Chapter 1 Studying Life
Cellular structure evolved in the common ancestor of life
An important step in the evolution of life was the enclosure of complex proteins and other biological molecules by membranes that contained them in a compact internal environment separate from the surrounding (external) environment. Fatty acid molecules played a critical role in membrane evolution because these molecules do not dissolve in water. Think of shaking an oil and vinegar salad dressing. The oil breaks up into small droplets, but the droplets do not dissolve in the vinegar and they rapidly coalesce. Similarly, fatty acids can form
membranous films on the surface of water. When these films are agitated, they can form spherical structures called liposomes. Such structures are now used for delivering drugs to cells (Figure 1.3A). In a primordial ocean, such membranous structures could have enveloped assemblages of complex biological molecules. The origin of an internal environment that concentrated reactants and products of chemical reactions led to the first cells with the ability to replicate themselves—the evolution of the first cellular organisms. For billions of years all the organisms existing on Earth were unicellular and were enclosed by a single outer membrane. Such organisms, like the bacteria that are abundant on you, in you, and all around you, are called prokaryotes (Figure 1.3B). Two main groups diverged early in life’s history: Bacteria and Archaea. The third major category of life on Earth, Eukarya, arose billions of years later, with contributions from both bacteria and archaea. (Note that we use initial capitals when referring to these three main groups, but lowercase when referring to members of the groups.) In addition to its outer membrane, a eukaryotic cell (Figure 1.3C) has internal membranes that enclose specialized internal
compartments called organelles. The organelle that gives eukaryotes their name is the nucleus the organelle that contains the cell’s genetic information. The word “eukaryote” comes from two Greek words meaning “a true kernel.” Other organelles carry out specific functions such as synthesizing biological molecules or providing energy. How might eukaryotes have arisen from prokaryotes? Infoldings of the prokaryotic cell membrane could have formed internal compartments—the organelles—that isolated cell functions from each other, resulting in greater integration or efficiency of cell functions. Another possibility, similar to that seen in the relationship between corals and dinoflagellates described at the beginning of this chapter, is that close, interdependent relationships may have developed between different prokaryotic cells, leading to a merger of sorts. Suppose a prokaryote good at converting energy was engulfed (but not digested) by a prokaryote good at synthesizing biological molecules. Each would supply a valuable service for the other, but now one would be an organelle inside the other. The structure of prokaryotic and eukaryotic cells, their membranes, and their evolution are the subjects of Part Two.
Photosynthesis allows some organisms to capture energy from the Sun Living cells require energy to function. The earliest prokaryotes supplied their energy needs—their metabolism—by taking in small molecules from their environment, breaking the chemical Life 12e Oxford University Press Dragony Media Group Life12e_01.03.ai Date 10-29-19 (B)
Prokaryotic cells (C) Eukaryotic cell Membrane of nucleus Cell membrane Cell membrane
Mitochondria (membrane-enclosed) (A) Liposomes
200 nm 1 µm 1 µm Figure 1.3 Membranes Enclose the Building Blocks of Life These photographs were taken with electron microscopes (see Figure 5.4) and enhanced with added color to highlight details. (A) Liposomes are tiny round bubbles known as vesicles and are made out of the same material as a cell membrane. Liposomes can be filled with drugs and used to deliver drugs to cells. (B) Two prokaryotic cells of an Enterococcus bacterium that lives in the human digestive system. Prokaryotes are unicellular organisms with genetic and biochemical material enclosed inside a single membrane. (C) A human white blood cell (lymphocyte) represents one of the many specialized cell types that make up a multicellular eukaryote. Multiple membranes within
the cell-enclosing outer membrane segregate the different biochemical processes of eukaryotic cells.
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Chapter 1 Studying Life 01_Life12e_Ch 01.indd 4
12/3/19 11:20 AM bonds of these molecules, and using the energy released from those chemical bonds to do cellular work. Many modern prokaryotes still function this way, and they function very successfully. But about 2.5 billion years ago, the emergence of photosynthesis changed the nature of life on Earth and also changed our planet. Photosynthesis transforms the energy of sunlight into a form of chemical energy that can be used to do work such as the synthesis of large molecules. These large molecules can then be used to build cell structures or can be broken down to provide metabolic energy. Photosynthesis is the basis of most of life on Earth today because its energycapturing processes provide food for other organisms. Early photosynthetic cells were probably similar to present-day prokaryotes called cyanobacteria (Figure 1.4). Over time, photosynthetic prokaryotes became so abundant that vast quantities of oxygen gas (O2 ), a by-
product of photosynthesis, began to accumulate in the atmosphere. Connect the Concepts The pathways that harvest chemical energy to do all the kinds of biological work necessary to support metabolism are presented in Chapter 9. During the early history of prokaryotic life, there was very little O2 in Earth’s atmosphere. In fact, O2 was toxic to many of the prokaryotes living at that time, and its buildup in the atmosphere resulted in a huge mass extinction. But those organisms that could tolerate O2 proliferated. Atmospheric O2 opened up vast new avenues of evolution because aerobic metabolism—a biochemical process that uses O2 to extract energy from nutrient molecules—is far more efficient than anaerobic metabolism (which does not use O2 ). Most organisms today use aerobic metabolism. Oxygen in the atmosphere also made it possible for life to move onto land. For most of life’s history, ultraviolet (UV) radiation falling on Earth’s surface was so intense that it destroyed any organism that was not shielded by water. The atmospheric accumulation of photosynthetically generated O2 over a period of more than 2 billion years gradually led to the production of a layer of ozone (O3 ) in the upper atmosphere. By about 500
million years ago, the ozone layer was sufficiently dense and absorbed enough of the Sun’s UV radiation to make it possible for organisms to leave the protection of the water and live on land. Cellular differentiation and specialization underlie multicellular life Single-celled organisms were the only forms of life for more than half of the history of life on Earth (see Figure 1.2). However, at some point the cells of some eukaryotes didn’t separate after cell division and instead remained attached to each other. Such colonial aggregations of cells made it possible for some of the associated cells to specialize in certain functions, such as reproduction, while other cells specialized in other functions, such as nutrient absorption or motility. This cellular specialization enabled multicellular eukaryotes to increase in size and become more efficient at gathering resources and adapting to specific environments. Single-celled organisms must provide for all of their own needs, but the cells of multicellular organisms can evolve specializations to carry out certain functions exclusively and efficiently because they can depend on other cells to carry out other functions. Thus the cells of a multicellular organism can have different developmental fates.
Similar cell types can develop together into tissues that accomplish tasks that a single cell cannot. For example, muscle cells develop cellular mechanisms for generating force. A single muscle cell cannot generate much force, but many cells of a muscle
Life 12e Oxford University Press Dragony Media
Group Life12e_01.04.ai Date 10-08-19 Stromatolites form as small grains of sediment are cemented together by communities of microorganisms, especially cyanobacteria. (A) Cyanobacteria (B)
Fossilized stromatolite (C) Living stromatolite 5 µm
Figure 1.4 Photosynthetic Organisms Changed Earth’s Atmosphere (A) Cyanobacteria are aquatic and photosynthetic: they live in the water and can manufacture their own food. Although they are quite small, they often grow in colonies large enough to see. (B) Colonies of photosynthetic cyanobacteria and other microorganisms produced structures called stromatolites that were preserved in the ancient fossil record. (C) Living stromatolites can still be found in appropriate environments. © Dr. Robert Calentine/Visuals Unlimited, Inc. © Armands
Pharyos/Alamy Stock Photo © Krystyna
Szulecka/Alamy Stock Photo KEY CONCEPT 1.1
Living Organisms Have a Common Origin and Share
Similarities 01_Life12e_Ch 01.indd 5 12/3/19 11:20
AM tissue can work together to generate considerable force, and working with structural tissues such as bones, they can produce large movements. Different tissue types develop together into organs that accomplish specific functions. The heart, brain, and stomach are each composed of several types of tissues, as are the roots, stems, and leaves of plants. Organs whose functions are interrelated can be grouped into organ systems; the esophagus, stomach, and intestines, for example, are all part of the digestive system. The hierarchy of biological organization from atoms to organisms is shown in Figure 1.5A. The biology of the two major groups of multicellular organisms, plants and animals, is discussed in detail in Parts Seven and Eight, respectively. Organisms extract energy and raw materials from the environment Living organisms acquire nutrients from the environment. Biochemical reactions break down complex nutrient molecules into smaller chemical units. Some of those smaller units are used as building blocks for structures the organism requires. The breakdown of nutrient molecules does more than supply raw materials. The breaking of the chemical bonds of
nutrient molecules releases energy that the cell transfers to high-energy molecules that it uses to do work. One kind of work cells do is mechanical— moving molecules from one cellular location to another, moving whole cells or tissues, or even moving the organism itself (Figure 1.6A). The cell also does biochemical work in the building, or synthesis, of new complex molecules and structures from smaller chemical units. For example, we are all familiar with the fact that carbohydrates eaten today may be deposited in the body as fat tomorrow (Figure 1.6B). Still another kind of work is the electrical work that is the essence of information processing in nervous systems. © Rolf Nussbaumer
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Life12e_01.05.ai Date 09-20-19 Large molecules
Cells Organism Multicellular organism (leopard frog, Rana pipiens) Water Atoms Small molecules
Methane Proteins Nucleic acids Cell specialization
Tissues Organs Organ systems Colonial organisms
Oxygen Unicellular organisms Carbon Hydrogen
Carbon dioxide (A) Atoms to organisms Population
Community Ecosystem Biosphere (B) Organisms to
ecosystems Figure 1.5 Biology Is Studied at Many Levels of Organization (A) Life’s properties emerge when DNA and other molecules are organized in cells, which form building blocks for organisms. (B) Organisms exist in populations and interact with other populations to form communities, which interact with the physical environment to make up the many ecosystems of the biosphere. View in
Achieve Activity 1.1 The Hierarchy of Biological
Organization Life 12e Oxford University Press
Dragony Media Group Life12e_01.06.ai Date 09-2019 (B) Urocitellus parryii (A) Coracias garrulus
Figure 1.6 Energy Can Be Used Immediately or Stored (A) Animal cells break down food molecules and use the energy contained in the chemical bonds of those molecules to do mechanical work, such as running, flying, and jumping. This image is a European Roller flying with mammal prey in its bill. (B) The cells of this Arctic ground squirrel have broken down the complex carbohydrates in the plants it consumed and converted those molecules into fats. The fats are stored in the animal’s body to provide an energy supply for the cold months.
Chapter 1 Studying Life 01_Life12e_Ch 01.indd 6
12/3/19 11:20 AM The many biochemical reactions
that take place in cells are integrally linked in that the products of one reaction are the raw materials of the next. These complex networks of reactions must be integrated and precisely controlled; when they are not, the result is malfunction and disease. Media Clip 1.1 Leaping Lemurs Life12e.com/mc1.1 Living organisms must regulate their internal environment If different specialized cells, tissues, and organs provide for different needs of the multicellular organism, how are these specific services shared by the cells of the whole organism? All of the cells of the body share an internal environment that is made up of extracellular fluids. Cells derive their nutrients from these extracellular fluids, and they deposit their wastes into the extracellular fluid. This internal environment serves the needs of all cells of the body, and therefore its physical and chemical composition must be maintained within a narrow range of physiological conditions that support survival and function. The maintenance of this narrow range of conditions is known as homeostasis. A relatively stable internal environment means that cells can function efficiently even when external conditions could not support the lives of individual cells. Homeostasis requires that the
activities of the cells and systems of the body be regulated. Regulation requires information information about internal conditions, external conditions, and what is optimal. Thus organisms must have sensory mechanisms to monitor conditions, effector mechanisms to alter those conditions, and signaling mechanisms to integrate information and enable communication between sensors and effectors. The major information systems of animals—nervous, hormonal, and immune—use chemical and electric signals to process information. Connect the Concepts The regulation of body temperature is an important example of homeostasis that is discussed in Chapter 38. The concept of homeostasis also applies to the intracellular environment. Both unicellular and multicellular organisms must regulate the composition of their intracellular environments within a range that allows those cells to survive and function. Individual cells regulate these properties through actions of their membranes and, in the case of eukaryotes, their organelles. Thus self-regulation to maintain a more or less constant internal environment is a general attribute of all life. Living organisms interact Organisms do not live in isolation.
Besides the internal hierarchy of the individual organism, there is also an external hierarchy of the biological world (Figure 1.5B). The populations of all the species that live and interact in a defined area are called a community. Communities together with their abiotic, or physical, environment constitute an ecosystem. Individuals in a population interact in many different ways. Animals eat plants and other animals and compete with other species for food and other resources. Some animals prevent other individuals of their own species from exploiting a resource, be it food, nesting sites, or mates. Animals may also cooperate with members of their own species, forming social units such as a termite colony or a flock of birds. Such interactions have resulted in the evolution of social behaviors such as communication and courtship displays. Plants also interact with their external biotic and abiotic environment. Plants living on land depend on partnerships with fungi, bacteria, and animals. Some of these partnerships are necessary to obtain nutrients, some to produce fertile seeds, and still others to disperse seeds. Plants compete with each other for light and water and have ongoing evolutionary interactions with the animals that eat
them. Through time, many adaptations such as thorns and toxins have evolved that protect plants from predation. Other adaptations such as flowers and fruits help attract the animals that assist in plant reproduction. The interactions of populations of plant and animal species in a community are major evolutionary forces that produce specialized adaptations. Life 12e Oxford University Press
Dragony Media Group Life12e_01.05.ai Date 09-20-
19 Large molecules Cells Organism Multicellular organism (leopard frog, Rana pipiens) Water Atoms
Small molecules Methane Proteins Nucleic acids
Cell specialization Tissues Organs Organ systems
Colonial organisms Oxygen Unicellular organisms
Carbon Hydrogen Carbon dioxide (A) Atoms to organisms Population Community Ecosystem
Biosphere (B) Organisms to ecosystems NASA image by Reto Stöckli, based on data from NASA and NOAA KEY CONCEPT 1.1 Living Organisms Have a Common Origin and Share Similarities
01_Life12e_Ch 01.indd 7 12/3/19 11:20 AM The major ecosystems of Earth that cover broad geographic areas with distinguishing physical features and communities of organisms are known as biomes. Examples of biomes include Arctic
tundra, coral reefs, and tropical rainforests. All of the biomes on our planet make up the biosphere. The ways in which species interact with one another and with their environment in populations, communities, and ecosystems is the subject of ecology, covered in Part Nine of this book. KEY CONCEPT 1.1 Recap and Assess All organisms are related by common descent from a single chemical origin of life. That origin involved the enclosure of complex biological molecules within an internal environment and led to the first cells with the ability to replicate themselves. Cells need energy and can extract that energy from the environment and use it to do biological work, including synthesizing complex molecules and structures. A major step in energy extraction was the evolution of photosynthesis, which changed our planet and made life on land possible. The rise of multicellularity enabled cellular specialization and the development of tissues, organs, and organ systems. Cellular specialization made it possible and necessary to have a regulated internal environment that serves the needs of all the cells of the body.
Evolution has produced an enormous diversity of organisms that interact with each other, forming the many communities and ecosystems that constitute
life on Earth. 1. Describe two ways in which the origin of photosynthesis influenced the history of life on Earth. 2. If we discovered life on another planet, how could we tell if it had a separate origin from life on Earth? 3. Why was maintenance of a regulated internal environment but not extracellular environment necessary for the evolution of large multicellular animals? 4. Explain the concept of homeostatic regulation. This key concept outlined some of the major features of life— features that we will cover in depth in subsequent chapters. But first, to better understand how the enormous diversity of life evolved, we look at two of the central principles of biology: genetics and evolution. KEY CONCEPT
Genetics and Evolution Are Fundamental 1.2
Principles of Biology Learning Objectives 1.2.1
Describe the relationships among genes, genomes, and genetics. 1.2.2 Explain how natural selection can produce evolutionary change. 1.2.3 Describe what information can be represented in a phylogenetic tree. Before there was a science of biology, humans recognized that offspring resemble parents, and this fact was used by plant and animal breeders to produce variants with desirable qualities. However, not until the famous plant breeding
experiments of the Austrian monk Gregor Mendel in the mid-1800s was it demonstrated that inherited traits existed in discrete units (Figure 1.7). These discrete units of inheritance were termed genes in the early 1900s, giving rise to the science of genetics. The science of genetics preceded discovery of the chemical nature of genes An enormous body of information about traits of organisms represented by single genes was created before the chemical structure of a gene was known, and hence before the nature of the information that is passed from parent to offspring was known. That mystery was solved in the mid-1900s with the discovery that the molecule deoxyribonucleic acid (DNA) is the genetic information that specifies what an organism will look like and how it will function. This “blueprint” for the existence of each individual organism is contained in the sum total of all the DNA molecules contained in each of the organism’s cells—its genome. DNA molecules are long sequences of four different subunits called nucleotides. Genes are specific segments of DNA that encode information about structure and function, including the information the cell uses Life 12e Oxford University Press Dragony Media Group
Life12e_01.07.ai Date 09-20-19 Short pea plant
Parental cross First generation: All tall pea plants
Second generation: 3 tall, 1 short Tall pea plant × Figure 1.7 Genetics Developed as the Study of Discrete Heritable Traits Gregor Mendel discovered the basics of genetics through breeding experiments with pea plants. When he crossed tall plants with short plants, all of the offspring were tall. Then when he crossed these offspring plants, he got one short plant for every three tall plants. From these experiments Mendel concluded that there was a tall factor and a short factor. Each plant inherited one factor from each parent, and the tall factor was dominant. Chapter 1 Studying Life 01_Life12e_Ch
01.indd 8 12/3/19 11:20 AM to build proteins. Therefore each gene is defined by a specific sequence of the four nucleotides. The genetic code spells out how sequences of nucleotides are translated into sequences of amino acids, which are the building blocks of proteins. This translation process involves first transcribing some of the DNA information of a gene into the structure of another smaller molecule called ribonucleic acid (RNA) (Figure 1.8). RNA serves as the template for synthesis of a protein. Protein molecules govern the
chemical reactions within cells and form much of an organism’s structure.
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