critique the mechanism of action and highlight either why that particular target would not work or why men would not be willing to take the drug (for instance, blocking testosterone would inhibit sperm production, but it would also negatively affect a whole host of male secondary sex characteristics).

By the end of this activity, students will have learned the details of the male reproductive system and will understand why it has taken so long to develop male oral contraceptives.

(Tip: The male contraceptive pill currently under development contains only testosterone.

The testosterone shuts off the release of pituitary FSH and spermatogenesis by negative feedback.)

Predictions: How Do Antibiotics Affect Non-human Reproduction?

These discussion topics and questions will encourage your students to think about reproduction in animals other than humans.

• Before discussing the reproductive patterns of cyclic parthenogens, like Daphnia, ask your students to recall the differences between asexual and sexual reproduction and to predict the main advantages and disadvantages of each of these modes of reproduction. Ask them how research on cyclical parthenogenetic species may provide insight into the use of cloning technology.

• Wolbachia is a bacterial symbiont of many arthropods that significantly affects the physiology of their host species. This intracellular bacterium is transmitted maternally. It is referred to as a reproductive parasite because it manipulates the reproductive patterns of its host. Ask students how the following manipulations favor the transmission of Wolbachia.

What are some potential short-term and long-term effects of these manipulations on the arthropod host species?

1. In some arthropods, Wolbachia induces parthenogenesis.

2. In some species, Wolbachia converts genetic males into females.

3. Wolbachia can induce cytoplasmic incompatibility, whereby all embryos that result from the mating of uninfected females with infected males die.

Ask students to predict how antibiotic treatment might affect an infected host species in each of the situations listed above.

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Answer: In each case, the reproductive manipulations of Wolbachia favor its vertical transmission to the next generation by passing from female to female. Infected host species thereby have a reproductive advantage over their uninfected counterparts because asexual reproduction is more efficient at producing a greater number of offspring over a given period of time than sexual reproduction.

Visualizing Gastrulation in 3-D: Activity for a Small Class (fewer than 50 students) or a TA-Led Study Section Estimated duration of activity: 5–10 minutes per demonstration Students have a difficult time learning to think in three dimensions. It is particularly difficult to think of cells on the outside of a sphere moving over a lip, into a depression, and into the interior in an orderly fashion and in predictable directions. Here are three ways to help students learn to think in 3-D: 1. Bring a hard-boiled egg or two to class. At the top of the long side of the egg, try to cut through to the surface of the yolk (a start on a cross section). Then cut the white all around the still-intact yolk. Next, put a large dot of ink on about the dorsal blastopore position. Then ask your students to imagine that cells are moving across the top, through this invagination, and into the inside. You could in fact make a dotted path on the white just above the yolk toward the future anterior. What is nice about the whole yolk is that you can then talk about how cells—yolky endoderm cells—are in this position as the involuting cells move between the future neural and gut regions. Pass the egg (or several eggs) around, and suggest that your students try doing this at home.

2. A balloon filled with a sand/water mixture works well to illustrate gastrulation in a frog embryo. If you want to go the extra mile, use a white balloon (underinflated so that you can press your hand far into the sand/water mixture) and paint half of it black. Hold the balloon so that the black half (animal pole) is on the bottom and the sand/water is in that section. You can then demonstrate formation of the gray crescent by picking a fertilization spot and rotating the balloon about 30 degrees away from that spot. This should shift the sand/water mixture so that it is visible through a portion of the white half of the balloon. In this “gray crescent,” use your hand to illustrate the path of invagination by pushing into the balloon and following the contour of the balloon as you press into the sand/water mixture. Of course, when you remove your hand, the “gastrula” will revert to a blastula, but after your illustration, you may pass the balloon around and have students demonstrate and explain the process to one another.

3. Large lima bean seeds cut carefully in half should give students a real glimpse into the parts of the plant seed, including the embryo. Have some other seeds germinated on a wet filter. Just seeing the real parts is so much better than any two-dimensional picture.

“Embryogenesis: The Movie” Estimated duration of activity: 20–30 minutes Divide the class into groups of three to five students. Instruct the groups that they are part of a production team to create an instructional video called Embryogenesis: The Movie. Each group is assigned one of the following four segments of the movie: 1. Fertilization 2. Cleavage

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3. Gastrulation 4. Early organogenesis The task is to create a series of diagrams representing the storyboard for the movie, with each diagram representing a different step. Have students draw the diagrams with markers on large sheets of paper so that they can be visible to the entire class during the second part of the activity. Since this is to be an instructional video, it will need labels and notes for a narrative to accompany each diagram. You or a teaching assistant will need to circulate and monitor group progress to keep students on task and to answer questions. This part of the activity should be completed in about 10 minutes.

Now bring the class back to order. Select four groups (one for each stage of embryogenesis) to share their part of the story with the rest of the class. These presentations are given in sequence. During the presentations, offer suggestions, correct any misconceptions, and encourage students to ask questions.

Activity Idea for Either a Large (50+ students) or Small Class (fewer than 50 students) Estimated duration of activity: 10–15 minutes This activity is completed with students working in small groups of two or three. Provide the following paragraph with the accompanying explanation. The task is for the groups to decide what conclusions regarding embryogenesis can be inferred from the data. Specifically, each group should answer the following questions: 1. What question was being addressed by the investigators?

2. What are the data?

3. What conclusions can be drawn?

Gross and Cousineau published the results of a key experiment on sea urchin development in 1963 (Biochem. Biophys. Res. Commun.). Their experiment took advantage of two drugs: cyclohexamide, which inhibits translation, and actinomycin D, which inhibits transcription.

Fertilized sea urchin eggs were divided into three groups. One group was left untreated (“Normal”), one group was treated with cyclohexamide, and the third group received actinomycin D. The group treated with cyclohexamide did not divide. Those treated with actinomycin D reached the blastula stage and then ceased to develop further.

In Vitro Fertilization: Discussion Idea for Any Class Size In vitro fertilization is rapidly changing and controversial. It also has important lessons for biology and ethics. The references listed below are a good starting point for a debate or class discussion. Ask students to read one or more of these papers and write a position paper (or come to class prepared to debate) on one of the following questions: • Should couples be allowed to sell their extra embryos to biotech companies or universities to be used for research? To be used for stem cell treatments?

• Should all viable embryos be implanted to avoid the issue of “extra” embryos?

• Should extra embryos be generated simply for the purpose of stem cell research?

• Should insurance companies be expected to pay for in vitro fertilization?

• As the human genome becomes better understood, should parents be able to choose which embryos to implant and which to destroy based on their genetic makeup?

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References This is just a small sample of the available articles. The Heady article has many of the classic references from Edwards and Steptoe and early discussions of ethics and technical difficulties.

Beyler, S. A., W. R. Meyer, and M. A. Fritz. “Disposition of extra embryos.” Fertility and Sterility 74(2): 213–215 (2000).

Heady, Judith E. “Ethics discussions about technologies such as those associated with in vitro fertilization, the genome project, and gene replacement.” In Science Discoveries and Science Teaching: The Link, edited by E. D. Siebert and C. R. Estee. Cedar City, UT: Society for College Science Teachers, pp. 19–26 (2004).

Heitman, E. “Social and ethical aspects of in vitro fertilization.” International Journal of Technology Assessment in Health Care 15(1): 22–35 (1999).

Kyle, B. L. In vitro fertilization: A right or a privilege? Journal of the Louisiana Medical Society 152(12): 625–629 (2000).

Mertens, Thomas R., and Jon R. Hendrix. “The popular press, scientific literacy in human genetics, and bioethical decision-making.” School Science and Mathematics, Bowling Green, 90(4): 317 (1990).

Reame, N. K. “Making babies in the 21st century: New strategies, old dilemmas.” Women’s Health Issues 10(3): 152–159 (2000).

Rao, R. A. “Assisted human reproductive techniques—emerging ethical and legal implications.” Medicine and Law 20(2): 295–299 (2001).

Rhodes, R. “Ethical issues in selecting embryos.” Annals of the New York Academy of Sciences 943: 360–367 (2001).

Wells, D., and J. D. Delhanty. “Preimplantation genetic diagnosis: Applications for molecular medicine.” Trends in Molecular Medicine 7(1): 23–30 (2001).

Wood, E. C. “Uses of embryo duplication in humans: Embryology and ethics.” Human Reproduction 15(3): 497–501 (2000).

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The Immune System in Animals

Learning Objectives: Students should be able to...

• Explain how the innate immune responses defend animals against broad groups of pathogens.

• Describe how the adaptive immune response recognizes pathogen-specific antigens.

• Compare and contrast the steps required for activating B and T cells.

• Compare and contrast how the adaptive immune system responds to extracellular and intracellular pathogens.

• Use examples to discuss the problems that can arise when human immune systems function improperly.

Lecture Outline • Infectious diseases threaten the survival of all living organisms.

• Organisms can recover from infections without medical intervention.

• Successful immune systems have three key functions: • Prevent the entry of potential pathogens • Detect the presence of a pathogen by distinguishing it as different from the animal’s own body • Eliminate the pathogen • There are two kinds of immune response: • Innate: Cells are always ready to induce a response.

• Adaptive: Cells that must be selectively activated to tailor a response to a particular pathogen.

• The different roles of innate and adaptive responses are summarized in Table 48.1.

I. Innate Immunity A. Innate immunity refers to immunity present from birth.

1. Cells involved in innate immunity are nonspecific in their response to antigens; they respond in the same way to all antigens, and they are always ready to respond.

B. Barriers to entry 1. In humans and other organisms, the exterior surface is the most important barrier. a. The thick armor (cuticle) of insects is covered in a waxy substance. b. Soft-bodied organisms are covered in mucus that traps pathogens and then sloughs off.

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c. Human skin is covered in a layer of dead cells reinforced with keratin. d. In addition to being a physical barrier, human skin is dry and slightly acidic, which makes it a chemical barrier as well.

2. How are openings in the body protected? a. Most body openings have mucus that traps pathogens; some have secretions (tears, earwax) that contain natural antibiotics (lysozyme, etc.). (Fig. 48.1) 3. How do pathogens gain entry? a. Some pathogens have adaptations to gain entry. For example, flu virus has an enzyme that disrupts the mucous lining of the respiratory tract. b. Other pathogens enter through wounds in the skin.

C. The innate immune response 1. The leukocytes involved in the innate response provide an immediate and generic immune response directed against the general type of pathogen encountered. a. Researchers studying fruit flies identified that the protein “toll” was important for defending against fungal infections. (Fig. 48.2) b. Toll-like proteins were also found in humans. c. Pathogens enter through a break in the skin.

2. Researchers studying antigen (foreign molecules that initiate an immune response) found that the toll and toll-like proteins served as receptors within a class of receptors known as pattern recognition receptors.

3. Toll-like receptors (TLR) are found in a wide variety of eukaryotes, and many different types of TLRs are found within an organism (e.g., 11 in humans).

4. Each TLR responds to a different antigen, none of which occur naturally in the host animal. Each antigen is a reliable indicator of attack for the innate immune system.

5. When pattern recognition receptors (like TLR) detect an invader, they trigger a signal cascade specific to the type of antigen detected.

6. Students should be able to return to Figure 48.2 and predict how the same two groups of flies would respond to a Gram-negative bacterial pathogen. Assume the survival rate of wild-type flies would be similar to what is shown in the figure.

7. Once the TLR is activated, a cascade of changes results in a fully engaged immune response.

8. The inflammatory response in humans (Fig. 48.3) a. Pathogens enter through a break in the skin. b. Platelets form a blood clot. c. The wounded cells and macrophages release chemokines, which form a gradient that marks a path to the wound. d. Mast cells arrive and release various signaling molecules, including histamine and chemicals that cause blood vessels near the wound to constrict. e. In response to the above signals, neutrophils begin to invade the area and destroy the foreign cells. f. Other leukocytes arrive that become macrophages. They secrete cytokines that attract other immune-system cells and induce fever. g. Table 48.2 summarizes the major cells and signaling molecules involved in the inflammatory response. .

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II. Adaptive Immunity: Recognition A. Key characteristics of the adapted immune response 1. The adaptive immune response has four key characteristics: specificity, diversity, memory, and self-nonself recognition.

2. Many of these features relate to the production of antibodies, proteins that bind to only a certain part of one certain antigen.

3. Experiments in the 1920s demonstrated that rabbits can produce antibodies against molecules that are not found in nature.

4. Researchers concluded that the immune system can produce an almost limitless array of antibodies.

B. An introduction to lymphocytes 1. Based on studies from chickens and mice, researchers identified that lymphocytes from the bursa (B cells) and thymus (T cells) have two different functions. a. Chickens that lack an avian immune organ called a bursa cannot mount an antibody response to Salmonella infection. These chickens had no B cells, which produce antibodies. b. Mice that lack a thymus do not reject skin grafts as normal mice do. These mice had no T cells, which detect and kill cells bearing foreign antigens (i.e. infected cells).

2. Where are lymphocytes found? (Fig. 48.4) a. In species that lack a bursa (e.g., humans), B cells are produced, and mature, in bone marrow. b. T cells are also produced in bone marrow, but they move to the thymus to mature. c. Lymphocytes circulate through the blood, lymph nodes, spleen, and lymph fluid. d. Large numbers of lymphocytes are also in the skin, gut, and respiratory tract. These immune system cells are called mucosal-associated lymphoid tissue (MALT).

C. Lymphocytes recognize a diverse array of antigens.

1. B-cells can produce antibodies to a diverse array of antigens.

2. The discovery of B-cell receptors (BCRs) a. The BCR is composed of two light chains, two heavy chains, and a transmembrane domain that anchors the protein in the membrane. (Fig. 48.5a) b. This is the same structure an antibody has, except that antibodies lack the transmembrane domain. c. Both BCRs and antibodies belong to a category of proteins called immunoglobulins. Immunoglobulins come in five classes that have different roles in the immune system. (Table 48.3) 3. The discovery of T-cell receptors (TCRs) a. TCRs bind to antigens only after they have been modified and displayed by other cells. b. TCRs are composed of a single alpha chain and a single beta chain, similar to one “arm” of an antibody. (Fig. 48.5b) 4. Antibodies and receptors bind to epitopes. a. Antibodies, BCRs, and TCRs bind to only a small part of the antigen, called an epitope. b. One antigen may have several epitopes. (Fig. 48.6) 512

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5. What is the molecular basis of antigen specificity and diversity? a. Each BCR and each TCR has a constant (C) region and a variable (V) region of the light chain that has a unique amino acid sequence, allowing it to bind to a unique epitope. (Fig. 48.7) b. The V regions of heavy and light chains of a BCR are adjacent to one another and face away from the plasma membrane. c. The V and C region of the TCR are arranged in a similar manner as the BCR. 6. The discovery of gene recombination a. The gene-recombination hypothesis states that immunoglobulin genes are shuffled together by combining a gene for the constant (C) segment with a gene for the variable (V) segment. b. As a B cell matures, the DNA content of the V + C region diminishes, as predicted by this hypothesis. c. The genes for light and heavy chains are now known to have many different regions that are mixed and matched to produce a unique receptor. (Fig. 48.8) (1) For example, in the light chain, one of the 40 V segments can recombine with one of the 5 J segments, resulting in 200 different light chains.

(2) In the heavy chain, any one of 51 V, 27 D, and 6 J segments can recombine.

(3) Light and heavy chain rearrangements occur independently, but the final BCR includes polypeptides from each gene to form the antigen-specific variable region. d. Students should be able to calculate the number of different antigen-specific BCRs that could be made by this process in (1) a human and (2) a single human B cell.

D. How does the immune system distinguish self from nonself?

1. Developing B cells and T cells that have anti-self receptors appear to be destroyed or inactivated in the bone marrow or thymus, although this process is not fully understood.

III. The Adaptive Immune Response: Activation A. Lymphocytes are normally in a resting or inactive state.

B. An inactive lymphocyte that does not encounter its specific epitope will eventually die.

C. The clonal selection theory 1. Certain lymphocytes are selected by binding to an antigen, and then the selected cells multiply. (Fig. 48.9) a. Antigens are recognized by receptors on B cells and T cells. b. Each receptor (and each B or T cell) recognizes just one antigen. c. Lymphocytes are activated when their receptor binds to an epitope in the antigen. d. Activated lymphocytes are cloned; that is, they divide to produce many identical copies of themselves. e. Some of the cloned cells persist after the infection has been cleared, “remembering” the antigen and enabling a quick response if the invader returns.

2. Students should be able to explain what “clonal” and “selection” refer to.

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D. T-cell activation 1. Dendritic cells collect antigens via phagocytosis and present them to T cells.

2. Antigen presentation links the innate and adaptive arms of the immune system, and without a transfer of information, the adaptive immune response would not be activated to respond to infection.

3. T cells come in two major types: a. CD8+ T cells have the CD8 protein on their membrane. b. CD4+ T cells have the CD4 protein on their membrane.

4. T cells can only bind to epitopes that have been processed and presented on the surface of other cells.

5. Antigen presentation via MHC proteins a. MHC proteins come in two types: class I and class II. b. The MHC protein structure promotes peptide binding. (Fig. 48.10) (1) Dendritic cells ingest antigens at the site of infection.

(2) The antigen is moved from the phagosome to the endosome, where is it broken them into peptide fragments.

(3) Some peptides bind to the groove in the class II MHC (major histocompatibility complex) molecules.

(4) The MHC-peptide complexes are transported to the surface.

(5) The vesicles fuse with the plasma membrane, displaying the MHC-peptide complex on the surface.

7. How are T cells activated by antigen-presenting cells? (Fig. 48.12) a. If a T cell has complementary receptors to that antigen, it is activated and divides to produce many genetically identical daughter cells (clonal expansion).

(1) CD8+ T cells interact with MHC−class-I−bound epitopes.

(2) CD4+ T cells interact with MHC−class-II−bound epitopes.

8. Cytotoxic T cells and helper T cells a. An activated CD8+ T cell undergoes clonal expansion to produce cytotoxic T cells (killer T cells).

(1) Cytotoxic T cells travel to the site of infection and directly kill cells that display the antigen (i.e., infected cells). b. CD4+ T cells divide to produce helper T cells.

(1) Helper T cells assist with the activation of other lymphocytes.

9. Students should be able to create a table that summarizes the roles of the class I and class II MHC proteins with regard to the origin of the antigens that are loaded onto them, type of T cells that bind to them, and the activity that is stimulated in these T cells via the interaction.

E. B-cell activation and antibody secretion 1. B cells undergo morphological changes when activated. a. Inactive B cells have a large nucleus, little cytoplasm, few mitochondria, and a ruffled membrane. (Fig. 48.13a) b. Active B cells produce large amounts of rough ER and a large number of mitochondria. (Fig. 48.13b)

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2. Activation requires several steps: (Fig. 48.14) a. If a B cell encounters the antigen to which its BCR can bind, it displays the antigen on a class II MHC protein. b. If a helper T cell has been activated by the same antigen, it binds to the B cell and the two cells stimulate each other. c. The helper T cell releases cytokines that provide a second signal to the B cell that fully activates it. d. The fully active B cell replicates, and some of the daughter cells differentiate into plasma cells that produce and secrete large quantities of antibodies.

3. Somatic hypermutation fine-tunes the immune response by producing BCRs that bind more tightly to the antigen than BCRs formed during maturation in the bone marrow.

IV. Adaptive Immunity: Response and Memory A. The mechanisms used by the adaptive immune system are grouped into two types of responses: 1. The cell-mediated response is promoted by TH1 cells, requires cytotoxic T lymphocytes, and takes place via cell-cell contact.

2. The humoral response is promoted by TH2 cells and involves the production of antibodies and other proteins secreted into the blood and lymph.

B. How are extracellular pathogens eliminated?

1. During the innate response, macrophages and dendritic cells phagocytize many invaders and present antigens on their surface via MHC proteins.

2. Dendritic cells, which bear bacterial antigen, move to the lymph nodes and activate any T cells whose receptors can bind to that antigen.

3. Activated helper T cells bind to the antigen-laden macrophages at the site of infection. a. Binding enhances the phagocytic activity of the macrophage. b. The helper T cell releases cytokines that increase the inflammatory response by recruiting additional phagocytic cells to the site. c. Meanwhile, antibodies from plasma cells begin attaching to extracellular bacteria, fungi, viruses, and foreign material. The antibodies interfere with infection in four ways. (Fig. 48.15) (1) Opsonization: Pathogens tagged with antibodies are destroyed by phagocytes.

(2) Neutralization: Coated pathogens are blocked from interacting/infecting host cells.

(3) Agglutination: Antibodies cause clumping, which makes them easy targets for phagocytes.

(4) Co-stimulation of complement proteins: Complement proteins circulate in the bloodstream and assemble at antigen-antibody complexes. When active, they punch holes in the plasma membrane of pathogens.

C. How are intracellular pathogens eliminated?

1. The cell-mediated response leads to cell suicide. (Fig. 48.16) a. Cells infected by a virus display viral antigen on their surfaces, bound to class I MHC protein. b. Activated cytotoxic T cells recognize the antigen, bind to the cell, and destroy it.

(1) Destruction of the host cell prevents the virus from reproducing. .

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c. For pathogens that are only able to replicate inside host cells (e.g., viruses), the cell-mediated response limits the spread of infection.

D. Why does the immune system reject foreign tissues and organs?

1. Just as the immune system recognizes and attacks foreign antigens on bacteria and viruses, it attacks transplanted tissues and organs that bear unfamiliar antigens.

Examples: blood transfusions, organ transplants E. Responding to future infections: immunological memory 1. In vertebrates, activated B cells and T cells produce memory cells that remain in the body for years, ready to mount another response if the invader returns.

2. The secondary immune response is faster and more efficient than the primary response. (Fig. 48.17) a. The response is strong because some memory cells undergo somatic hypermutation, which produces variations of their antibodies.

(1) Some of these antibodies bind more tightly to the antigen than the original antibodies.

3. Table 48.4 summarizes the activation and function of adaptive immune system cells.

4. Vaccination leads to immunological memory. a. A vaccine contains antigens from a pathogen or from killed or weakened versions of the pathogen itself. b. Three types of vaccines: (1) Subunit vaccines, consisting of isolated viral proteins (2) Inactivated viruses that have been damaged and cannot cause infection (3) Attenuated or “live” virus vaccines, containing complete virus particles that have been grown for so long in other species’ cells that they have lost the ability to grow rapidly in human cells c. Vaccination works because it triggers the production of memory cells without the person having to experience the infection and disease. d. Viruses such as HIV mutate so rapidly that memory cells are unlikely to be effective.

V. What Happens When the Immune System Doesn’t Work Correctly?

A. Allergies 1. An allergic reaction is an abnormal response to an antigen.

2. Some individuals produce IgE antibodies to molecules that are not harmful; these molecules are called allergens.

3. The IgE antibodies are then bound by mast cells and other leukocytes, which are then “sensitized.”

4. Upon the next exposure to the allergen, the sensitized cells initiate a hypersensitive reaction, releasing histamines, cytokines, and chemokines. a. Blood vessels dilate and become leakier; smooth muscles contract (sometimes in respiratory passageways). b. These reactions cause the symptoms of hay fever, hives, and asthma. c. In severe cases, blood vessels can dilate so much that blood pressure drops and the person becomes unconscious (anaphylactic shock); respiratory passages can also close off.

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d. The best treatment is to avoid the allergen. Other treatments include antihistamines that block histamine receptors or corticosteroids to reduce swelling.

B. Autoimmune diseases 1. Autoimmune reactions often result in disease due to the destruction of the body’s own cells and structures.

2. Examples include multiple sclerosis (MS), rheumatoid arthritis, and type I diabetes.

3. The hygiene hypothesis states that autoimmune and allergic responses arise due to hygienic practices that reduce exposure to pathogens and parasites.

C. Immunodeficiency diseases 1. Individuals with severe combined immunodeficiency (SCID) lack a normal immune system. a. People with SCID die at a young age from infections that other people fight off easily.

2. HIV causes a progressive failure of the immune system (AIDS) due to loss of CD4+ helper T cells and macrophages.

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text. acquired immune deficiency syndrome (AIDS) activation adaptive immune response agglutination allergen allergy anaphylactic shock antibody antigen antigen presentation antihistamine asthma attenuated virus autoimmune disease B cell basophil B-cell receptor (BCR) blood type bone marrow CD4+ T cell

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CD8+ T cell cell-mediated response chemokine class I MHC proteins class II MHC proteins clonal expansion clonal-selection theory complement proteins constant (C) region corticosteroids cytokine cytotoxic T cell/killer T cell dendritic cell effector T cell epitope gene recombination germinal center hay fever heavy chain helper T cell histamine hives

human immunodeficiency virus (HIV) humoral response hypersensitive reaction IgE immune immunity immunization immunodeficiency disease immunoglobulin (Ig) immunological memory inactivated virus inflammatory response innate immune response innate immunity inoculation leukocyte light chain lymph lymph node lymphatic duct lymphatic system lymphocyte

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lysozyme macrophage major histocompatibility complex (MHC) mast cell memory cell mucosal-associated lymphoid tissue (MALT) mucus multiple sclerosis myeloma neutralization neutrophil opsonization

pattern-recognition receptor phagocytosis plasma cell platelet primary immune response recognition rejection rheumatoid arthritis secondary immune response self-nonself recognition severe combined immunodeficiency (SCID) somatic hypermutation spleen subunit vaccine

T cell T-cell receptor (TCR) TH1 cell TH2 cell thymus Toll-like receptors (TLR) transmembrane domain Type I diabetes vaccination vaccine variable (V) region vasoconstriction vasodilation white blood cell

Lecture Activities Concept Mapping the Immune System Estimated duration of activity: 10–20 minutes Keeping track of the many different cell types and activation sequences of the adaptive and innate immune responses is always a challenge for students. Students will probably find it helpful to make their own concept maps of the immune system. If your students are unfamiliar with preparing a concept map, model the process by starting a small concept map of your own, and then have students add to it, working in pairs or small groups. Ask them to link the words together using terms that are as specific as possible, and encourage them to try to represent the order of the activation steps that occur in the process. Refer students to BioSkills 15 in Appendix B for a primer on how to make concept maps.

Immune-system concept maps may be divided into innate and adaptive responses. For the innate immune response, give students a “starter set” of the following list of terms, and encourage them to add more: leukocyte macrophage neutrophil mast cell bone marrow phagocytosis cytokine platelet vasodilation

vasoconstriction nearby cells chemokines lysozyme clot fever wound repair inflammatory response

For the adaptive immune response, use this starter set: dendritic cells inactive CD8+ T cell activation clonal expansion

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helper T cells antigen plasma cells

class I MHC class II MHC memory cells

Putting the Pieces Together: Immune Activation Estimated duration of activity: 10–20 minutes This activity challenges students to reconstruct the correct sequence of events in the adaptive immune response.

Make several copies of the following list of phrases (one copy per student group). Cut up each copy to create a set of strips, with each strip containing one of the phrases. Put the strips from one copy into one plastic bag and mix them up. After describing the adaptive immune response in class, divide students into small groups and give each group a bag (each bag contains one complete set of phrases describing the events of the adaptive immune response).

Ask each group to try to put the strips in order. (If students get stuck, have them first identify the first and last events and then look for clues to put the middle events in order.) Alert students that some events may occur simultaneously and especially that there are three different “subsequences” involving CD8+ T cells, CD4+ T cells, and B cells.

Note: The list below is a simplified sequence that does not discuss the two classes of MHC proteins or the two kinds of helper cells. Use this list to get students started, and then add further refinements later.

• A wound in the skin allows a pathogen to enter. Most of the pathogens stay at the wound site; a few get into the bloodstream.

• A leukocyte at the wound site ingests antigens from the pathogen, displays them on MHC proteins on its cell surface, and travels to a lymph node.

• A CD8+ T cell in a lymph node meets a leukocyte that is bearing a particular antigen—the antigen to which that T cell can bind. The T cell is activated.

• The activated CD8+ T cell divides to give rise to many cytotoxic T cells. (The cytotoxic T cells are not yet activated.)

• A B cell encounters a free-floating antigen that has gotten into the bloodstream. The B cell happens to have a receptor that can bind to this particular antigen. The B cell takes in the antigen and displays the antigen on its cell surface. (The B cell is not yet fully activated.)

• A CD4+ T cell in a lymph node meets a B cell that is bearing the antigen. It turns out that this particular CD4+ T cell can bind to the same antigen, so this CD4+ T cell is activated.

• The activated CD4+ T cell divides to give rise to many helper T cells.

• Some helper cells help activate the new cytotoxic T cells.

• Some helper cells activate the B cell that is bearing the same antigen.

• The activated cytotoxic T cells cruise through the body and destroy any infected cells— that is, cells that display the antigen on their surfaces.

• The activated B cell divides to produce many plasma cells and memory cells.

• Plasma cells produce large quantities of antibodies.

• Large numbers of antibodies circulate through the bloodstream, bind onto the antigens, and cause the antigens (and the cells bearing them) to clump.

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• Complement proteins and macrophages destroy any cells that are coated with clumped antigens and antibodies.

• Memory cells remain in the spleen and lymph nodes for years, ready to respond if the pathogen returns.

Student-Directed Research: Flu Vaccines Estimated duration of activity: 10–15 minutes Recent flu epidemics will have made students much more aware of flu vaccines. Ask students the following questions. If they do not know the answers, have them do some research at home and return with answers to the next class session.

• How many different flu vaccines are available in any given year?

• How do health researchers decide what epitopes to include in the coming year’s new flu vaccine?

• Why were young people particularly vulnerable to the recent H1N1 flu?

• What are the current options for this year’s flu vaccine? (live or killed? nasal spray or injection?)

• Why is the flu virus often grown in chicken eggs?

• How effective are the current vaccines?

• Why is there a new vaccine every year? Why can’t we just get vaccinated once, as children, and never get vaccinated again for flu?

Student-Directed Research: To Vaccinate or Not to Vaccinate?

Estimated duration of activity: 10–20 minutes Many students will know of people who refuse to have their children vaccinated for serious diseases that were once very common. Some students may have heard the (unconfirmed) rumor that vaccines are linked to autism. You can use this topic as a teaching exercise to help students understand how vaccines work and also to show them where to get good information about public health issues. Ask students in small groups to research a certain disease and to return to the next class session prepared to present the answers to the rest of the class.

(Note: You may wish to guide your students’ research to particular websites that are well researched.)

Diseases to research (one disease per student group): measles, mumps, rubella, polio, H1N1, diphtheria, tetanus, pertussis For each disease, the students should find answers to these questions: • How common was this disease before vaccination was available?

• What was the mortality rate? The rate of serious complications?

• Is a vaccine available? If so, what form is the vaccine? How is it made?

• How effective is the vaccine?

• Is there any firm scientific evidence that this vaccine is unsafe in any way?

If students come across websites that offer only anecdotal information about vaccine safety (i.e., websites that do not discuss controlled studies), you may use this as a teaching tool to discuss experimental design, the value of controlled experiments versus anecdotes, and the difference between primary and secondary literature. Students may also appreciate receiving pointers on how to assess the scientific validity of information found on a random website.

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An Introduction to Ecology

Learning Objectives: Students should be able to…

• Identify the levels of ecological study and give an example of a question that could be asked at each level.

• Explain how species distributions are affected by abiotic, biotic, and historical factors.

• Explain the main factors that establish climate patterns.

• Describe the factors that affect species variation and number within terrestrial ecosystems.

• Describe the factors that affect species variation and number within aquatic ecosystems.

Lecture Outline I. Levels of Ecological Study A. Ecologists study how organisms interact with their environment at different hierarchical levels. (Table 49.1) B. Organismal ecology 1. At the organismal level, researchers focus on how individual organisms interact with their environment.

2. Organismal ecologists study morphological, physiological, and behavioral adaptations that allow individuals to be successful in a particular habitat.

3. Example: Scientists are interested in studying salmon breeding habits, traits that make them reproductively successful, and how they adapt to the changing environment as they migrate.

C. Population ecology 1. A population is a group of individuals of the same species that live in the same area at the same time.

2. Population ecology seeks to understand how the numbers of individuals in a population change over time.

3. Example: Population ecology studies have predicted that salmon populations are in danger based on their recent decline due to man-made changes to their environment.

D. Community ecology 1. A biological community consists of the species that interact with one another in a particular area.

2. Community ecologists study the nature and consequences of the interactions among species in a community.

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3. Community ecologists are often called on to predict how human actions will affect ecological communities in the future.

4. Example: Salmon eat smaller fish and are eaten by both bigger fish and marine and land mammals. In both marine and freshwater habitats, salmon are affected by disease and parasites as well as human disturbances such as damming and overfishing.

E. Ecosystem ecology 1. An ecosystem consists of all the organisms that live in a certain area plus all nonliving, abiotic components such as air, water, and soil.

2. Ecosystem ecologists study how nutrients cycle and energy flows through an ecosystem.

3. Example: Salmon form a link between marine and freshwater ecosystems as they carry nutrients and energy from one ecosystem to the other.

F. Global ecology 1. Biologists define the biosphere as a thin zone surrounding the Earth where all life exists.

2. Global ecology studies the effects of human impacts on the biosphere.

3. Example: Salmon are sensitive to changes in water temperature, so who is accountable for the impacts of climate change on salmon?

G. Conservation biology applies to all levels of ecological study.

1. Conservation biology is the effort to study, preserve, and restore threatened populations, communities, and ecosystems.

2. Ecologists study how interactions between organisms and their environments result in a particular species being found in a particular area at particular population size. Conservation biologists apply these data to preserve species and restore environments.

II. What Determines the Distribution and Abundance of Organisms?

A. Biogeography is the study of how organisms are distributed geographically.

B. Abiotic factors 1. The most fundamental observation about the environmental range of organisms is that no one species can survive the full range of environmental conditions on Earth.

2. At present, humans have the broadest range of any organism on Earth.

3. The range of conditions that a species can tolerate and the range of resources it can use is called a niche.

4. Example: An artic microbe occupies a cold niche, whereas a hot springs microbe occupies a hot niche. (Fig. 49.1) (Making Models 49.1) C. Biotic factors 1. Interactions with other species can limit dispersal.

2. Examples: a. Townsend’s warbler and hermit warbler ranges do not overlap because the Townsend’s warblers have outcompeted the hermit warblers. b. Some species are limited by a specific food source, such as the yucca moth to the yucca plant. c. The range of domestic cattle in Africa has been limited by the tsetse fly, which transmits a deadly parasite. (Fig. 49.2) 522

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D. History matters: Past abiotic and biotic factors influence present patterns.

1. How a species came to live in a certain area can be explained by its history of dispersal. a. Dispersal refers to the movement of an individual from its place of origin to the location where it lives and breeds as an adult. b. If a species is missing from an area it could potentially inhabit, then it is likely that a physical barrier such as an ocean or mountain has blocked dispersal. c. Continental drift had an important impact on dispersal. (Fig. 49.3) 2. The Wallace line: Barriers to dispersal a. The Wallace line separates species with Asian and Australian affinities and exists because of a deep trench maintaining a water barrier to dispersal. (Fig. 49.4) 3. The influence of humans a. Humans transport seeds, birds, insects, and other species across physical barriers to new locations. b. A species introduced to an area where it would not normally live is called an exotic species. c. If that species begins to outcompete other species, it is called an invasive species.

(1) Some invasive species include kudzu, purple loosestrife, African honeybees, and zebra mussels.

E. Biotic and abiotic factors interact.

1. In many cases, biotic and abiotic factors interact to determine the distribution and abundance of organisms.

2. Example: The Argentine ant, Linepithema humile—native to South America and rapidly expanding around the world a. Menke, Haloway, and colleagues looked at factors affecting dispersal of these ants.

(Fig. 49.5) b. Argentine ants are at a competitive advantage in moist soils, but at a disadvantage compared to native ants in dry soils. c. This information helps scientists to predict dispersal patterns and design strategies to limit the spread of these invaders.

III. Climate Patterns

Climate is the prevailing long-term weather conditions found in an area, whereas weather consists of the specific short-term atmospheric conditions of temperature, precipitation, sunlight, and wind.

A. Why are the tropics warm and the poles cold?

1. Generally, areas that receive a lot of sunlight are warm and those that receive less sunlight are cold.

2. Over the course of a year, regions at or near the equator receive much more sunlight per unit area—and thus much more energy in the form of heat—than regions closer to the poles.

3. Earth is round, so the equator receives a lot of sunlight because the sunlight often hits the equator at a 90° angle.

4. Because Earth slopes away from the equator, sunlight strikes Earth’s surface at lower and lower angles as you move away from the equator. (Fig. 49.6)

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5. Sunlight arriving at lower angles results in much lower energy reception per unit area.

6. Therefore, average temperature drops as latitude increases.

B. Why are the tropics wet?

1. When the average annual rainfall is mapped for regions around the globe, it is clear that areas along the equator receive the most moisture, while regions about 30° latitude north and south of the equator are among the driest on the planet.

2. A major cycle in global air circulation, called a Hadley cell, is responsible for making the Amazon River basin wet and the Sahara Desert dry. (Fig. 49.7) a. At about 30° latitude north and south of the equator, this air mass has cooled so much that its density increases and it begins to sink. b. As the air mass sinks, the cell warms and gains water-holding capacity. c. The area then receives little rain and is bathed in warm air. d. This pattern of air movement is responsible for the world’s great deserts.

C. What causes seasonality in weather?

1. Seasons are regular annual fluctuations in temperature and precipitation.

2. These fluctuations occur because Earth is tilted on its axis. (Fig. 49.8) a. Due to the tilt, the Northern Hemisphere is closest to the Sun in June and July, creating a warm summer. b. Summer in the Southern Hemisphere is in November and December because that is when this region is tilted toward the Sun. c. The warmest months for the equator are March and September because then the equator faces the Sun most directly. d. If Earth did not tilt on its axis, there would be no seasons.

D. What regional effects do mountains and oceans have on climate?

1. The broad patterns of climate that are dictated by global heating patterns, Hadley cells, and seasonality are overlain by regional effects. The most important of these are due to the presence of mountain ranges and proximity to an ocean.

2. Mountains cool the air and wring the water out of the air as it cools. (Fig. 49.9) a. As the moist air from the Pacific Ocean hits the Cascade Mountain range, rain falls from northern California to southeast Alaska. b. Once the air has passed over a mountain crest, it is relatively dry; the land on the other side is in a rain shadow that receives little to no precipitation.

3. Oceans moderate temperatures. a. Because water has a high specific heat, it can absorb a lot of heat in the summer when the water temperature is cooler than the air. b. Conversely, in the winter, when the water is warmer than the air, it releases heat to warm the air. c. Massive ocean currents flow in cycles, or gyres, and can influence local climate patterns. (Fig. 49.10)

IV. Types of Terrestrial Biomes A. Natural biomes 1. Each biome in the world is associated with a particular set of abiotic conditions.

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2. The global distribution of the most common types of naturally occurring biomes is shown in Figure 49.11.

3. Weather consists of the specific short-term atmospheric conditions of temperature, moisture, sunlight, and wind. a. Temperature is critical because the enzymes that make life possible work most efficiently in only a narrow range of temperatures. b. Moisture is significant because it is required for life, and terrestrial organisms constantly lose water. c. Sunlight is essential because it is required for photosynthesis. d. Wind is important because it exacerbates the effects of temperature and moisture.

4. The nature of the biome that develops in a particular region is governed by the average annual precipitation and the annual variation in temperature and precipitation. Each biome contains species that are adapted to a particular temperature and moisture regime.

5. On land, photosynthesis and plant growth are maximized when temperatures are warm and conditions are wet; conversely, photosynthesis cannot occur efficiently at low temperatures or under drought stress.

6. Net primary productivity (NPP) is the total amount of carbon that is fixed per year minus the amount of fixed carbon oxidized during cellular respiration. a. NPP is often estimated by measuring aboveground biomass—the total mass of living plants, excluding roots.

7. The six terrestrial biomes that represent Earth’s most extensive natural vegetation types are shown in Table 49.2.

B. Terrestrial biomes: arctic tundra 1. The arctic tundra extends north (or south) of the subarctic all the way to the poles.

2. The growing season is 10 to 12 weeks long, at most.

3. Outside of the growing season, temperatures are below freezing.

4. Precipitation is very low.

5. Arctic tundras are completely treeless.

6. Biodiversity, productivity, and aboveground biomass are all low.

7. Most soils are perennially frozen in a state called permafrost, but lichens and herbaceous plants cover the ground.

8. Animal diversity is low, but insect diversity may be very high.

C. Terrestrial biomes: boreal forests 1. Boreal forests are found across most of Canada, Alaska, Russia, and northern Europe (subarctic); they are also called taiga.

2. They are characterized by very cold winters and cool, short summers. The annual temperature variation is extraordinarily high.

3. Due to the cold, evaporation is minimal; abundant moisture results.

4. Highly cold-tolerant conifers, most of which are evergreen, dominate the landscape.

5. Productivity is low, but aboveground biomass is high and biodiversity is exceptionally low.

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D. Terrestrial biomes: temperate forests 1. These forests are found in temperate areas that have moderately high precipitation.

2. There is a period when mean monthly temperatures fall below freezing and plant growth stops.

3. Precipitation is moderately high and relatively constant throughout the year.

4. Deciduous trees dominate these forests in North America and Europe, but broadleaved evergreens dominate the forests in New Zealand and Chile.

5. Temperate forests have productivity levels higher than those of deserts or grasslands, but lower than those of tropical forests.

E. Terrestrial biomes: temperate grasslands 1. Temperature grasslands are found throughout central North America and the heartland of Eurasia.

2. Precipitation is still quite low, but annual precipitation is much greater than the amounts recorded in deserts.

3. Temperatures are moderate (temperate) but highly seasonal. a. Temperature variation dictates a well-defined growing season. b. Plant growth occurs only in months that have adequate moisture and warmth.

4. These areas are also called prairies or steppes.

5. Grasses are the main life-form because conditions are too dry to support tree growth and too cold and seasonal to support drought-adapted desert species.

F. Terrestrial biomes: subtropical deserts 1. Subtropical deserts are found in bands about 30° latitude north and south of the equator.

2. Mean monthly temperatures vary but never fall below freezing.

3. The most striking feature is their low annual precipitation.

4. Desert species must cope with extreme temperatures and aridity.

G. Terrestrial biomes: tropical wet forest 1. Also called rain forests, tropical wet forests are found in equatorial regions where the temperature and rainfall are high and variation is low.

2. These forests are not seasonal (aseasonal). a. Plants grow all year long. b. There is no complete, seasonal loss of leaves.

3. Year-round growing conditions result in vigorous growth, high productivity, and large amounts of aboveground biomass.

4. Students should be able to explain why NPP is higher in tropical wet forests than in temperate forests.

H. Anthropogenic biomes 1. Ellis and colleagues have pioneered efforts to map anthropogenic biomes, or “anthromes.” (Fig. 49.12) 2. Humans are directly affecting the distribution of terrestrial ecosystems by physically changing the landscape as well as the global climate.

I. How will global climate change affect terrestrial biomes?

1. It is now well established that CO2 pollution is causing climate change.

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2. Biologists use four tools to predict how global warming will affect aquatic and terrestrial ecosystems: a. Simulation studies are based on computer models of weather patterns in local regions. b. Observational studies are based on long-term monitoring at fixed sites around the globe. c. Historical studies examine the relationship between CO2, climate change, and the distribution of organisms during events that occurred millions of years ago. d. Experiments are designed to simulate changed climate conditions and to record responses by the organisms present.

3. Experiments that manipulate temperature a. Many of these experiments have been done in the tundra biome because these regions are projected to experience dramatic changes in temperature and precipitation. b. Most experiments are performed as shown in Figure 49.13. c. Results support the conclusion that species diversity in the arctic decreases during experimental warming and arctic tundra environments are giving way to boreal forest.

4. Other simulation and observational studies a. Simulation and observational studies support the conclusion that increases in average global temperature increase the variability in temperature and precipitation.

Stated another way, global warming is making climates more extreme.

V. Types of Aquatic Biomes A. An organism’s environment has both physical and biological components.

1. The abiotic or physical components include temperature, precipitation, sunlight, and wind.

2. The biotic (“living”) components consist of other members of the organism’s own species as well as individuals of other species.

B. Salinity 1. The proportion of solutes dissolved in water determines its salinity.

2. The salinity of freshwater is from about 0.006 to 0.3 parts per thousand (ppt), whereas the open ocean is about 35 ppt.

3. Salinity has dramatic effects on osmosis and water balance in organisms.

C. Water depth and sunlight availability 1. Water depth dictates how much light reaches the organisms that live in a particular region.

2. Water removes light in the blue and red regions of the visible spectrum. (Fig. 49.14) 3. In pure seawater, the total amount of light available at a depth of 10 m is less than 40% of the amount available at the surface, and virtually no light reaches depths greater than 40 m.

4. Biologists describe the structure of an ocean by naming six regions: (Fig. 49.15) a. The intertidal zone (littoral zone) is along the shore. b. The neritic zone extends from the intertidal zone to an ocean depth of about 200 m (the edge of the continental shelf). .

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c. The oceanic zone encompasses the remainder and largest of the ocean environments. d. The benthic zone is the area at the ocean bottom. e. The photic zone receives sunlight to support photosynthesis. f. The aphotic zone is the area that is too dark to support photosynthesis.

5. Biologists describe the structure of lakes and ponds by naming five zones: (Fig. 49.16) a. The littoral zone consists of shallow waters near shore, where flowering plants are rooted. b. The limnetic zone is offshore and is composed of water that receives enough light to support photosynthesis. c. The benthic zone is made up of the substrate. d. Regions of the littoral, limnetic, and benthic zones that receive sunlight are part of the photic zone. e. Portions of a lake or pond that do not receive sunlight make up the aphotic zone.

6. The turbidity of water is also an important determination of light penetration.

D. Water flow 1. Water movement presents a physical challenge; it can literally sweep organisms off their feet.

2. Marine intertidal organisms are exposed to air periodically.

E. Nutrient availability 1. Levels of nutrients, such as nitrogen and phosphorus, limit growth rates in photosynthetic organisms.

2. Ocean upwellings bring nutrients to the surface. (Fig. 49.17) 3. Each year, lakes experience fall and spring turnovers, which redistribute freshwater nutrients as well as oxygen. (Fig. 49.18) Turnovers occur because of heating and cooling of water and the fact that water is most dense at 4°C. F. Aquatic biomes>Freshwater>lakes and ponds (Table 49.3) 1. Ponds are small, while lakes are big enough that the water in them can be mixed by the wind.

2. As the temperature of the surface changes throughout the year, water from different temperatures can mix. This allows well-oxygenated water from the surface to reach the benthic zone and nutrient-rich water from the benthic zone to enter the littoral and limnetic zones.

3. Cyanobacteria, algae, and other microscopic organisms are collectively called plankton and live in the photic zone. Animals that consume detritus are common in the benthic zone.

G. Aquatic biomes>Freshwater>wetlands 1. Wetlands are shallow-water habitats where the soil is saturated with water for at least part of the year.

2. Wetlands are distinct from lakes and ponds for two reasons: a. They have only shallow water. b. They have emergent vegetation⎯that is, plants that grow above the surface of the water.

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3. Marshes and swamps are wetland types characterized by a slow but steady flow of water.

4. Bogs develop depressions where water flow is low or nonexistent. Bog water is oxygen poor and acidic.

5. Marshes lack trees and typically feature grasses.

6. Swamps are dominated by trees and shrubs.

H. Aquatic biomes>Freshwater>streams 1. Streams are bodies of water that move constantly in one direction.

2. Creeks are small, and rivers are large.

3. The structure of a stream varies along its length. a. At its mountain origin, a stream is usually cold, narrow, and fast. b. As it reaches its ocean or lake destination, it becomes larger, warmer, and slower.

4. The organisms in a stream vary along its length. a. Small, fast-moving streams rarely have photosynthetic organisms. b. Wider, slower streams have algae and plants.

I. Aquatic biomes>Freshwater/Marine>estuaries 1. Estuaries are where rivers meet the ocean, which means that freshwater mixes with saltwater.

2. Because the water is shallow in estuaries and nutrients are constantly renewed by the incoming stream, estuaries are very productive environments.

3. Salinity has dramatic effects on osmosis and water balance, so species that live in estuaries have adaptations that allow them to cope with variations in salinity.

J. Aquatic biomes>Marine>oceans 1. The world’s oceans form a continuous body of saltwater and are remarkably uniform in chemical composition.

2. Tides and wave action are major influences in the intertidal zone.

3. Large-scale currents circulate water in the oceanic zone in response to prevailing winds and Earth’s rotation.

4. Each zone is populated by distinct species.

5. In the tropics, shallow portions of the neritic zone may support coral reefs—among the most productive environments in the world.

6. Students should be able to predict which zones of oceans and lakes have the highest NPP and explain their logic.

K. How are aquatic biomes affected by humans?

1. Direct effects—Humans cause physical changes to aquatic ecosystems by filling or draining wetlands, damming streams, and removing water from ponds and rivers for agricultural irrigation.

2. Indirect effects—Global climate change influences aquatic ecosystems by the same mechanisms discussed for terrestrial ecosystems. a. In addition, increased CO2 is causing waterways to become more acidic, limiting the ability of organisms like coral to build skeletons.

3. While the oceans are deep and vast, water represents a small fraction compared to the volume of the earth. (Fig. 49.19)

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Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text. emergent abiotic vegetation epiphytes aboveground biomass estuaries adiabatic cooling exotic aphotic zone Hadley benthiccell intertidalzone zone benthic invasive species biogeography lakes biomass latitude biomes limnetic biotic littoral bogs marshes boreal forests/taiga neritic canopyzone net primary productivity climate community conservation biology continental shelf coral reefs detritus dispersal ecology ecosystem

productivity

(NPP) oceanic zone permafrost photic zone plankton ponds population prairies

rain shadow range sage-steppe seasons specific heat steppes streams subtropical desert swamps taiga temperate temperate forests temperate grasslands thermocline tropical wet forest tundra turnovers weather wetlands

Lecture Activities Ecology Case Study: Highlighting the Effect of Exotic Species Estimated duration of activity: May be done completely outside of class or used to prepare students for an in-class debate that can last 30 minutes or so This case study is about purple loosestrife, an exotic species brought to the United States from Europe in the 1800s as a perennial garden plant. It has spread to most temperate regions in the United States and Canada. It is a hearty, fast-growing plant that seriously affects the wetland communities it invades. Its invasion has been so significant that several states have laws prohibiting its importation and distribution. If asked, your students will likely be hardpressed to explain why they should care about purple loosestrife. This activity helps them realize how important the delicate balance of the ecological communities around them is to all of our lives. This case may be assigned to students as they begin this unit on ecology. Students will need to do some Internet research outside of class. The assignment may culminate in an in-class debate, or you may choose to have the students write position papers.

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The Case June: Jackie was glad to have a break from school. Chemistry was difficult last semester, and she looked forward to delving into her biology classes in the fall. As her mom drove along the Wisconsin highway to her grandparents’ house, Jackie watched the landscape and daydreamed. She suddenly realized that she had been looking at beautiful deep-pink flowers that blanketed the landscape. “Wow,” she said, “these flowers stretch for miles! Hey, Mom, what are those pink flowers out there? They are beautiful!”

Jackie’s mom sighed and replied, “Talk about a wolf in sheep’s clothing. Those pretty little flowers are called purple loosestrife, and they are devastating Wisconsin’s wetlands.

They have taken over! Many other plants have been squelched by this pretty little invader, and many animals have died or left the swamp because their food source is now gone.”

Puzzled, Jackie asked, “Well, how did it get here, and why doesn’t someone just kill it?”

“I’m not sure,” Jackie’s mom replied. “I have heard Grams and Gramps complaining about it, but I don’t know how it got here.”

Now Jackie was curious. When she got to her grandparents’ house, she got out her laptop and started doing some Internet research. How could something that pretty be so bad?

Use an Internet search engine to find credible websites to answer these questions: 1. What is purple loosestrife?

2. How did it arrive in the United States?

3. Why is it considered an invasive species?

4. What habitats does it typically invade?

5. What abiotic factors in those habitats make them suitable for purple loosestrife?

6. How does the presence of loosestrife change the biotic structure of the habitats it invades?

Interesting, Jackie thought as she finished her Internet research on purple loosestrife. She enjoyed the rest of her visit with her grandparents and didn’t really think much more about those pretty little flowers until several months later, after she had returned to school.

October: Jackie rushed into biology class just in time to find a seat and glance over the handout her professor had given her as she came in. “Oh man, a research paper!” Jackie groaned internally. “I hate boring research papers.”

Jackie listened as her biology professor explained the assignment. He said the paper was to be a position paper. Each student was to choose any subject in biology that involved a dilemma, research all sides of the dilemma, choose a position, and defend it.

A smile slowly spread across Jackie’s face. She sat up straight in her chair and wrote the title of her paper in block letters across the top of the page: CONTROLLING PURPLE LOOSESTRIFE: THE BEAUTIFUL KILLER. As Jackie researched her paper, she found that there are two options for controlling purple loosestrife: biocontrol or herbicides.

Now it’s your turn. Using a search engine to find credible websites, research the biocontrol and herbicide options available for the control of purple loosestrife. Answer these questions and choose a position.

1. What are the available biocontrols for purple loosestrife?

2. What are the advantages and disadvantages of introducing these organisms into the habitats that purple loosestrife has invaded?

3. What herbicides are available for controlling purple loosestrife?

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4. What are the advantages and disadvantages of using these chemicals in these habitats?

5. How have biocontrols and herbicides been used to control other invasive plant species?

In defending your position, you must be able to tell the class why they should care about controlling purple loosestrife. To do that, identify at least three plant and three animal species that are displaced by the introduction of purple loosestrife to a wetland. Describe why each species has been displaced. Are these organisms simply dying, or are some being driven to different habitats? With all of this in mind, prepare a statement about the potential environmental and economic impacts of uncontrolled purple loosestrife in North America.

Using a Concept Map to Show Students Why Ecology Is Important Estimated duration of activity: 5–10 minutes Many students have a general understanding that ecology is important to study, but sometimes they are so focused on human biology that they fail to understand the importance of studying species that do not have an obvious link to humans. As you introduce the unit on ecology, you could use the salmon example in the text chapter to illustrate the different levels of ecological study while showing how studying sockeye salmon at all levels of ecology is relevant to humans. You may draw a concept map as you discuss this part of the chapter.

By the end of your discussion, the concept map should give the students the “big picture” (quite literally) of how studying and protecting a single species are imperative to preserving the human habitat. If you use mural paper for the concept map, leave a lot of space in your diagram; then you can add to it over several class periods as you study each level of ecology in more depth.

If you are unfamiliar with drawing concept maps or how to use them as a teaching tool, consult these articles: http://en.wikipedia.org/wiki/Concept_map http://academic.wsc.edu/frc/innovations.htm http://www.udel.edu/chem/white/teaching/ConceptMap.html

Discussion Ideas for All Class Sizes Ecotones and Taxol The following discussion suggestions will engage students and allow them to apply what they are learning to subjects that will interest them.

• An ecotone is a region where two or more major biomes meet. For example, the tundra and boreal forest meet at one type of ecotone, tropical rain forests and subtropical deserts at another, and freshwater and saltwater systems at still another (these ecotones are referred to as estuaries). What do you think the major characteristics of ecotones are? How is an ecotone different from either of its parent biomes, and what might be better or worse about the ecotone? How might the ecotones in the examples just given differ from one another, and which of the three do you think would be the most ecologically beneficial? Explain your answer.

• Taxol, an effect anticancer drug prescribed to women who have breast and ovarian cancer, was originally extracted from the Pacific yew tree. Between 1989 and 1992, Bristol Myers was given the exclusive rights to harvest the bark from Pacific yew trees. However, many people opposed the harvesting of Pacific yew trees because it takes many trees to extract 532

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enough taxol to be used (three trees per patient) and would dramatically affect forest communities. Removing the bark kills the tree. The Pacific yew is a slow-growing tree that is hard to grow from seed. It is food for deer, elk, and other grazing mammals in forests. After presenting this background to students or after having them read the Pacific Yew Act of 1992 (https://www.govtrack.us/congress/bills/102/s2851), initiate a short debate on the benefits of taxol as an anticancer drug and the ecological impact of the removal of Pacific yew trees.

You may conclude the discussion or debate with an update: Taxol has also been identified in the endophytic fungus Taxomyces andreanae as well as in another tree, the African fern pine.

The new promise of this tree is that it can be grown in a nursery, allowing for taxol harvesting without a large ecological impact.

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CHAPTER

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Behavioral Ecology

Learning Objectives: Students should be able to…

• Explain proximate and ultimate causation of behavior.

• Use examples to describe how foraging decisions can maximize energy gain and minimize costs.

• Contrast the proximate and ultimate causes of mate choice.

• Describe the cues that animals use to navigate.

• Use examples to describe how animals communicate.

• Explain why animals might cooperate or help others.

Lecture Outline I. An Introduction to Behavioral Biology A. Proximate and ultimate causation 1. Proximate causation explains how actions occur.

2. Ultimate causation explains why actions occur.

3. For example, Argentine ants live in colonies and defend their feeding areas against other colonies, yet where they are introduced, they are friendly to other Argentine ant colonies and form supercolonies. How does this work? (Fig. 50.1) 4. The proximate cause is that ants can identify each other by smelling waxy hydrocarbon tags on each other’s exoskeleton.

5. The ultimate cause is that a supercolony might create a higher overall fitness compared to isolated smaller colonies.

B. Types of behavior: An overview 1. Behaviors vary in their flexibility. (Fig. 50.2) a. Highly inflexible behavior patterns are called fixed action patterns (FAPs). b. FAPs are examples of innate behavior—that is, types of behavior that are inherited and show little variation based on learning. c. Most behaviors lie in between the two extremes of inflexibility and flexibility.

2. Behaviors vary in the extent to which they are learned. a. Innate behavior is inherited. b. Learning is an enduring change in behavior that results from a specific experience.

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c. Although all species studied to date show some degree of innate behavior, it is much more common for an individual’s behavior to change in response to learning and show flexibility in response to changing environmental conditions.

3. Flexible learned behaviors often involve choice. a. Animals take information from the environment and make decisions about what to do based on that information; animals make choices. b. Each behavior has different costs and benefits, resulting in fitness trade-offs—two traits that cannot be optimized simultaneously c. Cost-benefit analysis: Animals appear to weigh the costs and benefits of responding in various ways, based on their impact on fitness. d. Choices are not usually conscious. e. The behavior of individuals varies within populations.

II. Choosing What, How, and When to Eat A. Animals forage for food.

1. Animals forage in a way that maximizes the amount of usable energy they take in, given the costs of finding and ingesting their food and the risk of being eaten while they’re at it.

B. Proximate causes: Foraging alleles in Drosophila melanogaster 1. Marla Sokolowski discovered an important behavioral trait controlled by a single gene. She noticed that some fruit fly larvae tended to move away from food after eating, while others remained in place.

2. By breeding “rovers” and “sitters” that possessed other distinct genetic markers, Sokolowski and colleagues were able to map the gene responsible for the behaviors.

(Fig. 50.3) C. Ultimate causes: Optimal foraging 1. Optimal foraging is the concept that animals maximize their feeding efficiency.

2. Fitness is proportional to feeding efficiency, which is proportional to the benefits divided by the costs.

3. The specific costs and benefits of foraging depend on the feeding strategy and the environmental conditions.

4. Making models of a feeding scenario can help visualize the problem and identify the variables. (Making Models 50.1) 5. To test whether optimal foraging actually occurs, researchers studied the eating behavior of desert gerbils.

6. Foraging is significantly reduced in the presence of predators suggesting gerbils weigh the relative risk of predation and benefits of extra seeds when they forage. (Fig. 50.4)

III. Choosing a Mate A. Proximate causes: How is sexual activity triggered in Anolis lizards?

1. Males become active in January and begin establishing territories. (Fig. 53.5a) 2. Females lay an egg every 10 to 14 days. By the end of the breeding season, females will lay an amount of eggs equal to twice their body mass.

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3. What causes these dramatic seasonal changes in behavior? (Fig. 53.5b) At the proximate level, these seasonal changes in behavior are caused by the sex hormones testosterone and estradiol.

4. Testing the effects of light and social stimulation a. Researchers captured inactive Anolis lizards and placed them in treatment groups that varied light and social combinations. b. Females exposed to springlike conditions began producing eggs; females in the field under winter conditions did not. c. Females exposed to breeding males began producing eggs much earlier than females that were alone, with other females, or with castrated males.

5. Cues from males trigger female readiness a. Researchers exposed females to intact males and males with dewlaps (a brightly colored patch of skin on the throat) surgically removed. b. Females living with dewlap-less males were slow to produce eggs. c. The data suggest that the dewlap is a key visual signal that triggers hormone production and the onset of sexual behavior.

B. Ultimate causes: sexual selection 1. Sexual selection favors individuals possessing traits that increase their ability to obtain mates.

2. Females are usually the gender that is pickiest about mate choice and choose males that contribute good alleles/resources to their offspring.

3. When females choose males, intersexual selection is occurring. (Fig. 50.6) 4. When males compete with one another for mates, intrasexual selection is occurring.

(Fig. 50.6b) 5. Although lizard dewlap brightness does not appear to correlate with nutritional health, it does correlate with territory defense success. (Fig. 50.7)

IV. Choosing a Place to Live A. Animals select their habitats.

1. Biologists have explored an array of questions related to habitat selection. a. Whether or not juveniles should disperse to a new area b. How large a territory should be defended c. Whether high-quality, crowded habitats are better than low-quality, uncrowded habitats 2. Migration is the long-distance movement of a population associated with a change in seasons.

B. Proximate causes: How do animals navigate?

1. Many species use piloting, or using familiar landmarks, to find their way. a. Young offspring follow their parents during migrations and appear to memorize the route. b. Homing pigeons can always find their way back home, but if their eyes are covered, they fail to navigate just the final stage, suggesting they may use piloting for the final part of a journey.

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2. Compass orientation is using a reference to determine where north is. a. To determine where north is, homing pigeons appear to use the sun during the day and the stars at night. During cloudy conditions, these birds appear to use Earth’s magnetic field to orient. b. To use the sun as a reference, the animals must have a circadian clock that maintains a 24-hour rhythm of chemical activity. c. Although research on mechanisms of compass orientation continues, one point is clear: Birds and perhaps other organisms have multiple mechanisms of finding compass direction. d. At least some species can use a sun compass, a star compass, and a magnetic compass. Which systems they use depends on the weather and other circumstances. e. Dung beetles can use the band of light caused by the Milky Way galaxy to navigate.

(Fig. 50.8) 3. True navigation or map orientation means that displaced animals could always find their way home. a. Lohmann hypothesized that green sea turtles use Earth’s magnetic information as positional information. (Fig. 50.9) b. He captured green sea turtles and released them in a circular pool in which the magnetic information was changed. c. Turtles swam in the direction of “home” based on the magnetic information of the pool; they therefore possess map orientation.

C. Ultimate causes: Why do animals migrate?

1. Migratory movements can be spectacular.

2. Arctic terns (Fig. 50.10) a. Arctic terns nest along the Atlantic coast of North America, fly south along the coast of Africa to wintering grounds off Antarctica, and then fly back along the eastern coast of South America. b. They feed on fish in different parts of the world in different seasons.

3. Salmon a. Salmon that hatch in rivers along the Pacific Coast of North America and northern Asia migrate to the ocean when they are a few months to several years old. They return to the stream where they hatched to mate and then die. b. Salmon eggs and young are safer and thrive better in freshwater habitats, whereas adults can find more food in ocean habitats.

V. Communicating with Others A. Communication is a social process.

1. Communication is defined as any process in which a signal from one individual modifies the behavior of a recipient individual.

2. A signal is any information-containing behavior.

B. Proximate causes: How do honeybees communicate?

1. Karl von Frisch suspected that successful food finders communicate the location of food to other individuals.

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2. He observed bees displaying a “round dance” that contained information about the location of food. Workers got the information about the location of food through the dancer’s movements.

3. He also observed a “waggle dance” that tells the distance the workers must fly, as well as the direction. (Fig. 50.11) 4. Von Frisch was able to confirm that the bees were varying the orientation of the waggle to communicate the position of the food relative to the sun. (Fig. 50.12) 5. Honeybees do not have large brains, yet they are capable of symbolic language.

C. Ultimate causes: Why do honeybees communicate the way they do?

1. Bees communicate to share information about food resources which maximizes hive fitness.

2. Communication can be acoustic, visual, olfactory, or tactile.

3. One of the most general observations about communication is that the type of signal used by an organism correlates with its habitat.

4. Each mode of communication has advantages and disadvantages.

D. When is communication honest or deceitful?

1. Deceiving individuals of another species a. Hognose snakes play dead to avoid being eaten. (Fig. 50.13a) b. Predatory Photuris fireflies can mimic the pattern of flashes by the females of several other species. (Fig. 50.13b) 2. Deceiving individuals of the same species a. In some cases, natural selection has favored the evolution of lying to the same species. b. A male bluegill imitates a female and gets close to another male while he is courting an actual female. The male bluegill thinks he is courting two females, and while he does this, the female mimic secretly fertilizes the real female’s eggs but does not have to care for them. (Fig. 50.14) c. In analyzing deceitful communication in bluegill sunfish and other species, researchers point out that in most cases, lying works only when it is relatively rare. d. If deceit becomes extremely common, then natural selection strongly favors individuals that can detect and avoid or punish liars. e. If liars are rare, then natural selection favors individuals that are occasionally fooled but are more commonly rewarded by responding to signals in a normal way.

VI. Cooperating with Others A. Altruistic behavior appears to be paradoxical.

1. Altruism is an act that has a cost to the actor, in terms of his or her ability to survive and reproduce.

2. Altruism also has a benefit to the recipient of the altruistic act.

B. Kin selection 1. Black-tailed prairie dogs perform a behavior called alarm calling. This call alerts all prairie dogs in the area to hide from the threat, but giving the call is very risky.

2. In an altruistic act, the coefficient of relatedness is a measure of how close the actor and beneficiary are related.

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3. Calculating the coefficient of relatedness (Quantitative Methods 50.1) a. The coefficient of relatedness (r) varies between 0.0 and 1.0, with 1.0 being identical twins and 0.0 being individuals that have no identical alleles. (Fig. 50.15a) b. In each parent-to-offspring link of descent, the probability of any particular allele being transmitted is 1/2. c. The probability that full siblings share alleles as a result of inheriting them from one parent is 1/4. (Fig. 50.15b) (BioSkills 4) 4. Hamilton’s rule (Br > C) states that altruistic behavior is most likely when three conditions are met: a. The fitness benefits of altruistic behavior are high for the recipient (high B). b. The altruist and the recipient are close relatives (high r). c. The fitness costs to the altruist are low (low C).

5. The key is that close relatives are very likely to have copies of the altruistic allele, which allows these relatives to produce more offspring than they could produce without help.

6. Inclusive fitness is the combination of indirect and direct fitness components. a. Direct fitness is derived from an individual’s own offspring. b. Indirect fitness is derived from helping relatives produce more offspring than they could produce on their own.

7. Kin selection is natural selection that acts through benefits to relatives.

8. Students should be able to explain why it is likely that kin selection is common in humans.

9. The kin-selection hypothesis predicts that individuals who do not have close genetic relatives nearby will rarely give an alarm call. a. Experimental data indicate that black-tailed prairie dogs are much more likely to call if they live in a coterie that includes relatives. (Fig. 50.16) 10. This same pattern—preferentially dispensing help to kin—has been observed in many other species of social mammals and birds. Most cases of self-sacrificing behavior that have been analyzed to date have been consistent with Hamilton’s rule and are hypothesized to be the result of kin selection.

C. Manipulation 1. In eusociality, workers sacrifice most or all of their direct reproduction to help rear the queen’s offspring.

2. Since diploid queens mate with haploid males, all the workers are highly related, with a coefficient of relatedness of 0.75.

3. This suggests it would be more adaptive for workers to raise sisters than to raise their own offspring.

4. Students should be able to suggest a proximate cause of manipulation by the queen in insect colonies.

D. Reciprocal altruism 1. Reciprocal altruism is an exchange of fitness benefits that are separated in time. a. Vervet monkeys are most likely to groom unrelated individuals that have groomed or helped them in the past.

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b. Vampire bats are most likely to donate blood meals to non-kin that have previously shared food with them.

E. Cooperation and mutualism 1. Some behaviors appear altruistic but actually may be selfish—that is, the benefits of assisting often exceed the costs. a. African wild dogs cannot hunt large prey alone, but they can succeed by hunting as a pack. b. Female horses that spend more time in grooming relationships with other females tend to suffer less harassment from males and tend to then have more foals.

F. Individuals do not act for the good of the species.

1. Some apparent acts of cooperation are misunderstood.

2. “Self-sacrificing” alleles in individuals would likely die before passing them on and thus would be unlikely to perpetuate in a population.

3. Organisms behave altruistically only when doing so either increases their direct fitness or their inclusive fitness, or when they are coerced.

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text. altruism behavior causation circadian clock classical conditioning coefficient of relatedness cognition communication compass orientation cost-benefit analysis critical/sensitive period

echolocation eusociality fixed action patterns (FAPs) Hamilton’s rule imprinting innate behavior kin selection learning migration optimal foraging orientation

phonotaxis phototaxis piloting proximate (mechanistic) reciprocal altruism releasers/sign stimuli signal taxis true navigation ultimate (evolutionary)

Lecture Activities Common Behavioral Examples and Exercises Activity Ideas for All Class Sizes Estimated duration of activity: 10–20 minutes • Ask your students to consider the adaptive value of emotions, and behaviors associated with emotions, that are presumably specific to humans. Good examples are guilt, spite, and revenge. These emotions often lead to terrible tragedies, so why have they persisted in the human population? Why hasn’t natural selection eliminated them? What are the disadvantages of these emotions that often lead to tragedy? Do they have advantages? If so, what 540

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are they, and why might they be overlooked? Could these emotions and associated behaviors be classified as cooperative, altruistic, selfish, or spiteful behavior?

• If you are brave, ask your students to consider that someday they will die. As far as we know, only humans comprehend their own mortality. And, although cultures vary, humans are the only species to practice different forms of religion or belief in the supernatural.

Recent research suggests a genetic basis for religious or spiritual belief; that is, humans may be predisposed to believe in the supernatural and the behaviors associated with such a belief. What pressures might generate alleles associated with such beliefs in the first place?

Why might natural selection favor alleles associated with religious and spiritual beliefs?

What is the adaptive value of such beliefs? How could you test your hypothesis?

• Animal communication is a subdivision of animal behavior gaining popularity in the scientific community. Ask students to think about the four basic modes of communication— vocal, visual, chemical, and tactile. What are the advantages and disadvantages of each mode? In which groups of organisms is each mode found most predominantly? Why? Many organisms exhibit complex communication that relies on combinations of some or all of these modes. Is this kind of complex communication a language? If so, what distinguishes language from other types of communication, and where do you draw the line? Is it possible for humans to then learn these other “languages”? If such communication is not a language, what characteristics does language have that distinguishes it from other types of complex communication? Is language better or worse than other forms of communication? Why or why not?

• A great historical experimental archive is devoted to discussing how researchers went about trying to discover how homing pigeons navigate. Although many of these experiments were gruesome and wouldn’t be permitted today, they clearly demonstrate that pigeons can home when they are not allowed to use their eyes. Before revealing these and many other experiments to the class, ask students how they would determine these types of concepts experimentally. How do pigeons navigate? They could form groups and come up with a series of experiments. You could also use information on sea turtles to address these concepts. Hatchlings somehow know where water is and in which direction to swim once in the ocean.

• Another great behavioral example is the classic game Prisoner’s Dilemma, in which players choose either to cooperate or defect. The premise is that each player gains if they all cooperate, but if one defects, that one will gain more. This is a great game for small groups and underscores types of selfish and altruistic behavior.

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Population Ecology

Learning Objectives: Students should be able to…

• Describe how individuals within populations are distributed geographically.

• Describe the relationship between survivorship, fecundity, and other life-history traits.

• Calculate the growth rate of a population.

• Compare and contrast exponential and logistic growth.

• Describe how and why population size changes over time.

Lecture Outline • A population is a group of individuals from the same species that live in the same area at the same time.

• Population ecology is the study of how and why the number of individuals in a population changes over time.

I. Distribution and Abundance A. Geographic distribution 1. Abiotic and biotic factors both determine the range of a species.

2. The range of a species can also be considered at different scales. a. The population density of the common lizard varies throughout its range.

(Fig. 51.1) 3. In general, individual organisms can be arranged in different patterns. a. Random—position of each individual is independent of the others b. Clumped—organisms associate in groups c. Uniform—even distribution usually associated with competition 4. A metapopulation is a population of populations connected by migration.

B. Sampling methods 1. Immobile organisms are easy to assess by using quatrats or transects and simply counting individuals.

2. Mobile animals are more difficult to assess. Mark-recapture studies are one method.

(Fig. 51.2) (Quantitative Methods 51.1) a. The relationship between marked and unmarked individuals can be expressed as M/N = m/n. (Eq. 51.1)

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b. M is the number of marked animals; m is the number of marked individuals in the recapture; n is the total number of individuals in the recapture; N is the total population size. c. The total population size is N = Mn/m (Eq. 51.2) 3. Students should be able to complete this example: Suppose you trap 255 stickleback fish in a lake and mark them by clipping the first spine on their dorsal fins. One month later you recapture a total of 162 individuals, 78 of them marked. Estimate the total population size.

4. Suppose you re-trapped the fish population every month and noticed that the percentage of marked individuals decreased each time. What are the two possible explanations for the decline?

II. Demography A. Demography is the study of factors that determine the size and structure of populations through time.

1. Analyzing birth rates, death rates, immigration rates, and emigration rates is fundamental to demography.

2. A generation is the average time between a mother’s first offspring and her daughter’s first offspring.

B. Life tables 1. Formal demographic analyses of populations are based on a type of data set called a life table, which summarizes the probability that an individual will survive and reproduce in any given year over the course of its lifetime.

2. Zootoca vivipara: a case study a. Researchers set out to estimate the life table of a low-elevation population of Z. vivipara in the Netherlands with the goal of comparing the results to data from other Z. vivipara populations. b. The data allowed them to calculate the number of individuals that survived each year as well as the number of individuals produced per female. An age class is a group of individuals of a given age. (Table 51.1) c. A cohort is a group of animals of the same age that can be followed through time.

3. Survivorship a. Survivorship is the proportion of offspring produced that survive, on average, to a particular age (lx, where x represents the age class being considered). b. Survivorship is calculated by dividing the number of individuals in a given age class by the number of individuals that existed as offspring: lx = Nx /No. (Quantitative Methods 51.2) (Eq. 51.3) c. To recognize general patterns in survivorship and make comparisons among populations or species, biologists plot the logarithm of the number of survivors versus age, which is called a survivorship curve. (Fig. 51.2) (BioSkills 5) (1) Type I curve: Survivorship throughout life is high.

(2) Type II curve: Constant mortality occurs throughout life.

(3) Type III curve: The death rate is high early in life.

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4. Fecundity a. Fecundity is the number of female offspring produced by each female in the population. b. The net reproductive rate of a population is calculated as: R0 = Σ lx mx.

(Quantitative Methods 51.2) (Eq. 51.4) c. Lifetime reproduction is a function of fecundity at each age (mx) and survivorship to each age class (lx). If R0 is greater than 1, the population is increasing. If R0 is less than 1, the population is declining. d. Age-specific fecundity is the number of female offspring produced by a female in age class x.

5. Students should be able to use the data in Table 51.1 to determine how many female offspring an average L. vivipara female produces over the course of her lifetime, and describe whether the population is growing, stable, or declining.

C. The role of life history 1. An organism’s life history consists of how it allocates resources to growth, reproduction, and activities related to survival.

2. It is not possible for an organism to have both high fecundity and high survivorship.

A trade-off exists between survival and reproduction.

3. Fitness trade-offs occur because every individual has a restricted amount of time and energy at its disposal; that is, its resources are limited.

4. If a female lizard devotes a great deal of energy to feeding a large number of offspring as they develop, it is not possible for her to devote that same energy to her immune system, growth, nutrient stores, or other traits that increase survival.

5. In almost all cases, biologists find that life history is shaped by natural selection in a way that maximizes an individual’s fitness in its environment.

6. In general, organisms with high fecundity tend to grow quickly, reach sexual maturity at a young age, and produce many small eggs or seeds.

7. In contrast, organisms with high survivorship tend to grow slowly and invest resources in traits that reduce damage from enemies and increase their own ability to compete for water, sunlight, and food.

8. An Arabidopsis thaliana plant and a coconut palm represent the two ends of a broad continuum of life-history characteristics. (Fig. 51.4)

III. Population Growth

1. Populations are affected by birth and immigration rates as well as death and emigration rates.

2. A population’s growth rate is the change in the number of individuals in the population (ΔN) per unit time (Δt).

3. If no immigration or emigration is occurring, then the growth rate is equal to the number of individuals (N) in the population times the difference between the birth rate per individual (b) and the death rate per individual (d).

A. Exponential growth 1. An exponential growth equation a. The difference between the birth rate and the death rate per individual is called the per capita rate of increase and is symbolized r.

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b. A species with a certain life history has an intrinsic rate of increase (rmax) that does not change. But at any time, each population of that species has an instantaneous growth rate, or per capita rate of increase, symbolized by r.

2. Using growth models to predict population growth (Quantitative Methods 51.3) a. From life history data—The per capita rate of increase, r, is related to the net reproductive rate, R0, which means that r can be estimated from life table data: r = ln R0/g. (Eq. 51.5) (BioSkills 5) b. Continuous growth—The size of the population at time t is calculated as N t = N0ert (Eq. 51.6); this equation summarizes how populations grow when they breed continuously. c. Discrete growth—Biologists use N to symbolize population size. N0 is the population size at time zero, and N1 is the population size one breeding interval later. The growth rate is calculated as λ = N1/N0, where λ is the finite rate of increase.

(Eq. 51.7) d. The size of the population at time t is calculated as N t = N0λt (Eq. 51.8); this equation summarizes how populations grow when breeding takes place seasonally. The size of the population at time t is equal to the starting size times the finite rate of increase multiplied by itself t times. e. The relationship between the finite rate of increase and the per capita growth rate is λ = er, where e is the natural logarithm, or about 2.72. (Eq. 51.9) f. Because r represents the growth rate at any given time and because r and λ are so closely related, biologists routinely calculate r even for species that breed seasonally. g. Students should be able to solve the problems at the end of Quantitative Methods 51.3.

3. Graphing exponential growth a. Exponential growth occurs when r does not change over time. It does not depend on the number of individuals in the population. Therefore, it is density independent.

(Fig. 51.5) (Making Models 51.1) 4. Exponential growth is density independent a. In nature, exponential growth has been observed in two circumstances: (1) A few individuals found a new population in a new habitat.

(2) A population was devastated by a storm or other type of catastrophe and then began to recover, starting with a few surviving individuals. b. In reality, it is not possible for growth to continue indefinitely because the habitat would not have enough resources for the number of individuals present. c. When a population stabilizes at the maximum number that can be supported by the resources available, that population has reached the habitat’s carrying capacity.

C. Logistic growth 1. Growth is often density dependent.

2. The carrying capacity (K) of a habitat depends on a large number of factors: food, space, water, soil quality, resting or nesting sites, and the intensity of disease and predation. Carrying capacity may change from year to year, depending on conditions.

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3. The logistic growth equation describes logistic population growth, or changes in the growth rate that occur as a function of population size. Just as exponential growth is density dependent, logistic growth is density dependent.

4. In summary, biologists have developed several ways of calculating and expressing a population’s growth rate. λ has the advantage of being easy to understand, and R0 has the advantage of being calculated directly from life table data. Although r is slightly more difficult conceptually, it is the most useful expression for the growth rate because it is independent of generation time and is relevant for species that breed either seasonally or continuously.

5. Graphing logistic growth a. Density-dependent graphs have three sections: (Fig. 51.6a) (1) Initially growth is exponential, meaning that r is constant.

(2) N increases to the point at which competition for resources or other densitydependent factors begins to occur. Growth rate begins to decline.

(3) Eventually the growth rate reaches 0; that is, the habitat’s carrying capacity is reached. b. For example, both species of Paramecia exhibit logistic growth, but the carrying capacities differ. (Fig. 51.6b) D. What limits growth rates and population sizes?

1. Density-independent factors change birth rates and death rates regardless of the number of individuals in a population.

2. Density-dependent factors are usually biotic and change in intensity as a function of population size.

3. Density-dependent factors a. Table 51.2 lists some of the major types of density-dependent factors that reduce survivorship, fecundity, or both. b. For example, a new population of Arabidopsis will thrive at high density when they are seedlings. As the population becomes crowded, each seedling has reduced access to water and light, thus survivorship declines. c. Crowded populations are also vulnerable to infectious disease and parasites as well as increased predation rates. d. Density-dependent factors can be based on intraspecific (“within-species”) interactions and interspecific (“between-species”) interactions.

4. Carrying capacity is not fixed. It varies in space and time because conditions in some years are better than in others.

IV. Population Dynamics A. Population dynamics are changes in populations over time.

1. Researchers have uncovered a wide array of patterns in natural populations in addition to exponential and logistic growth.

B. Why do some populations cycle?

1. Population cycles are regular fluctuations in size that some populations exhibit.

2. For example, the snowshoe hare and lynx populations in northern Canada exhibit regular cycling. (Fig. 51.7)

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3. Two hypotheses were proposed for an experiment: (Fig. 51.8) a. Bottom-up hypothesis—Hares use up all of their resources when population density is high, and they starve. In response, lynx also starve. b. Top-down hypothesis—Lynx populations reach high density in response to an increase in hare density. Lynx eat so many hares that the prey population crashes.

4. The data support the hypothesis that hare populations are limited by the availability of food as well as by predation and that these two factors interact. This explains why when hares are at high density, individuals are weakened by nutritional stress and therefore more susceptible to predation.

C. How do metapopulations change through time?

1. Because humans are reducing large, contiguous areas of forest and grasslands to isolated patches or reserves, more and more species are forced into a metapopulation structure⎯a population of populations. (Fig. 51.9) 2. Ilkka Hanski and colleagues showed that metapopulations exist in nature. They performed a mark-recapture study on the Glanville fritillary butterfly. They found a migration rate of 9%, which was high enough to recolonize extinct populations. They also confirmed that some populations had gone extinct while others had been created.

3. Given enough time, each population within the larger metapopulation is expected to go extinct. The cause could be catastrophic, such as a storm or oil spill, or a disease outbreak or a sudden influx of predators.

4. Migration from nearby populations can reestablish populations in these empty habitat fragments. In this way, there is a balance between extinction and recolonization.

V. Human Population Growth A. Age structure in human populations 1. Analyzing an age pyramid can give biologists important information about a population’s history.

2. In countries where industrial and technological development is advanced and average incomes are relatively high, the age pyramid of the population tends to be even.

(Fig. 51.10a) 3. The age pyramid predicted for developed countries in 2050 highlights their major public policy concern: how to care for an increasingly aged population.

4. In contrast, the age distribution is bottom-heavy in less-developed countries.

(Fig. 51.10b) 5. The age pyramid predicted for less-developed countries in 2050 illustrates their major public policy concern: providing education and jobs for an enormous influx of young people who will want to be starting families.

6. Overall population size will increase dramatically in developing countries. Even though the number of children per female (fecundity) will drop, there will be so many women having children that the number of offspring born per year is expected to stay high.

B. Analyzing change in the growth rate of human populations 1. The growth rate for humans has been increasing, leading to a steeply rising curve over the past few centuries. Most recently, it took only 12 years to add 1 billion people to the population to reach 7 billion. (Fig. 51.11)

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2. It is not possible to overstate the consequences of recent and current increases in human population. It is the primary cause of habitat loss, declines in living standards, mass movement of people, political instability, and acute shortages of water and other resources.

3. The one encouraging trend in the data is that the growth rate of the human population has already peaked and begun to decline.

4. Will the size of the human population peak in your lifetime? a. Population growth has slowed over the past few years; now it is 1.2% per year.

Humans are ending a period of rapid growth that lasted well over 500 years. How quickly growth rates decline and the maximum population size is ultimately reached will depend on changes in fertility rates and the course of the AIDS epidemic. b. Extrapolating the world’s population to the year 2050 is based on three different fertility rates: (Fig. 51.12) (1) High fertility⎯nearly 11 billion (2) Medium fertility⎯about 9.3 billion (3) Low fertility⎯about 8.3 billion c. The future of the human population hinges on fertility rates—on how many children each of the women living today decides to have.

VI. How Can Population Ecology Help Conserve Biodiversity?

A. Using life table data 1. Population projections enable biologists to alter values for survivorship and fecundity at particular ages and assess the consequences.

2. Using the life table data in Figure 51.13a, calculations are shown to predict how many adults (of 1,000 introduced lizards) will survive and how many offspring from each age class will be produced. (Fig. 51.13b) 3. Figure 51.13c extends the calculations by predicting what will happen as the offspring of the original females begin to breed.

4. Students should be able to continue the analysis into the third year and predict whether the population of lizards will stay the same, decline, or increase over time.

5. Studies performed on life table data support some general conclusions. a. Whooping cranes, sea turtles, spotted owls, and many other endangered species have high juvenile mortality, low adult mortality, and low fecundity. (Fig. 51.14) b. Even though many invasive species have high reproductive capacity, some have adaptive capacity—sufficient genetic variation in life-history traits to evolve in new habitats. c. Climate change has enormous implications for the life-history traits of populations.

Example: Temperature determined sex in reptiles—significant implications for growth rates. d. Direst measurements of life-history traits of populations in the field are essential.

For example, climate has reduced the insulating snow layer in Colorado. Therefore, the fitness of alpine marmots has declined because they use more energy staying warm at the expense of fecundity.

6. In some cases, projections made from life tables may be too simplistic to be useful.

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B. Preserving metapopulations 1. Conservationists draw heavily on concepts and techniques from population ecology when designing programs to save species threatened with extinction.

2. A small, isolated population—even one within a nature reserve—is unlikely to survive over the long term. (Fig. 51.15) 3. Populations are most likely to persist when: a. They have larger population sizes. b. They occupy larger geographical areas. c. They are closer to neighboring populations. d. They have higher genetic diversity.

4. Results on the Glanville fritillaries have important messages for conservation biologists. a. Areas being protected need to be large enough in area to maintain large populations. b. If that is not possible, smaller tracts of land should be connected by corridors of habitat so that migration is possible. c. Land where populations have emigrated should be preserved so that immigration is possible.

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text. age class age pyramid age structure age-specific fecundity carrying capacity (K) cohort demography density-dependent density-dependent factors density-independent density-independent factors emigration

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evolutionary ecology exponential growth fecundity finite rate of increase (λ) fitness trade-offs generation immigration intrinsic rate of increase (rmax) life history life table logistic growth equation logistic population growth mark-recapture study

metapopulation net reproductive rate per-capita rate of increase (r) population population cycles population density population dynamics population viability analysis (PVA) quadrats replacement rate survivorship curve zero population growth (ZPG)

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Lecture Activities Population Exercises Activity Ideas for Any Class Size Estimated duration of activity: 5–10 minutes • The human population has grown at a nearly exponential rate during the past several centuries and, although it has shown recent signs of slowing, continues to increase at an impressive rate. Is the human population close to reaching Earth’s carrying capacity, or have we already reached it? What would happen if the human population surpassed Earth’s carrying capacity? Is it possible for any population to do this? How and when would you go about calculating the carrying capacity of the environment for any given species in its particular habitat? What is the carrying capacity of Earth most dependent on, and how might it be calculated?

• At high numbers, population growth becomes density dependent and is limited by factors that operate better under high-density situations such as disease, competition, and predation. But ecologists know that population growth at all levels is also limited by density-independent factors. What is meant by the term density-independent? How would density-independent factors affect a population and work to inhibit or even stop population growth? What are some examples of density-independent factors that operate in this capacity?

• It is the year 2525, and the human race can travel quickly and easily between the stars and planets. While out exploring the universe one day, you discover a new planet that is essentially identical to Earth, except it has no humans, cities, or constructs of any kind. Other- wise, the planet has all the same wildlife, plants, geographical features, and climate. A few years later your superiors tell you that they have found thousands of volunteers who are eager to inhabit this new planet. It is your job to organize the initial colonization of this new planet.

Where on this “earth” would you start your colony? Why have you chosen your particular location? Would you colonize in several different locations or just one? What factors enter into your decision? What kind of age structure and sex ratio would you want to start with? Why? What is likely to happen to the human population once it becomes established in your location?

What kind of growth and growth rate will the new population exhibit? After several years, how will its growth rate, survivorship, and age structure change?

• Try an exercise related to the survivorship/fecundity graph. First, choose some animals with high fecundity, such as an octopus. Lead the students to discover that this animal lays an incredibly large number of eggs per reproductive event. Then ask the students to guess the life span of this species. After a few of these examples, see whether students can see the relationship between massive reproductive output and short life expectancy. Now, if time permits, switch to high-survivorship, low-fecundity species—like an elephant. Continue the same exercise with these types of species, ending with humans. At this point, students should realize that humans have low fecundity but high survivorship. Does this change with different regions of the world, or is it all the same? How would this change if most women on Earth had 20 children over the course of their lifetime and this continued for 500 years? Can you find a correlation within humans in the literature that supports the survivorship/fecundity relationship, or doesn’t this occur within species?

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Community Ecology

Learning Objectives: Students should be able to…

• Use examples to explain commensalism, competition, consumption, and mutualism.

• Explain the importance of the niche concept to ecology.

• Explain whether community structure is predictable or unpredictable over time and why.

• Describe the factors that affect ecological succession following a disturbance.

• Discuss why species richness is higher in the tropics than in the poles and higher on large islands near the mainland than on small, isolated islands.

Lecture Outline • A biological community consists of interacting species, usually within a defined area.

I. Species Interactions A. Four types 1. Commensalism occurs when one species benefits and the other is unaffected (+/0).

2. Competition occurs when individuals use the same resources (–/–).

3. Consumption occurs when one organism eats another (+/–).

4. Mutualism occurs when two species interact in a way that contributes fitness benefits to both (+/+).

B. Three key themes 1. Species interactions can affect the distribution and abundance of a particular species.

2. Species act as agents of natural selection when they interact.

3. The outcome of interactions among species is dynamic and conditional.

C. Commensalism 1. Commensalism (+/0) is the least studied and very difficult to demonstrate because absence of fitness benefits is difficult to quantify. Commensalism is also very conditional.

2. An example is the relationship between epiphytic orchids and their host trees. They have no measureable fitness effect on their host trees, but gain from the association through greater access to light. (Fig. 52.1) D. Competition 1. Competition is a –/– interaction that occurs when individuals use the same resources and those resources are limited. a. Intraspecific competition is competition between members of the same species. .

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b. Interspecific competition occurs when individuals from different species use the same limiting resources.

2. Using the niche concept to analyze interspecific competition a. A niche can be thought of as the range of resources that the species is able to use or the range of conditions it can tolerate. b. G. Evelyn Hutchinson proposed that a species’ niche could be envisioned by plotting habitat requirements along a series of axes. (Fig. 52.2) (Making Models 52.1) c. Interspecific competition occurs when the niches of two species overlap.

3. What happens when one species is a better competitor? a. The principle of competitive exclusion states that it is not possible for species within the same niche to coexist or overlap. b. When two species of Paramecium are put in the same culture together, only the P. aurelia population exhibits a logistic growth pattern, and P. caudatum is driven to extinction. c. When asymmetric competition occurs, one species suffers a much greater fitness decline than the other species. (Fig. 52.3) d. Under symmetric competition, each of the interacting species experiences a roughly equal decrease in fitness. e. The fundamental niche is the combination of conditions that the species occupies in the absence of competitors, and its realized niche is that portion of the resources used when competition occurs. (Fig. 52.4) 4. Experimental studies of competition in nature a. In one of the first experimental examples of how competition can modify the distribution of a species, Joseph Connell noticed two species of barnacles on an intertidal rocky shore of Scotland. b. The poorer competitor (Chthalamus) is restricted to the upper intertidal zone, which is a more severe habitat. (Fig. 52.5) 5. Students should be able to sketch and label a graph of the fundamental and realized niche of the two barnacle species as a function of depth in the intertidal zone.

6. Fitness trade-offs in competition a. The ability to compete for a particular resource, like space on rocks or edible seeds of a certain size, is only one aspect of an organism’s niche. If individuals are extremely good at competing for a particular resource, then they are probably less good at enduring drought conditions, warding off disease, or preventing predation.

7. Mechanisms of coexistence: niche differentiation a. A change in resource use is called niche differentiation; a change in a species’ traits is called character displacement. b. An evolutionary change in traits reduces the amount of niche overlap and thus the amount of competition. (Fig. 52.6) c. Competition encourages natural selection, and the characteristics of a species change in a way that reduces competition.

(1) Character displacement was measured in Galápagos finches in response to severe drought. (Fig. 52.7)

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d. Students should be able to sketch the frequency of beak depths in G. fortis before (see top graph in Figure 52.7) and many years after the arrival of G. magnirostris on Daphne Major.

E. Consumption 1. Consumption is a +/− interaction that occurs when one organism eats another.

2. There are three major types of consumption: a. Herbivory takes place when herbivores consume plant tissues. b. Predation occurs when a predator kills and consumes most or all of another individual. c. Parasitism occurs when a parasite consumes small amounts of tissues from another organism, or host.

3. Constitutive defenses a. Some species run or hide in response to predators; others sequester or spray toxins or use weaponry (standing or constitutive defenses) for defense. (Table 52.1) b. Mimicry occurs when one species closely resembles another species. (Fig. 52.8) (1) In Batesian mimicry, a harmless species resembles a harmful one. (Fig. 52.9a) (2) In Müllerian mimicry, harmful species resemble each other. For example, brightly colored wasps resemble each other, so predators tend to avoid all wasps that have those colors. (Fig. 52.9b) c. Prey have adaptations that reduce their likelihood of becoming victims. These adaptations are responses to natural selection exerted by predators. Constitutive defenses are expensive, however, in terms of the energy resources that must be devoted to producing and maintaining them.

4. Inducible defenses a. Inducible defenses (produced in response to a predator) are efficient energetically, but they are slow: It takes time to produce them. b. Researchers hypothesized that if blue mussels have inducible defenses against crabs, then they should live in the low-flow area where predation pressure is higher.

They found that mussels in the high-predation area had thicker shells. (Fig. 52.10) c. Even without direct contact, mussels can sense the presence of crabs and increase their investment in defenses.

5. Can parasites manipulate their hosts? a. When a tropical ant species is infected with parasitic nematodes, its abdomen changes from black to a reddish color and the ant changes its behavior to hold its abdomen in the air. (Fig. 52.11) b. This causes infected ants to be more likely eaten by birds, which are needed to complete the nematode life cycle.

6. Using consumers as biocontrol agents a. The use of predators and parasites as biocontrol agents is a key part of integrated pest management. b. These strategies maximize crop and forest productivity and minimize pesticide use.

7. Consumption is an agent of natural selection a. When predators and prey or herbivores and plants interact over time, coevolutionary arms races result. Consumers evolve traits that increase their efficiency; in

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response, prey evolve traits that make them unpalatable or elusive, which leads to selection in consumers for traits that overcome the prey adaptation, and so on.

F. Mutualism 1. Mutualism is a +/+ interaction that involves a wide variety of organisms and rewards.

2. Examples a. Bats (and many insects) visit flowers to harvest nectar and pollen, and the flowers benefit by being fertilized. (Fig. 52.12a) b. There are mutualistic associations between mycorrhizal fungi and roots where fungi receive sugars in exchange for plants receiving nitrogen or phosphorus. c. Mutualism also occurs between nitrogen-fixing bacteria and the plant species that house them. d. Mutualism is common between bacteria and animals, particularly in the digestive systems of the hosts. For example, the bacteria in the guts of termites allows them to digest cellulose. e. Cleaner shrimp pick external parasites from the jaws and gills of fish. (Fig. 52.12b) 3. Biologists compared the growth and survival of treehoppers on plants with and without ants and found that ants protect the treehoppers. (Fig. 55.13) 4. Even though mutualism benefits both species, the interaction does not involve individuals from different species being altruistic.

5. The benefits received in mutualism are a by-product of each individual pursuing its own self-interest⎯by maximizing its ability to survive and reproduce.

6. Because the costs and benefits of species interactions are fluid, an interaction between the same two species may range from parasitism to mutualism to competition.

(Table 52.2)

II. Community Structure A. Key attributes 1. The total number of species, the sum of the interactions among all species, the relative abundance of those species, and the physical attributes of the community 2. One way to study complex communities is to look at the interactions between pairs of species. A food chain is sequentially linked whereas a food web is a summary of all the consumption interactions in a community. (Making Models 52.2) B. Why are some species more important than others in structuring communities?

1. Bottom-up influences on community structure a. When the amount of nutrients, sunlight, water, and other abiotic factors determine the abundance of primary producers b. For example, smooth cordgrass is the dominant marsh grass in all salt marshes on the Atlantic coast. Its removal is devastating to the entire wetland community.

2. Top-down influences on community structure a. The presence of certain consumers can significantly change the number of species present in a community. b. Experiments have shown that, in some cases, the structure of an entire community can change dramatically if a single species of predator or herbivore is removed from a community or added to it.

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c. A keystone species has a much greater impact on the surrounding species than its abundance would suggest. d. Wolves, sea stars, and sea otters are examples of species that vary in abundance over space and time, which significantly affects their communities. e. The reintroduction of wolves to Yellowstone caused a trophic cascade. (Fig. 52.14) (1) Wolves feed on elk. Elk numbers declined and aspen, cottonwood, and willow have increased.

(2) Changes in willow triggered an increase in beavers.

(3) Beaver damming increased habitat for frogs and turtles.

(4) Coyotes declined, mouse increased, which led to increase in hawks.

C. How predictable are communities?

1. The Clements-Gleason dichotomy a. Frederick Clements promoted the idea that communities are stable, integrated, and orderly entities. b. Biologically communities have a highly predictable composition. c. The diversity and abundance of a species in a particular community are the same both before and after a disturbance. d. Communities develop by passing through a series of predictable stages, culminating in a stable climax community. e. Henry Gleason contended that communities found in a particular area are neither stable nor predictable. f. Plant and animal communities are ephemeral associations of species that just happen to share similar climatic requirements. g. It is largely a matter of chance whether a similar community develops in the same area after a disturbance occurs.

2. An experimental test a. Researchers studied whether similar communities of animals develop in the plankton of similar ponds. (Fig. 52.15) b. The researchers constructed 12 identical ponds. Each pond was filled at the same time with water that contained enough chlorine to kill any preexisting organisms. c. Results (1) A total of 61 species were found in all of the ponds, but each individual pond contained only 31 to 39 species.

(2) Each pond in the study developed a different assemblage of species even though all were identical at the outset. d. Conclusions (1) Some species are good at dispersing and likely to colonize most of the habitat (ponds).

(2) The arrival of competitors and/or predators early in the colonization process greatly affects which species are able to successfully invade later.

(3) The specific details of community assembly and composition are contingent on many factors and are thus unpredictable.

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3. Mapping current and past species’ distributions a. If communities are predictable assemblages, then the ranges of species that make up a particular community should be congruent; that is, the same group of species should almost always be found growing together. b. Studies of fossils confirmed that the distribution of species and communities in North America has changed radically since the end of the last ice age. c. Groups of species have not changed their distributions in close associations.

(1) Species tended to change their ranges independently of one another.

(2) The composition of plant communities has been dynamic and unpredictable over time rather than static. d. All data support Gleason’s view of communities and contradict Clements’s view. e. Although both biotic interactions and climate are important in determining which species exist in a certain site, chance and history also play a large role.

III. Community Dynamics A. Disturbance and change in ecological communities 1. A disturbance is any event that removes some individuals or biomass from a community. a. A disturbance alters light levels, nutrients, unoccupied space, or some other aspect of resource availability. b. Fires, windstorms, floods, the fall of large-canopy trees, disease, and herbivore outbreaks are all disturbances.

2. The impact of a disturbance is a function of three factors: the type, frequency, and severity of the disturbance.

3. Each community type experiences a characteristic type, frequency, and severity of disturbance, known as the disturbance regime.

4. Why is it important to understand disturbance regimes? a. Researchers studied the fire history of giant sequoia groves in California.

(Fig. 52.16) (1) They obtained samples of cross sections through the bases of trees from different groves. (Fig. 52.17a) (2) They found that 20 to 40 fires had occurred each century during the past 1,500 to 2,000 years. Each tree had been burned an average of 64 times. (Fig. 52.17b) (3) Conclusion: Fires are extremely frequent and not very severe in this community. b. Biologists now set controlled fires or let low-intensity natural fires burn instead of suppressing them immediately. c. Disturbance regimes in the Colorado River and in southwestern North America have been studied.

(1) Land managers were inspired to release a huge pulse of water from reservoirs behind dams on the waterway.

(2) The flood that resulted was designed to mimic a natural disturbance event because it appears to have beneficially affected the plant and animal communities downstream.

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B. Succession: the development of communities after disturbance 1. The recovery and development of communities after a disturbance are known as succession. a. Primary succession occurs when a disturbance removes the soil and its organisms as well as the organisms that live above the surface. Examples: glaciers, floods, volcanoes, landslides b. Secondary succession occurs when a disturbance removes some or all of the organisms from an area but leaves the soil intact. Examples: fire, logging 2. A distinct sequence of communities develops as succession proceeds. (Fig. 52.18) a. Early successional communities are dominated by species that disperse their seeds long distances, are short-lived, and are small. b. Late successional communities are dominated by species that tend to be long-lived, large, and good competitors for resources such as light and nutrients. c. The specific sequence that appears over time in a specific area is called a successional pathway.

3. Biologists focus on three factors that predict the outcome of succession in a community: a. The particular traits of the species involved b. How species interact c. Historical and environmental circumstances, such as the size of the area involved and short-term weather conditions 4. Species traits are important early in succession. a. Recently disturbed sites are colonized by plants and animals with good dispersal ability. b. These pioneering species must be able to withstand harsh environmental conditions.

5. The role of species interactions a. Once colonization is under way, the course of succession depends less on how species cope with the abiotic environment and more on how they interact with other species. b. Facilitation takes place when an early-arriving species makes conditions more favorable for the arrival of certain later species. c. Tolerance means that existing species do not alter the establishment probability of subsequent species. d. Inhibition occurs when the presence of one species inhibits the establishment of another.

6. The role of chance and history a. The pattern and rate of succession depend on their historical and environmental context. For example, communities that develop after a fire depend on the size of the burned patch and how hot the fire was. b. Succession is also affected by the particular weather or climate conditions that occur during the process. Variation in weather and climate causes different successional pathways to occur in the same place at different times.

7. A case study: Glacier Bay, Alaska a. Extensive glacial regression has provided a great site for studying succession.

(Fig. 52.19) .

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b. Researchers found that at least two distinct successional pathways have occurred in Glacier Bay. (1) Species traits are important in explaining certain details about each pathway.

(2) Species interactions are important in each pathway. (a) One species may facilitate the growth of another. (b) Competition is another important interaction that is involved.

(3) The environmental context influenced the succession at all levels and helped determine exactly how the community developed.

8. Species traits and species interactions tend to make succession predictable, while history and chance events contribute a degree of unpredictability to succession.

IV. Patterns in Species Richness A. The diversity of species can be quantified in two ways: 1. Species richness is a simple count of how many species are present in a given community.

2. Species diversity is a weighted measure that incorporates a species’ relative abundance as well as its presence or absence. a. Species diversity involves more complex calculations than species richness.

(Quantitative Methods 52.1) b. The Shannon index of species diversity for the hypothetical communities in Figure 52.20 was calculated by: (1) Computing the proportion of individuals in each community that belong to each species (2) Taking the natural logarithm of these proportions (BioSkills 5) (3) Multiplying each natural logarithm by the proportion for each species (4) Summing the total across the six species in the community and multiplying by –1 3. Students should be able to do these calculations and get the same result given in Figure 52.20.

B. Predicting species richness: the theory of island biogeography 1. Robert MacArthur and Edward O. Wilson came up with a theory to explain why islands in the ocean have smaller numbers of species than areas of the same size on continents. a. There is equilibrium between the arrival of new species and the extinction of existing ones. (Fig. 52.21a) b. Species richness is higher on large islands than on small islands and higher on nearshore islands than on remote islands. (Fig. 52.21b, c) 2. This is an important model because: a. It is relevant to a variety of island-like habitats, such as alpine meadows, lakes and ponds, and caves. b. It is relevant to metapopulations. c. It makes specific predictions that can be tested, so species diversity can be quantified. d. It can help inform decisions about the design of natural preserves.

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C. Global patterns in species richness 1. Data compiled in the intervening years have confirmed the existence of a strong latitudinal gradient in species diversity—for communities as a whole as well as for many taxonomic groups. a. Species diversity declines as latitude increases. Examples: birds, mammals, fish, reptiles, aquatic and terrestrial invertebrates, orchids, trees (Fig. 52.22) b. This pattern is not universal, but it is widespread.

(1) A few species show a positive relationship between latitude and diversity.

(2) Others show no relationship.

2. Why does species diversity decline with latitude? a. The cause may be abiotic because Earth’s shape determines latitude. b. Species diversity is the sum of speciation, extinction, immigration, and emigration—and these may be affected by abiotic factors.

3. The high-productivity hypothesis states that high productivity in the tropics could promote high diversity. a. This hypothesis is supported by global patterns of productivity. b. It is not supported by the experimental addition of fertilizer to aquatic or terrestrial communities. c. It is not supported in some highly productive habitats such as estuaries.

4. The area and age hypothesis states that the tropical regions have had more time and more space for speciation to occur than other regions have.

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text. asymmetric competition Batesian mimicry character displacement climax community coevolutionary arms race commensalisms community competition competitive exclusion consumption defenses disturbance disturbance regime energy hypothesis facilitation

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food chain food web fundamental niche herbivores herbivory high-productivity host hypothesis inducible defenses inhibition intermediate disturbance interspecific competition intraspecific competition keystone species meta-analysis mimicry

Müllerian mimicry mutualism niche niche differentiation parasite parasitism pioneering species plant-defense hypothesis poor-nutrition hypothesis predation predator prey primary succession principle realized niche secondary succession

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Shannon index species diversity species richness standing/constitutive

successional pathway symmetric competition tolerance top-down control

trophic cascade weed

Lecture Activities Succession and Key Stone Species Activity Ideas for Any Class Size Estimated duration of activity: 5–10 minutes • Compare and contrast succession in aquatic environments with succession in terrestrial systems. What abiotic factors most critically drive the course of succession in each case?

What biotic factors are most important in each case? How does the succession of animal species compare to that of plant species? Which plays a more essential role in succession?

Why? • Primary succession occurs in regions that have had a previous community and all of its soil completely removed by a disturbance. It can also occur in areas that are recently formed, have no soil, and never contained a community at all. How is new soil formed in such regions? Besides the presence or absence of initial soil, what are the major differences between primary and secondary succession? Which one occurs more often? Why? Was this always true? Explain your answer.

• A sea otter is considered a keystone species. Help students identify what sea otters eat and how removing sea otters would affect this population of prey. You should finally reach the conclusion that allowing sea urchins to grow suppresses the growth of kelp forests.

• If it is not covered in regular lecture notes, discuss the past practice of preventing wildfires in sequoia groves. This practice allowed the understory to grow unchecked. Help the class reach the conclusion that the result was larger and more damaging fires when they did happen. Smaller fires left alone would burn and not damage sequoia groves. Discuss the current practice of controlled burning and how this might help the situation.

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Ecosystems and Global Ecology

Learning Objectives: Students should be able to…

• Discuss how energy flows in a food web from producers to consumers to decomposers.

• Describe the factors that limit the productivity in aquatic and terrestrial ecosystems.

• Describe the major reservoirs of the nutrient cycles and the processes that move elements from one reservoir to another.

• Explain how humans are altering the nutrient and water cycles.

• Explain how the burning of fossil fuels has led to rapid global warming.

• Describe several ways that climate change affects organisms, citing examples.

Lecture Outline • Ecosystems consist of all the species that live in an area together with all the components of their physical, or abiotic, environment.

I. How Does Energy Flow through Ecosystems?

A. Primary producers: the conduit where energy enters ecosystems 1. Primary producers (autotrophs) use solar or chemical energy to manufacture their own food.

2. The total amount of chemical energy produced in a given area and time period is called gross primary productivity (GPP).

3. The primary producers use this energy in two ways: a. Cellular Respiration—use chemical energy to create ATP to fuel a diverse array of metabolic processes b. Growth and Reproduction—chemical energy that isn’t used for cellular respiration can be used for growth and reproduction; the energy invested in new tissue is called net primary productivity (NPP) B. How efficient are autotrophs at capturing energy?

1. Gross primary productivity is the total amount of photosynthesis in a given area and time period.

2. Gross photosynthetic efficiency is the ratio of gross primary productivity to the total incoming solar radiation (in kcal/m2).

3. How does energy flow through the Hubbard Brook system? a. Researchers measured how energy flows through a forest ecosystem.

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b. They found that 0.8% of the light was captured by photosynthesis. Plants used 45% of this energy for growth and 55% for maintenance (lost as respiration/heat).

(Fig. 53.1) C. What happens to the biomass of autotrophs?

1. NPP represents the amount of energy available to the other living components of an ecosystem: consumers and decomposers.

2. Consumers include all organisms that eat other organisms. a. Herbivores eat plants. b. Carnivores eat animals.

3. Decomposers obtain energy by feeding on the dead remains of other organisms. a. They may also feed on the waste products of other organisms.

4. All ecosystems have four components linked by the flow of energy: primary producers, consumers, decomposers, and the abiotic environment. (Fig. 53.2) 5. Trophic structure: Food chains and food webs a. Organisms that obtain their energy from the same type of source occupy the same trophic level. b. A food chain describes the species that occupy each trophic level in a particular ecosystem. (Fig. 53.3) c. Food chains are usually embedded in more complex food webs because multiple species are present at several trophic levels. d. A grazing food chain is the collection of organisms that eat plants, along with the organisms that eat the herbivores. e. Food webs are a way of summarizing energy flows and documenting the complex trophic interactions that occur in communities. (Fig. 53.4) 6. Energy flow to grazers versus decomposers a. In forest ecosystems, the percentage of NPP consumed alive may be as low as 5 to 7%, whereas in marine habitats, it can be 35 to 40%. b. The dead-versus-alive percentage is highly variable across space and time.

D. Energy transfer between trophic levels 1. All ecosystems share a characteristic pattern: The total biomass produced each year declines from one trophic level up to the next.

2. In chipmunks at Hubbard Brook, 80.7% of energy intake goes into maintenance, 17.7% is excreted, and only 1.6% goes into the production of new tissue (secondary production).

3. The amount of biomass produced at the second trophic level must be less than the productivity at the first trophic level. This pattern holds true for the entire food chain and produces a pyramid of productivity.

4. The pyramid of productivity a. Although biomass production varies among communities, the data are representative and show that the biomass production at the upper trophic levels is often only 10% of the production at the next lower level. (Fig. 53.5) b. Students should be able to explain why it takes about 10 times more energy to grow a kilogram of beef than a kilogram of wheat.

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5. Efficiency varies a. Large mammals are more efficient than small mammals at producing biomass. b. Biomass production is more efficient in ectotherms than in endotherms. c. Students should be able to explain why it may be more efficient to eat lobster than chicken, even though the two animals feed at the same trophic level.

6. Biomagnification a. Persistent organic pollutants (POPs) tend to increase in concentration at higher levels in the food chain, as in DDT. (Fig. 53.6) b. Students should be able to explain how you could get mercury poisoning from eating tuna (top predators) every day but not from eating the same quantity of sardines (primary consumers) every day.

E. Global patterns in productivity 1. How does net primary productivity vary among ecosystems?

2. NPP data have been collected from around the globe. (Fig. 53.7) a. The terrestrial ecosystems with the highest productivity are located in the wet tropics. b. Marine productivity is highest along coastlines.

3. NPP data can also be organized by biome. (Fig. 53.8) a. Tropical wet forests and tropical dry forests cover less than 5% of Earth’s surface but together account for more than 30% of the NPP. b. In aquatic ecosystems, the most productive habitats are algal beds and coral reefs, wetlands, and estuaries.

4. Humans currently appropriate 24% of potential NPP. Of this amount: a. 53% is directly harvested and used. b. 40% is lost due to suburbanization. c. 7% is lost due to human-induced fires.

II. How Do Nutrients Cycle through Ecosystems?

A. The biogeochemical cycle 1. Energy is not the only quantity transferred when one organism eats another; nutrients also cycle through ecosystems.

2. The path an element takes as it moves from abiotic systems through living organisms and back again is called its biogeochemical cycle.

B. Nutrient cycling within ecosystems 1. The generalized terrestrial nutrient cycle (Fig. 53.9) a. Nutrients are taken up from the soil by plants and incorporated into plant tissue. b. If the plant is eaten, the nutrients pass to the animal members of the ecosystem. c. If the plant dies, the nutrients and plant biomass become litter, or dead biomass. d. Once ingested by an animal, nutrients are excreted in fecal matter or urine, taken up by a parasite or predator, or enter the dead biomass pool when the individual dies. e. Decomposers break down the nutrients in plant litter, animal excretions, and dead animal bodies. f. The nutrients then become part of the organic matter of soil. Soil organic matter is a complex mixture of partially decomposed detritus. .

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g. The nutrients in soil organic matter are converted into an inorganic form. h. The nutrients are once again available for uptake by plants.

2. What factors control the rate of nutrient cycling? a. Of the many links in a nutrient cycle, the decomposition of detritus usually limits the overall rate of nutrient movement the most. b. The decomposition rate is influenced by three types of factors: (1) Abiotic conditions (a) These include temperature and precipitation. (b) An example is the detrital accumulations in boreal versus tropical forests.

(Fig. 53.10) (2) The quality of detritus as a nutrient source for decomposers (a) Decomposers can be limited by the presence of large compounds that are difficult to digest. (b) The growth of decomposers is also inhibited if detritus is low in nitrogen.

(3) The abundance and diversity of detritivores present. c. Students should be able to explain why farms created by clearing tropical rain forests tend to perform poorly.

3. Sources of local nutrient export and import a. Nutrient availability has a profound effect on productivity. b. The rate of nutrient loss is an important characteristic of any ecosystem.

(1) CO2 is released in the atmosphere by cellular respiration.

(2) If an herbivore eats a plant and moves out of the ecosystem, the nutrients are exported.

(3) Nutrients leave when water or wind removes them. c. Many human activities accelerate nutrient loss. d. Nutrients can be gained or replaced by: (1) Ions released as rocks weather (2) Nutrients can blow in or arrive dissolved in streams (3) Carbon fixation via photosynthesis (4) Nitrogen-fixing adds nitrogen 4. An experimental study a. Researchers tested the effect of vegetation removal on nutrient cycling. (Fig. 53.11) (1) They compared nutrient loss in vegetated and devegetated sites over the course of four years.

(2) Before removing the vegetation, they documented that 90% of the nutrients in the ecosystems were in soil organic matter, with an additional 9.5% in plant biomass.

(3) After the vegetation was removed, they monitored the concentrations of nutrients in the streams exiting the two watersheds. b. Nutrient losses were typically 10 times higher in the devegetated site than in the control site. c. Conclusions (1) Devegetation has a huge impact on nutrient export.

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(2) Without plants to recycle them, nutrients are quickly washed out of the soil and lost to the ecosystem.

(3) Devegetated areas are much less productive than intact, vegetated sites.

C. Global biogeochemical cycles 1. When nutrients leave one ecosystem, they enter another.

2. The movement of nutrients among ecosystems links local biogeochemical cycles into one massive global system. Biogeochemical cycles can be very complex.

(Making Models 53.1) 3. To understand how biogeochemical cycling works, researchers ask four fundamental questions: a. What are the nature and size of the reservoirs—areas or “compartments” where elements are stored for a period of time? b. How fast do elements move between reservoirs, and what natural processes are responsible for moving elements from one reservoir to another? c. How are human activities affecting the movement of elements among reservoirs? d. How does one biogeochemical cycle interact with another biogeochemical cycle?

4. The global water cycle (Fig. 53.12) a. Water typically evaporates from the ocean and then precipitates back into the ocean.

Evaporation exceeds precipitation; that is, there is a net gain of water into the atmosphere. b. This vapor moves over land and precipitates. The cycle is completed by water that moves from land to the oceans via streams and groundwater. c. The water table is the upper limit, below the surface, at which the ground is saturated with stored water; it is dropping on every continent.

(1) The Ogallala-High Plains Aquifer is being depleted at a much higher rate than it is being recharged. (Fig. 53.13) d. About 40% of the world population has insufficient water resources. Desalination is increasing but requires an enormous amount of energy.

5. The global nitrogen cycle (Fig. 53.14) a. Plants are able to use nitrogen only in the form of ammonium or nitrate ions, so the vast pool of molecular nitrogen (N2) in the air is not available to plants. b. Nitrogen is added to ecosystems in a usable form only when it is “fixed.”

(1) This means it is converted from N2 into either ammonium or nitrate ions.

(2) Fixing can be achieved through lightening reactions in the atmosphere or by enzymes in bacteria that live in the soil and oceans. c. Humans have altered this cycle.

(1) Nitrogen fixation from human sources is now about equal to fixation from natural sources.

(2) Human-fixed nitrogen comes from the cultivation of crops (such as soybeans and peas), industrially produced fertilizers, and the release of nitric oxide during fossil fuel combustion. d. Adding nitrogen to terrestrial ecosystems usually increases productivity.

(1) This can be beneficial in some cases.

(2) It can also be detrimental. .

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(a) Nitrogen runoff can be harmful to aquatic systems. (Fig. 53.15) (b) Nitrogen inputs can lead to a significant loss of biodiversity. (c) Nitrogen from burning fossil fuels causes acid rain, climate change, and depletion of the ozone layer.

6. The global phosphorus cycle a. Tracks movement of phosphorous among terrestrial and aquatic ecosystems. b. Unlike nitrogen, the main reservoir of phosphorus is in rock in the Earth’s crust; this element does not occur in gas form in the atmosphere. c. It is normally mobilized by weathering of rocks and when organisms die, they and their phosphorus get carried by water to the oceans where it eventually settles and returns to rock. d. Humans are mining unprecedented amounts of phosphorus and introducing it into ecosystems as agricultural fertilizer. The runoff is causing eutrophication of aquatic ecosystems. e. Students should be able to make a simple “box and arrow” model of the main reservoirs and processes involved in the phosphorus cycle.

7. The global carbon cycle (Fig. 53.16) a. The global carbon cycle involves the rapid movement of carbon among terrestrial ecosystems, the oceans, and the atmosphere. b. The ocean is the largest pool or reservoir of carbon. c. Carbon moves into and out of the atmospheric pool at exceptionally high frequencies. d. Carbon frequently moves through atmospheric pools through organisms. e. In both terrestrial and aquatic systems, photosynthesis is responsible for taking carbon from the atmosphere and incorporating it into tissue. f. Cellular respiration, in contrast, releases carbon that has been incorporated into living organisms back into the atmosphere in the form of carbon dioxide. g. Humans have altered the carbon cycle through: (Fig. 53.17) (1) Burning fossil fuels (2) Land-use changes (e.g., deforestation) (3) The use of fire h. These changes in the global carbon cycle are important because carbon dioxide is a greenhouse gas; that is, it traps heat that has been radiated from Earth and keeps it from being lost in space. This process is warming Earth’s climate by increasing the atmosphere’s heat-trapping potential.

8. Some of our most pressing environmental problems result from recent and massive alterations in biogeochemical cycles. Even local changes can have large-scale consequences.

III. Global Climate Change A. Terminology 1. Global warming refers to the increase in the average temperature of the planet.

2. Global climate change refers to the sum of all the changes in local temperature and precipitation patterns that result from global warming.

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3. Weather refers to the short-term highly variable atmospheric conditions such as temperature, moisture, sunlight, and wind at a specific time and place.

4. Climate refers to long-term averages of regional or global weather.

B. What is the cause of global climate change?

1. CO2 is a greenhouse gas: It traps heat that has been radiated from Earth and keeps it from being lost into space. (Fig. 53.18) 2. Keeling and colleagues collected data on CO2, temperature, and solar radiation in Hawaii for more than 40 years. (Fig. 53.19) a. There is no indication that the CO2 increase is slowing. b. Temperature fluctuates, but there is a clear increase in average temperature. c. Solar energy cycles, but there is no net increase over time.

3. Total CO2 emissions are related to population size. (Fig. 53.20) C. How much will the climate change?

1. The latest IPCC report in 2014 concluded that the average global temperature increased 0.85°C from 1880 to 2012.

2. Current models suggest that the average global temperature will increase by another 1.5 to 2.0°C by the year 2100 and 0.5–7.0°C by the year 2300. The low number is based on models that assume no further increase in levels of greenhouse gases; the higher number is based on increased use of fossil fuels and increases in levels of greenhouse gases. (Fig. 53.21) 3. Average temperatures rising doesn’t mean that all areas will be equally affected. The polar regions will experience a larger average change than the tropics.

4. Climate models also predict that temperature extremes will increase.

5. The interaction of temperature and moisture will determine how much soil moisture is available in ecosystems. (Fig. 53.22) 6. Warmer temperatures have caused precipitation to become more variable and sea level to rise due to melting of glaciers and polar ice caps (which affect ocean currents and cloud cover).

7. Positive feedback occurs when changes caused by global warming result in the release of additional greenhouse gases. a. Warmer and drier climate conditions have increased the frequency of forest fires in the Rocky Mountains and boreal forests. Fires release CO2, which leads to further warming. b. In tundra, decomposition rates have increased to release more carbon stored in soil organic matter.

8. Negative feedback occurs when changes caused by global warming result in increased uptake and sequestration of CO2 and other greenhouse gases. a. The growth rates of several tree species have increased in response to increased CO2.

D. Biological effects of climate change 1. Even though global temperatures have risen only slightly in comparison with projections for the next 50 to 100 years, biologists have already documented dramatic impacts on organisms. (Table 53.1)

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2. The geographic ranges of many organisms are shifting. a. Mild temperatures have triggered massive expansions and infestations of pine beetles. b. Warming ocean temperatures have reduced the number of cold-water copepod species and their respective ranges.

3. The timing of seasonal events, or phenology, is changing in many biomes. a. Snowshoe hares rely on daylight length to change coat color and this has become mismatched with the environment—that is, they have a white coat when there is no snow. b. Peak caterpillar biomass is occurring earlier, but the reproductive timing of the great tit (the passerine bird that feeds on them) has not changed.

4. Climate change and species interactions are already causing allele frequencies to change. a. Increasing temperatures has caused evolution in the two-spot ladybird beetle. The black morphs have lower fitness under warmer conditions.

5. Biologists are concerned that rapid changes in climate coupled with other stressors will continue to cause high extinction rates. a. Twelve percent of local lizard populations have gone extinct in Mexico since 1975 because the temperature is changing to rapidly. b. Alpine tundra habitats are disappearing, along with the animals that inhabit them. c. Amphibians and coral reefs are on the decline.

6. The ocean absorbs one-third of the global CO2 emissions, but its ability to absorb CO2 is not limitless. a. Increasing CO2 levels will cause the oceans to increase in carbonic acid and therefore lower pH. 7. Students should be able to predict what happens to the pH of a glass of water when you blow bubbles into it with a straw.

8. Although biologists are discovering that ecosystems are resilient, they each have a tipping point, beyond which the changes are not recoverable.

E. Consequences to net primary productivity 1. The average annual NPP has increased globally in terrestrial environments between 1982 and 1999. However, in a data set from 2000 to 2009 (the warmest decade on record), global terrestrial NPP declined. This was likely due to widespread droughts.

(Fig. 53.23) 2. Although there is considerable variation of local productivity in the oceans, data show a convincing correlation between increased surface water temperatures and decreased productivity.

3. When water at the surface cannot cool below 48°C, it will not sink and mix with the more nutrient-rich benthic water. (Fig. 53.24) 4. What are the consequences of global reduction in NPP? a. If terrestrial and marine NPP continue to decline, the rate of CO2 absorption will decline and accelerate climate change. b. Changes in NPP will continue to alter food web dynamics and thus the ecosystem.

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Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text. autotroph

food webs

humus

biogeochemical cycles biomass carnivores climate change consumers decomposer food chain decomposers detritus ecosystem efficiency food chain

global carbon cycle global nitrogen cycle global warming global water cycle grazing food chain greenhouse gas gross photosynthetic efficiency gross primary productivity groundwater herbivores

net primary productivity (NPP) nitrogen fixation primary consumer primary decomposer primary producer pyramid of productivity reservoirs secondary consumer secondary production soil organic matter watersheds

Lecture Activities Nutrient Cycles Activity Ideas for All Class Sizes Estimated duration of activity: 5–10 minutes • This chapter discusses the global carbon and nitrogen cycles. Although these are two very important, major cycles, they are by no means the only global biogeochemical cycles. For what other elements do you think such global cycles exist? Why? Make a list of all the major elements that have global biogeochemical cycles, and rank them according to their importance to living organisms. What are the sources of input and output for the element in each cycle? That is, how does each element get into the biotic environment, and how is it released back into the abiotic environment? What are the major pools or reservoirs for the element in each cycle? Do some elements have only local biogeochemical cycles? If so, name some, and discuss why their cycles are not global.

• While walking in the woods, you notice a dead squirrel on the side of the trail. It has died only recently and has not yet begun the process of decomposition. What will become of the nitrogen in this animal? Trace its flow through the ecosystem, and explain what happens to it at each step. How and for what is nitrogen used in living organisms? How is nitrogen released back into the abiotic environment as N2? During decomposition, is some nitrogen released into the atmosphere and some into the soil, or does all of it go to one place?

Explain your answer.

• Secondary production is always lower than primary production, but it also varies based on the type of ecosystem involved. What types of ecosystems have the highest amounts of secondary production? Why? Is it possible to increase secondary production in an ecosystem? If so, how? If not, is it possible to decrease it? Why or why not?

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• For this activity, either use the class as a whole or split them up into small groups. Create a background scenario similar to Lord of the Flies. Stranded on an island like Fiji in size and isolation, how will the students go about producing a consistent source of food? Will it be better for them to be primary consumers or secondary consumers? You could also turn this activity into a spirited debate of vegetarianism versus omnivorism—be careful to keep this one under control.

• The movie An Unfortunate Truth is relevant to this chapter. Encourage your class to watch it on their own time. Afterward, have your students speculate and discuss why prominent figures in the media cast doubt on the idea of global warming caused by greenhouse gases.

This could also turn into a class debate, since there are plenty of media articles by cynics of the global-warming phenomenon.

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CHAPTER

54

Biodiversity and Conservation Biology

Learning Objectives: Students should be able to…

• Discuss the different levels at which biodiversity is quantified.

• Identify the major threats to biodiversity and evaluate the evidence suggesting loss is occurring on a wide scale.

• Explain why the conservation of biodiversity and ecosystem function is important for • ecosystems and for humans.

Discuss strategies for preserving biodiversity and ecosystem function.

Lecture Outline • Orangutans have been one of the major casualties of the massive loss of forests in Borneo. (Fig. 54.1)

I. What Is Biodiversity?

A. Biodiversity defines all the distinctive populations and species living today—the phylogenetic tree of all organisms.

B. Biodiversity can be measured and analyzed at several levels.

1. Genetic diversity—the total genetic information contained within all individuals of a species a. Genetic diversity has advanced due to the ability to sequence entire genomes as well as environmental sequencing.

2. Species diversity⎯the variety of life-forms on Earth, which includes species richness and diversity (Making Models 54.1) a. The red panda and Indian river dolphin have few close relatives; therefore, they are species-poor lineages. (Fig. 54.2) 3. Ecosystem diversity⎯the variety of biotic communities in a region along with the abiotic components 4. Biodiversity changes through time due to mutations, speciations, and extinctions.

C. How many species are living today?

1. Given that only a fraction of the organisms alive have been discovered to date, how can biologists estimate the total number of species on Earth?

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2. Taxon-specific surveys a. Biologists conduct an intensive search for a specific taxon in a well-defined area. b. For example, Bouchet and colleagues conducted a massive survey of marine mollusks along the west coast of New Caledonia. It represented one of the most thorough sampling effort ever made on mollusks. They sampled 127,000 individuals comprising 2,738 species.

3. All-taxa surveys a. Biologists attempt to find and catalog all species present in an area. b. To obtain a more direct estimate of total species numbers, the Census of Marine Life is an effort to record the global diversity of life in the seas. c. More than 30 million records have been collected, including 6,000 new species.

(Fig. 54.3) D. Where is biodiversity highest?

1. Mapping species richness and endemism a. Biologists use the term hotspot to denote areas in the tropics that have exceptionally high species richness. b. In terms of bird species, the Andes Mountains, the Amazon River basin, portions of East Africa, and southwest China are important hotspots. (Fig. 54.4a) c. Researchers are also interested in finding out which regions have a high proportion of endemic species⎯that is, species found in one area and nowhere else.

(Fig. 54.4b) d. Tropical rain forests represent 7% of Earth’s land area but contain at least 50% of all species present.

2. Mapping biodiversity hotspots a. Mapping species richness hotspots and centers of endemism can inspire interesting questions—principally, why certain regions contain many species or a high proportion of endemic taxa. b. In addition, biologists are studying the geographic distribution of biodiversity as a way of focusing conservation efforts. c. In 1999, a team set out to identify regions of the world that meet two criteria: (1) They contain at least 1,500 endemic plant species.

(2) At least 70% of their traditional or primary vegetation has been lost. d. Although these areas cover only 2.3% of Earth’s land area, they contain more than 50% of all known plant species and 42% of all known vertebrate species.

(Fig. 54.5)

II. Threats to Biodiversity A. The high rate of extinction 1. Today species are vanishing faster than at virtually any other time in Earth’s history.

Modern rates of extinction are 100 to 1,000 times higher than the average rate recorded in the fossil record over the past 550 million years.

2. The IUCN has estimated that 13% of birds, 26% of mammals, and 41% of amphibians are threatened. (Fig. 54.6) 3. Endangered species are those that are almost certain to go extinct unless effective conservation programs are put in place.

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B. Multiple interacting threats 1. A recent analysis of 488 endangered species native to Canada reached several conclusions. (Fig. 54.7) a. Habitat loss is the single most important factor in the decline of these species. b. Virtually all the endangered species are affected by more than one factor. c. Overexploitation is the dominant problem for marine species, whereas pollution plays a larger role for freshwater species. d. Factors beyond human control can be important; background extinctions will continue to occur.

2. Habitat destruction a. Humans cause habitat destruction by logging and burning forests, overhunting, damming rivers, dredging and trawling the oceans, plowing prairies, grazing livestock, filling in wetlands, and excavating and extracting minerals to build housing developments, golf courses, shopping centers, office complexes, airports, and roads.

(Fig. 54.8) 3. Habitat degradation a. Humans can degrade habitats in many ways. b. Habitat fragmentation turns large, contiguous areas of natural habitats into small, isolated fragments. c. Habitat fragmentation is a big concern to biologists.

(1) Habitats can be too small to support some species (more vulnerable to catastrophic events as well as inbreeding depression).

(2) Fragmentation reduces the ability of some animals to disperse.

(3) It creates a large amount of vulnerable edge habitat. d. Experiments show that biomass declines sharply along edges of fragments.

(Fig. 54.9) 4. Overexploitation a. Overexploitation refers to any unsustainable removal of wildlife from the natural environment for use by humans. (Table 54.1) b. Overexploitation is the dominant threat for marine species. (Making Models 54.2) 5. Invasive species a. A nonnative species introduced into a new area is called an exotic species. When that population grows and disrupts native species, it is an invasive species. b. Burmese pythons were set free by pet owners in Florida and are now dramatically affecting wildlife there.

6. Pollution a. Humans change both the biotic and abiotic environment by releasing chemicals into the ecosystem. b. Pollutants can include industrial waste (sulfur, arsenic, mercury, CO2), pharmaceuticals, pesticides, herbicides, fertilizers, and solid waste.

7. Climate change a. Polar and alpine habitats are disappearing. b. Coastal habitats are threatened by sea level rise. c. Biomes are becoming redistributed. .

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d. Species distributions of communities are changing. e. Coral reefs are suffering from bleaching.

C. How will these threats affect future extinction rates?

1. The formulation of biodiversity projections is a rapidly growing area of research and an important player in decisions about conservation.

2. Estimating the probability that an individual species will go extinct a. Researchers use an approach called Population Viability Analysis (PVA) to estimate the probability that a particular species will go extinct. b. A population is considered viable is the analysis predicts that it has a 95% probability of surviving at least 100 years.

3. Estimating the effect of habitat area on species richness a. Species–Area relationships compare the rates of habitat destruction to the projected rates of species loss. (Quantitative Methods 54.1) b. Researchers used species–area curves to analyze the number of bird species found on islands in the Bismarck Archipelago near New Guinea. (Fig. 54.10) (BioSkills 5) 4. Students should be able to predict the species richness of an island in the Bismark Archipelago that is 5000 km2.

5. Estimating the effect of global climate change on species distributions a. Sinervo and colleagues collected extinction rates of 48 species of lizards at 200 sites in Mexico since 1975. (Fig. 54.11) b. 12% of local populations had already gone extinct because the temperature is changing too rapidly for populations to adapt. c. Extrapolating this data worldwide, they predict lizard species extinctions could be as high as 20% by 2080.

III. Why Is Biodiversity Important?

A. Biological benefits of biodiversity 1. Biodiversity increases productivity. a. David Tilman and colleagues classified 32 grassland plant species into five categories. They compared the productivity of plots with different mixes of species numbers and functional groups and found that both the number and type of species present had important effects. (Figs. 54.12) b. Follow-up experiments in an array of ecosystems not only supported the conclusion that species richness has a positive impact on NPP but also showed that other mechanisms may be at work.

(1) Resource use efficiency: Diverse assemblages of plant species make more efficient use of sunlight.

(2) Facilitation: Certain species facilitate the growth of other species by providing them with nutrients, partial shade, or other benefits.

(3) Chance inclusion of high-productivity species: In many habitats, one or two species are extremely productive. Therefore, if the number of species in a study is low, the likelihood of missing a big producer is high.

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2. Does biodiversity lead to stability? a. The stability of a community is its ability to: (1) Recover to former levels of productivity or species richness after a disturbance.

(2) Maintain productivity and other aspects of ecosystem function as conditions change over time. b. Resistance is a measure of how much a community is affected by a disturbance, and resilience is a measure of how quickly a community recovers following a disturbance. (Fig. 54.13) c. In North American grasslands, communities that are more diverse appear to be more productive, more resistant to disturbance and invasion, and more resilient than communities that are less diverse. d. Increased biodiversity increases the services provided by ecosystems. By implication, biologists can infer that if ecosystems are simplified by extinctions, then productivity and other attributes might decrease.

B. Ecosystem services: Economic and social benefits of biodiversity and ecosystems 1. All of the direct and indirect benefits that humans derive from organisms and the ecosystems they compose are called ecosystem services. a. Ecologists and economists estimate the value of ecosystem services at $125 trillion per year. (Table 54.2) 2. Provisioning services a. Humans rely on other organisms for most of their food and material needs. b. Research programs collectively known as bioprospecting focus on assessing bacteria, archaea, plants, and fungi as novel sources of drugs or ingredients in consumer products.

3. Regulating services a. Services provided by functioning ecosystems such as water purification, flood control, and waste decomposition.

4. Cultural services a. People integrate nature into their lives in diverse cultural ways, ranging from artistic celebrations to recreation, education, and spiritual contemplation.

5. Supporting services a. Some ecosystem services support all the other categories of services. Primary productivity is a key example. b. The production of almonds, apples, cherries, chocolate, alfalfa, and an array of other crops depend on the presence of wild pollinators. Some areas now need human pollinators. (Fig. 54.14) C. An ethical dimension?

1. Humans have an ethical obligation to preserve species and ecosystems. a. Organisms have intrinsic worth. b. Industrialized nations are responsible for most of the environmental harm, yet poor countries are disproportionately affected by the consequences of these actions. c. It is unethical for the current generation to deprive future generations of ecosystem services.

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IV. Preserving Biodiversity and Ecosystem Function A. Addressing the ultimate cause of loss 1. Extinction is irreversible and biodiversity loss is not slowing.

2. It will be impossible to preserve biodiversity if two things happen: a. The human population grows to 10 billion or more. b. The rate of consumption of fossil fuels and other resources does not decline.

3. The challenge is to create ways for humans to live off the resources being produced continuously rather than using species to extinction.

4. Small changes, like eating less beef, can make a big difference. (Fig. 54.15) B. Conservation strategies to preserve genetic diversity, species, and ecosystem function (Table 54.3) 1. Education campaigns a. Large, charismatic species such as pandas, tigers, and elephants tend to mobilize more effort than unfamiliar species such as copepods. By preserving familiar species, conservationists can succeed in preserving the ecosystems where they live.

2. Genetic restoration a. Small, isolated populations are vulnerable to loss of genetic diversity from chance events. b. Artificial gene flow can help restore genetic diversity, such as in the case of Florida panthers.

3. Seed banks a. Scientists are preserving diverse strains of crop plants in seed banks as well as using breeding programs to improve crop traits.

4. Management plans for invasive species a. Global awareness of the problem has led to limit the exchange of species. Prevention is more effective than treatment.

5. Management plans for endangered species a. The Endangered Species Act of 1973 was designed to prevent species from becoming extinct. b. One well-known success story is the bald eagle—it was listed in 1967 and delisted in 2007 6. Ex situ conservation and reintroduction a. Ex situ conservation is the preservation of species in zoos, aquaria, wildlife ranches, seed banks, or other artificial settings. b. The California condor, whooping cranes, and the Arabian oryx have been rescued from extinction through the use of these settings.

7. Wildlife corridors a. Strips of undeveloped habitat, called wildlife corridors, can connect populations that would otherwise be isolated. b. Research has shown that connected patches of grassland have been steadily gaining species over time, as compared to unconnected patches.

8. Designing effective protected areas a. The most effective strategy is to protect entire areas of land and sea from exploitation, however, this is not usually possible.

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b. Researchers are using a gap analysis program (GAP) that tries to identify gaps between geographic areas that are particularly rich in biodiversity and areas that are actually managed for conservation.

9. Ecosystem restoration a. How successful are ecosystem restoration projects in general? b. Smits restored a 2,000-hectare clear-cut rain forest in Samboja, and while it is not as valuable as primary rain forest, great successes were made. c. In general, restoration success is positively correlated with the size of the area restored.

10. Quantifying ecosystem services a. Quantifying ecosystem services might be a great approach to conservation efforts; however, it is difficult to do. b. Ecosystem restoration efforts in Costa Rica have produced a popular ecotourist destination.

Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first mention in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text. biomass deforestation ecosystem diversity ecosystem services endangered species endemic species environmental sequencing

exotic species genetic diversity habitat destruction habitat fragmentation hot spot net primary productivity (NPP) resilience

resistance species diversity species–area relationships sustainability wildlife corridors

Lecture Activities Biodiversity and Preservation of Species Activity Ideas for All Class Sizes Estimated duration of activity: 5–10 minutes • Suppose you are given the task of increasing the biodiversity in a relatively old forested region currently used as a nature center. How would you accomplish this task? What are some techniques you could use to increase biodiversity in this forest? Design a conservation plan to accomplish this goal. What supplementary factors might influence the plan you design? How would you deal with the ethical, aesthetic, and public policy issues likely to arise when you try to implement your plan? Explain your answer.

• Is it ever beneficial to decrease the biodiversity in a particular region? Why or why not? Is it possible to decrease only a small percentage of the biodiversity without disrupting the entire ecosystem? If so, under what circumstances and in what types of habitat? .

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• Human activities are causing a rapid loss of biodiversity around the world. How are natural selection and evolution in general affected by this loss of biodiversity? In the organisms most affected by the habitat destruction and fragmentation accompanying this loss, what traits do you suppose natural selection might be favoring? What traits are being selected against? Why? Explain your answer.

• Choose a megareserve in Africa, and present a picture of it to the class. Show the class what species the reserve is attempting to preserve. Ask students to evaluate the reserve. Is the size appropriate? Can corridors be introduced into other reserves? If time permits, you could address political and practical issues such as the size of the country and the people power needed to guard against possible poachers.

• Inbreeding depression is a possible outcome of a small, isolated population. After reading this chapter, the class will probably agree that inbreeding is bad. Have them list reasons to avoid inbreeding depression. Some argue that inbreeding is actually good for increasing the numbers of an endangered species in captivity. See whether the class has any ideas about how this can be.

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