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A Textbook of

BIOLOGY

11


National Curriculum 2006

A TEXTBOOK OF

BIOLOGY

11

Dr. Sarwat Jawaid Prof. Wasiullah Khan Prof. Jawaid Mohsin Malik

National Book Foundation

Islamabad Lahore - Rawalpindi - Multan - Bahawalpur - Karachi - Sukkur Hyderabad - Larkana - Peshawar - Abbottabad - Quetta Mardan - Saidu Sharif - Bannu - Faisalabad - Kohat - Jakobabad Dera Ismail Khan - Wah Cantt..


Textbook of Biology - 11 c 2009, National Book Foundation, Islamabad. All rights reseved. This volume may not be reproduced in whole or in part in any form without prior written permission from the publisher. First Edition: .............................................. Quantity:................................ Price:..................... ISBN:................. For information about other National Book Foundation publications, visit our web site http://www.nbf.org.pk. or call 92-51-9261125 or Email at: books@nbf.org.pk Managing Author and Editor Prof. Jawaid Mohsin Malik Written by Dr. Sarwat Jawaid, M.B.B.S, M.P.H (Chapter 11,12,13) Prof. Wasiullah Khan, M.Sc., M.Phil (Chapter 10) Prof. Jawaid Mohsin Malik, M.Sc (Chapter 1,2,3,4,5,6,7,8,9) Reviewed by: Prof Naubahar Bano, M.Sc


Members of the National Review Committee (NRC)

1.

Prof. Hamid Saeed Department of Biologial Sciences, Forman Christian College University Lahore

2.

Mr. M. Nadeem Asghar Subject Specialist (Biology) Punjab Textbook Board, Lahore

3.

Shahnawaz Cheema Professor, Department of Botany, Forman Christian College University, Lahore

4.

Prof. Dr. Zaheer-ud-Din Khan Professor of Botany, Government College University, Lahore

5.

Mr. Farrukh Shahbaz Hussain Associate Professor of Botany Girls Primary Education Project-II, Lahore

6.

Prof. Mohammad Riaz Khan Principal Government Degree College, Baatgram

7.

Dr. Javed Kausar Head Zoology Deptt., Islamabad College for Boys, Islamabad.

8.

Prof. Israr Ali Vice Principal Islamabad Model College for Boys, Islamabad

9.

Mr. Mohammad Hanif Senior Research Officer, Ministry of Education (CW), Islamabad.


Preface

With the advancement of science and technology the status, lifestyle and culture of today’s man have considerably changed during the last two decades. Due to which the subject of biology has emerged as an integral part of education in general and science education in particular. The teaching of ‘biology’ at school and college levels has become inevitable. Now, it holds prime importance in National Education Policy, curriculum development and text book writing. The textbook for class XI has been developed in accordance with the demands of the National Curriculum 2006 which targets all students to develop their individual capacities as successful learners, confident individuals, responsible citizens and effective contributors of social development. The curriculum is based on attainment of standard, benchmark and learning outcomes. The standard includes higher thinking, deep knowledge, problem solving substantive conversation and corrections to the world beyond the class room and achieve the target set by the curriculum. The special features of the textbook are: §

Each chapter begins with a brief recalling statement i.e. introduction to the chapter.

§

The textbook has coloured illustrations to capture the students’ attention. Where necessary, concept mapping has also been incorporated.

§

Necessary ‘Tit Bits’ and ‘Critical Thinking’ have been added in each chapter for motivating the students to apply their intelligence and acquire more knowledge.

§

The exercises include multiple choice questions, short answer questions and extensive questions.

§

At the end of the book a glossary and an index have been annexed.

In each chapter Science, Technology and Society connections are explained in accordance with the curriculum. These interventions will serve as a guide for evaluating the students’ skills development through the chapter knowledge and their abilities to apply knowledge to the scientific and social problems. The duration or the number of periods is also allocated to complete each chapter, so that the teachers can develop their teaching strategy and plans in an effective manner accordingly. Aknowledgement and references are also included. Efforts have been made to contact the holders of copyright but without success. To them the authors offer their apologies hoping they will take our liberty in good faith. The authors would appreciate information that will anable us to acknowledge the copyright holders in future editions of this book. September 2010

Authors


CONTENTS Chapter

1

2

3

SECTION 1 — Cell Biology CELL STRUCTURE AND FUNCTIONS 1.1 1.2 1.3 1.4

Techniques used in Cell Biology Cell wall and Plasma Membrane Cytoplasm and Organelles Prokaryotic and Eukaryotic Cells

BIOLOGICAL MOLECULES 2.1 2.2 2.3 2.4 2.5 2.6 2.7

Biological Molecules in Protoplasm Importance of Water Carbohydrates Proteins Lipids Nucleic Acids Conjugated Molecules

ENZYMES 3.1 Structure of Enzymes 3.2 Mechanism of Enzyme Action 3.3 Factors Affecting the Rate of Enzymatic Action 3.4 Enzyme Inhibition 3.5 Classification of Enzymes

2 2 8 14 31

38 39 42 44 52 55 62 72

78 79 80 84 87 89


4

BIOENERGETICS 4.1 Photosynthesis 4.2 Cellular Respiration 4.3 Photorespirtion

95 96 110 123

SECTION 2 — Biodiversity

5

ACELLULAR LIFE 5.1 5.2 5.3 5.4

6

Viruses: Discovery and Structure Parasitic Nature of Virus Life cycle of Bacteriophage Life cycle of Human Immunodeficiency Virus 5.5 Viral Diseases 5.6 Prions and Viroids

PROKARYOTES 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9

Taxonomy of Prokaryotes Archaea Bacteria: Ecology and Diversity Structure; Shape and Size of Bacteria Modes of Nutrition in Bacteria Growth and Reproduction in Bacteria Importance of Bacteria The Bacterial Flora of Human Control of Harmful Bacteria

132 132 138 141 144 149 153

157 157 158 160 163 169 171 175 183 184


7

8

9

PROTISTS AND FUNGI 7.1 Protists – The Evolutionary Relationships 7.2 Major groups of Protists 7.3 General characteristics of Fungi 7.4 Diversity among Fungi 7.5 Importance of Fungi

DIVERSITY AMONG PLANTS 8.1 8.2 8.3 8.4

Evolutionary Origin of Plants Non-Vascular Plants Seedless Vascular Plants Seed Plants

DIVERSITY AMONG ANIMALS 9.1 9.2 9.3 9.4

Characteristics of Animals Criteria of Animal Classification Diversity in Animals Chordates

190 190 192 201 203 209

220 220 222 227 236

251 251 253 260 278

SECTION 3 — Life Processes

10

FORM AND FUNCTIONS IN PLANTS 10.1 10.2 10.3 10.4 10.5

Nutrition in Plants Gaseous Exchange in Plants Transport in Plants Homeostasis in Plants Support in Plants

302 303 306 307 317 321


VII

11

10.6 Growth and Development in Plants 10.7 Growth Responses in Plants

DIGESTION 11.1 Digestive System of Man 11.2 Disorders: Digestive System and Food Habits

12

CIRCULATION

13

IMMUNITY

12.1 12.2 12.3 12.4 12.5

Blood Circulatory System of Man Blood Vessels Blood Pressure and its Measurement Cardiovascular Disorders Lymphatic System of Man

13.1 First Line of Defence 13.2 Second Line of Defence The Non-specific Defence 13.3 Third Line of Defence The Specific Defences

323 327

339 339 358

366 367 376 385 388 394

405 406 408 414

Acknowledgement and References

430

Glossary

431

Index

443


1

1

SECTION Cell Biology


1

CHAPTER

CELL STRUCTURE AND FUNCTIONS Major Concepts: 1.1

Techniques used in Cell Biology (2 Periods)

1.2

Cell wall and Plasma Membrane (2 Periods)

1.3

Cytoplasm and Organelles (10 Periods)

1.4

Prokaryotic and Eukaryotic Cells (2 Periods)

Number of allotted teaching periods: 16

You are quite familiar with the word “cell”. A cell is the smallest unit of living matter. According to German biologist Rudolf Virchow (pronunciation: Firkoh) “every cell comes from a pre-existing cell”. By the middle of the nineteenth century, biologists clearly recognized that all living things are composed of cells. This is known as cell theory. The cell theory is one of the unifying concepts of biology. A cell is also the functional unit of the organism. Cells can take in nutrients, break them down to release energy, and get rid of wastes. They can reproduce, react to stimuli, and maintain internal environment different from their surroundings. This chapter will help you to become familiar with the structure of cells and how they work.

1.1 TECHNIQUES USED IN CELL BIOLOGY How can you study the structure and functions of cell and its organelles? To know the structure and functions etc. of cell and cell organelles now different techniques are used. The techniques that will be discussed here in brief are: cell fractionation, centrifugation, differential staining, microdissection, tissue culture, chromatography, electrophoresis, and spectrophotometry.


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

3

Cell Fractionation A common approach for studying functions of a cell is to isolate a particular cell organelle from other cell components and try to make it perform its normal functions in a test tube. Generally cells are broken apart as gently as possible. A common procedure is to grind up i.e. to homogenize cells in a suitable medium (with correct pH, ionic composition and temperature). This is done with a homogenizer (food mixer). The mixture is then centrifuged. Centrifugation Centrifugation is the process to separate substances on the basis of their densities. It is done by the machine called centrifuge (fig. 1.1) This machine can spin the tubes. Contents are kept in tubes that are much like the test tubes. Spinning the tubes exerts a centrifugal force on the contents. As the number of revolutions per minute increase so does the centrifugal force (measured as G, which is equal to the force of gravity) increase.

Fig: 1.1 A Centrifuge

Differential Staining Most biological structures are transparent. To differentiate between different structures some methods must be used. The most common method is staining. Certain stains when used in low concentrations are non-toxic to living tissues and can therefore be used on living material. These are called vital stains e.g. methylene blue. When only one stain, such as borax carmine is used it is called single staining. When two stains, one that will stain nucleus e.g. haematoxylin and other that will stain cytoplasm e.g. eosin are used, the process is called double staining or differential staining. Microdissections When dissection is made under microscope it is called microdissection. It is done to remove tumour or granules from delicate tissue or cells like, brain, heart and nerve cells. These days the image is seen on large TV screen or monitor while dissecting.


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BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

Tissue Culture Cell cultures are a suspension of cells in a liquid medium. Tissue cultures are small pieces of plant or animal organ grown in liquid or on solid medium. In plant tissue the root and shoot tips are taken and cultured in a suitable medium to see any infection. Phloem tissue of plants is removed and placed in a germ free medium containing adequate food supply, mineral salts and growth substances. The cell will develop into a new plant, which will flower and produce seeds. Animal tissue culture is usually set up by growing individual cells to form a single layer of cells over the surface of a glass container. Tissue cultured cells are used to see any abnormality in the cell e.g. cancer etc. Chromatography Chromatography is a procedure through which various substances in a mixture are separated from each other and identified. Separation involves the use of two phases, one of which is stationary and the other is mobile. Separation depends upon the differential movement of the test substances between two phases. Can you find out names of seven types of chromatography? You may consult books or search on the internet. Paper chromatography is a simple and most widely used technique.

Fig: 1.2 Chromatography Chamber

Electrophoresis It is a laboratory procedure that separates molecules according to their size, shape, molecular weight and surface charge whether (+) or (–). Macromolecules such as nucleic acids or proteins can be separated in a mixture. Often the gel is sandwiched between glass or plastic plates to form a viscous slab (fig. 1.3). The two ends of the slabs are suspended in two salt solutions that are connected by electrodes to a power source. When voltage is applied to the apparatus, the molecules present in the gel migrate through the electric field according to their individual charge and they move away from


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

Spectrophotometry

Upper buffersolution

Movement of proteins

one another in the gel. The negative charged molecule will move towards the positive pole and the molecule having positive charge will move towards the negative pole. Later on the molecules can be pin pointed by staining the gel.

5

Electrode Glass tube or plates containing gel Gel Lower buffer solution

Power supply Fig: 1.4 Spectrophotometer

Electrode Fig: 1.3 Gel Electrophoresis

A spectrophotometer (fig. 1.4) is an instrument that measures the amount of light that passes through the sample and from this it can be calculated how much light was absorbed. The amount of light absorbed at each wavelength is plotted in a graph and the result is what we call the absorption spectrum (fig. 4.6). The procedure is called spectrophotometry. It can be used to determine the wavelengths of light that take part in photosynthesis. It is also used to know the turbidity or cloudness. The more cells e.g. microorganisms are in suspension the greater will be turbidity. Resolution and Magnification in Microscopy Most animal cells and plant cells are between 10 µm and 30 µm. µ is the Greek letter mu. The unit of microscopic measurement is micrometre (preffered to micron). The correct symbol is µm (preferred to µ) i.e. the µm is the abbreviation for micrometre (American spelling: micrometer). (1 µm = 1/10000 mm). When two objects are closer together than about 100 micrometres, the two light beams fall on the same “detector” cells at the rear of the eyes. When the two dots are farther apart than 100 micrometres, the two beams fall on different cells, only then our eyes resolve them i.e. tell that they are two objects and not one. Resolution is the minimum distance that two points can be separated and be distinguished as two separate points by an optical instrument.


6

BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

One way to increase resolution, is to increase magnification to make small object seen larger. The increase in the apparent size of an object is called magnification. Robert Hooke was able to see small cell by magnifying i.e. enlarging the size, so the cells appeared larger than 100Âľm limit imposed by the structure of human eye. Graticule and Micrometre Graticule is a photographically produced gird, cemented between two glasses. In order to use the graticule for measurement it must be calibrated so that we know what each square represents when a particular object is used. Measurement of microscopic objects is called micrometry. This can be done using specially designed scales or micrometre. There are two types of micrometres: Ocular graticule or micrometer and stage micrometre. Ocular Micrometre Fig: 1.5 Graticule

Ocular micrometre is also known as eyepiece micrometre (fig. 1.6). It is a circular glass piece. It can be put between the two lenses of the eyepiece. A scale showing 100 arbitrary divisions, have been photographically produced, cemented between two glasses.

Fig: 1.6 Ocular Micrometre or Graticule


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

7

As this micrometre is put between the two lenses of the ocular so, it is called ocular micrometre. The micrometre acts as ruler and its scale is used for direct measurement of the object. Stage Micrometre As this scale is placed on the stage of the microscope, so it is called stage micrometre. This is a plane slide. On the centre for the slide, a scale has been produced photographically or engraving. This scale is usually 1 mm having 100 divisions. 1 mm

=

100 divisions

100 division

=

1000 micrometres

1 division

=

20 Angstrom

=

1000 = 10 micrometres (0.01 mm) 100 1 Nanometre.

Fig: 1.7 Stage Micrometre, Total Length is 1mm

The resolution of an electron microscope is about 0.5 nm in practice, compared with 200 nm for light microscope. Infact, the most powerful modern electron microscope can distinguish objects as small as 0.2 nanometre (abbreviated nm, 1 1nm = 1,000,000 mm), a thousand fold improvements over the light microscope.


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BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

1.2 CELL WALL AND PLASMA MEMBRANE The plasma membrane is the outer living boundary of the cell. Many cells have an extracellular component that is formed exterior to the membrane, which is called cell wall. Cell Wall The cell wall is present in plant cells, prokaryotes and fungi. Plant cell walls differ in chemical composition from those of the prokaryotes and fungi. Do you know the differences? We will discuss here only plant cell wall. The cell wall is secreted by the cell. The structure, thickness and chemical nature of the cell wall varies with type of cell and its function. The plant cell wall consists of three main layers, primary cell wall, middle lamella and secondary cell wall. Primary cell wall is a true wall and develops in newly growing cell i.e. during cell division. Each cell produces a primary cell wall. The young growing cells, storage cell and photosynthesising cells of leaves have primary cell wall. The cell wall surrounds the protoplast and plasma membrane. The primary cell wall is thin i.e. 1-3 mm in thickness.


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

Not in most plant cell

Fig: 1.8 Electron Microscopic Structure of an Animal Cell

Fig: 1.9 Electron Microscopic Structure of a Plant Cell

9


10

BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

Fig: 1.10 Plant Cell Wall

The primary cell wall consists of cellulose microfibrils, running through the matrix of other polysaccharides. The microfibrils show a crisscross (fig. 1.11) arrangement. The primary cell wall is adapted to growth. The wall stretches plastically i.e. irreversibly. The cell wall is porous and allows free passage of water and dissolved material. Secondary cell wall is formed between the primary cell wall and plasma membrane. It is found in most of the cells specially that provide support for the plant. The protoplast secretes extra layers of cellulose on the inner surface of the primary cell wall i.e. outer surface of the plasma membrane. Its microfibrils also show crisscross arrangement (fig. 1.11). The secondary cell wall develops only when the cell has reached maximum size i.e. completes its growth.The secondary cell wall consists of cellulose, hemicelluloses and lignin. Lignin cements and anchors cellulose fibres together. Lignin is a complex polymer, not a polysaccharide. Lignin is much rigid than cellulose and cannot be easily compressed and resists changes in form. It provides the cell wall extra tensile and compressional strength. The secondary cell wall provides mechanical support to the cell and those to the plant as it is present in xylem and Fig: 1.11 Crisscross Arrangement of Microfibrils sclerenchyma.


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

11

Middle Lamella is present between adjacent primary cell walls of two cells. It is the first layer to be formed. It is formed of sticky, gel-like magnesium and calcium salts (pectates) of proteins. The middle lamella holds neighbouring cell walls together. Q. Why cell wall is not present in animal cell? Plasma Membrane Every cell is bounded by plasma membrane. The membrane is also called cell membrane or cell surface membrane. It gives shape and mechanical support to the cell. Chemically cell membrane consists of proteins 60-80%, lipids 20-40% and small quantity of carbohydrates. The most common lipids are phospholipids. Membranes also contain glycolipids, cholesterols and glycoproteins. The membrane is about 7nm thick. Fluid Mosaic Model of Plasma Membrane In 1972 Sanger and Nicholson put forward the fluid mosaic model of membrane structure. The model proposes that the membrane is a phospholipids bilayer in which protein molecules are either partially or wholly embedded. The protein are scattered throughout the membrane in an irregular pattern that can vary from membrane to membrane. The mosaic distribution of protein is supported especially by electron micrograph of freeze fractured membrane (see Glossary).

Fig: 1.12 Fluid Mosaic Model of Plasma Membrane


12

BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

At body temperature, the phospholipid bilayer of the plasma membrane has the consistency of olive oil. When the concentration of unsaturated fatty acid residues becomes greater, the bilayer becomes more fluid. The fluidity of a phospholipid bilayer means that the cells are pliable i.e. flexible. In general most membrane proteins are observed to drift sideway in the fluid bilayer. Many plasma membrane proteins are glycoproteins (see page 72), which have an attached carbohydrate chain. The proteins within a membrane determine most of the functions. The proteins form different patterns according to the particular membrane and also within the same membrane at different times, i.e. the membrane is a mosaic.

Science Titbits The fluidity of membrane is dependent on its lipid components, including phospholipids, glycolipids and cholesterol.

Critical Thinking Why the cell surface membrane is described as fluid mosaic?

The head of the lipid has a variety of sugars (glycolipids) joined to form a straight or branching carbohydrate chain. Glycolipids have protective function (see page 72). The steroid, cholesterol (see page 62), wedged into the bilayer, helps stabilize the phospholipids at a body temperature but helps keep the membrane fluid at lower temperature. The plasma membrane is asymmetrical i.e. the two surfaces and the two halves are not identical. Functions of Plasma Membrane Protein The phospholipids bilayer provides the basic structure of membrane. It also restricts entry and exit of polar molecules and ions. The other molecules specially variety of protein have various function. 1. Channel Proteins and Carrier Proteins: Certain plasma membrane proteins are involved in the passage of molecules through the membrane. Some of those have a channel through which a substance simply can move across the membrane, other are carriers that combine with a substance and help it to move across the membrane. 2. Enzymes: Some plasma membrane proteins have enzymatic functions. They perform metabolic reactions directly, for example the membrane protein, adenylate cyclase, is involved in ATP metabolism.


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

13

Fig: 1.13 Some of the Functions Performed by Protein in the Plasma Membrane

3. Receptor Molecules: Some proteins in the plasma membrane are receptors. Each type of receptor has a shape that allows a specific charge to bind it. The binding of a molecule can cause the proteins to change its shape and bring about an intracellular response. For example, hormones circulate in the blood, but bind to specific target cells, which have the correct receptor sites. Neurotransmitters the chemicals that enable nerve impulses to pass from one nerve cell to the next also fit into specific proteins in nerve cells. 4. Antigens: Antigens are glycoproteins. The glycoproteins have an enormous number of possible shapes, so each type of cell can have its own specific markers. This enables cells to recognize other cells and to behave in an organized way, for example during development of tissues and organs in multicellular organisms. It also means that the foreign antigens can be recognized and attacked by immune system. Role of Glycolipids and Glycolproteins as Cell Surface Markers Glycolproteins, are proteins with branching carbohydrate side chain work like antennae. They act as cell identity markers or same tags. Glycolipids have also branching carbohydrate side chains and are involved in cell to cell recognition. They may act as receptor sites for chemical signals. With glycolproteins they are also involved in sticking the correct cells together in tissues.


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BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

Role of Plasma Membrane in Regulating CellÂ’s Interaction with its Environment Transport across plasma membrane occurs to: (1) obtain nutrient (2) excrete waste substances (3) secrete useful substances (4) generate ionic gradients essential for nervous and muscular activity (5) maintain a suitable pH and ionic concentration within the cell for enzyme activity. For movement across the cell surface membrane there are four basic mechanisms. Diffusion and osmosis are passive processes i.e. they do not require the expenditure of energy. Active transport and bulk transport (endocytosis and exocytosis) are energy consuming processes. Skills: Analyzing, Interpreting and Communication 1. Draw and label fluid mosaic model of plasma membrane.

1.3 CYTOPLASM AND ORGANELLES The living contents of eukaryotic cells are divided into cytoplasm and nucleus, the two forming the protoplasm (Greek: protos, first plasm, form). Cytoplasm is an aqueous (water containing) substance containing a variety of cell organelles and other structures such as insoluble waste or storage products. The soluble part of the cytoplasm forms the background material or ground substance between the cell organelles. It contains a skeleton of very fine fibres but otherwise appears transparent and structureless in the electron microscope. The chemical nature of cytoplasm is about 90% water and form a solution that contains all the fundamental biochemicals of life. Some of these are ions and small molecules in true solution, such as salts, sugars, amino acids, fatty acids, nucleotides, vitamins and dissolved gases. Others are large molecules, such as proteins, which form the colloidal solutions. A colloidal solution is one in which the solute molecules are relatively large. It occurs in two forms, sol or plasmasol which are non-viscous fluid like and gel or plasma gel which are viscous and jelly like. The two forms are interchangeable and help in cytoplasmic movement. Often the ectoplasm is formed of gel and endoplasm is formed of sol. The outer regions of cytoplasm are more gel-like. The metabolic role of cytoplasm is that the ground substance of the cytoplasm is the site of certain metabolic pathways e.g. glycolysis


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

15

Cell Organelles The highly organized cellular bodies are called organelles. Specific roles e.g., heat production, cellular maintenance, repair, storage and protein synthesis are carried out within the organelles. The organelles in the cytoplasmic matrix of a cell are: endoplasmic reticulum, ribosomes, Golgi complex, peroxysomes and glyoxysomes, lysosomes, mitochondria, chloroplast etc. Endoplasmic Reticulum It is the system of membranes running through the cytoplasm of all eukaryotic cells. This network or reticulum of membrane is called endoplasmic reticulum (ER). It can be seen only with an electron microscope. The ER consists of membranous tubes, sacs and flattened channels called cisternae. The ER forms a distinct compartment within the cytoplasm. It is composed of double layer of lipids with various enzymes attached to its surface. The ER is continuous with plasma membrane, nuclear membrane and Golgi complex or apparatus. In terms of appearance and functions there are two type of ER, rough ER and smooth ER. Most cells contain both types of ER although the relative proportions vary considerably among different cells. Rough ER has ribosomes attached to the sides facing the cytoplasm and has rough appearance under electron microscope. Rough ER provides mechanical support to the cell. RER is concerned with the transport of proteins. Proteins are formed at ribosomes. A receptor in the membrane of the ER provides a channel through which a protein can pass into the ER. For example, secretory proteins is an antibody, a defensive molecule made and synthesized by white blood cells. Inside the ER short chains of sugars are then linked to the polypeptide making the Rough ER

Smooth ER

Fig: 1.14 Rough ER and Smooth ER


16

BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

molecule a glycolprotein. The ER packages the glycolprotein into a tiny sac called transport vesicle. Then the vesicles buds from the ER membrane. The secretory proteins will now travel to the Golgi apparatus for further processing. From there a transport vesicle containing the finished molecule will make its way to plasma membrane and its contents from the cell. The enzymes in lysosomes also follow this route. Other vesicles formed at RER function as storage vessels in the cytoplasm. Some vesicles merge with the smooth ER. Smooth ER is continuous with the RER and is a network of interconnected tubules. Ribosomes are not attached to it. In muscle cells a specialized form of smooth ER called sarcoplasmic reticulum is present. One of the chief functions of smooth ER is lipid synthesis. For example in the epithelium of the intestine the smooth ER makes lipids from the fatty acids and glycerol absorbed from the gut and passes them to Golgi complex for export. Smooth ER also makes steroids. Some smooth ER transport proteins from rough ER. Also, enzymes within the smooth ER of liver cells inactivate or detoxify a variety of chemicals. The function of some ER is to breakdown energy rich glycogen or fat molecules. Ribosomes Ribosomes are among the smallest structures suspended in the cytoplasm found in large number of living cells both prokaryotic and eukaryotic. They can be seen only under the electron microscope. They are roughly spherical granular bodies about 17 to 21 nm in diameter in prokaryotic cells of bacteria and about 20- to 24 nm in diameter in eukaryotic cells. They remain attached with RER or freely dispersed in the cytoplasm. Ribosomes are numerous in actively synthesizing cells e.g., endocrine cells and meristimatic cells. A eukaryotic cell may contain about half a million ribosomes. Ribosomes are made of almost an equal amount of RNA and protein so they are ribonucleoprotein. Being so small they are the last organelle to be sedimented in centrifuge. The 70S ribosomes are found in prokaryotes chloroplasts and mitochondria. All ribosomes in eukaryotes are composed of two subunits (particles) of different sizes, the The S unit has been named after large and the small. The larger Svedberg. When the speed of one is 60S particles and the centrifugation is more than the speed of smaller one is 40S particles. The gravity, it is called ultra centrifugation. two subunits on attachment form The larger sedimentation unit S, the 80S particles. The attachment is larger are the particles.

Science Titbits


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

17

Large subunit of ribosome

small subunit of ribosome Fig: 1.15 Polysome

controlled by presence of magnesium ions concentration or forming salt bonds between phosphate group of RNA and amino group of amino acid or both by magnesium ions and salt bonds. When several ribosomes are attached to one mRNA strip it is called polysome. (poly = many and soma = body) or poly ribosomes. Ribosomes are formed in the nucleolus. Then these are transported to the cytoplasm through the nuclear pore. The ribosomes are the sites of synthesis of proteins from amino acids. Golgi Complex It is also known as Golgi bodies or Golgi apparatus. It was discovered by Italian biologist Camillo Golgi in 1898 for which he was awarded Nobel Prize. However its structure was only revealed by electron microscope. It is found in all eukaryotic cells. Golgi complex consists of a stack of flattened, membrane bound sacs called cisternae, together with system of associated vesicles (small sacs) called Golgi vesicles (fig: 1.16). It is believed that a complex system of interconnected tubules is formed around the central stack. At one end of the stack a new cisternae are constantly being formed by vesicles from the smooth ER. This ‘outer’ or forming, face is convex, while the inner end is the concave ‘inner’ or maturing face where the cisternae break up into vesicles again. The whole stack consists of a number of cisternae moving from the outer face to the inner face. The function of Golgi complex is to transport and chemically modify the materials contained within it. It is particularly important in secretory cells. For example in pancreas specialized cells secrete digestive enzymes of the


18

BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

(Outer surface)

complex

Golgi complex (inner surface) Fig: 1.16 Golgi Complex

pancreatic juice into the pancreatic duct, which pass to the duodenum. After concentration in the Golgi complex the protein is carried in Golgi vesicles to the cell surface membrane. The final stage in the pathway is secretion of the inactive enzyme by exocytosis. In general, proteins received by the Golgi complex from the ER have short carbohydrate chains added to make them glycolproteins. These carbohydrate ‘antennae’ can be remodelled in the Golgi complex, possibly to become markers that direct the protein to correct their destination. The Golgi complex is also sometimes involved in the secretion of carbohydrates, an example being provided by the synthesis of new cell walls by plants. The membrane of the vesicle becomes new cell surface membrane of the daughter cells, while their contents contribute to the middle lamella and new cell walls. Cellulose is added separately and involves microtubules, not the Golgi complex. In addition to secretion, the second important function of the Golgi apparatus is the formation of lysosomes. Peroxisomes and Glyoxisomes Microbodies are similar to lysosomes and are a single membrane enclosed cytoplasmic organelle. Microbodies include peroxisomes and glyoxisomes. Peroxisomes In 1965 De Duve and his coworkers isolated particles from liver cells


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

19

and other tissues. These particles were enriched with some oxidative enzymes such as peroxidase, catalase, glycolic acid oxidase and some other enzymes. As this organelle is specifically involved in the formation and decomposition of hydrogen peroxide so they were named peroxisomes. These are found both in animal and plant cells. These are also present in protozoa, and yeast. Peroxisomes are opproximetely 0.5 to 1 micrometre in diameter. The enzymes oxidize certain organic substances with the formation of hydrogen peroxide (H2O2). Hydrogen peroxide is a toxic molecule, which is immediately broken down to water and oxygen by another enzyme called catalase. Peroxisomes are abundant is cells that metabolise lipids and in liver cells that metabolise alcohol. They help to detoxify alcohol, and convert fats to carbohydrates. Glyoxisomes These organelles are found in plants. These have in addition to glycolic acid oxidases and catalases, a number of enzymes that are not found in animal cells. These organelles are more abundant in plant seedlings, which rely upon saturated fatty acids to provide them with energy and material to begin the formation of a new plant. The germinating seedlings convert stored fatty acids to carbohydrates. This is achieved through a cycle called glyoxylate cycle, the enzymes of which are located in the glyoxisomes. Lysosomes Lyso means splitting and soma means body. So lysosomes are structures that break down other major macromolecules in the cell. De Duve discovered it in 1949. Almost all animal cells contain lysosomes. The lysosomal enzymes are manufactured on the RER. Then these enzymes are transported to Golgi apparatus. Here the enzymes are enclosed in membrane. The enclosed enzymes are pinched off as Golgi vesicles. These vesicles are called primary lysosomes. Once a lysosome has fused with a vesicle containing material to be digested, the structure is called secondary lysosomes. Lysosomes are roughly spherical structures bounded by a single membrane. They vary in size, and usually 0.2-0.5 Âľm in diameter. In plant cells large central vacuole may act as lysosome. Lysosomes are sacs or vesicles that contain various hydrolytic enzymes. Such as proteases, nucleases and lipases. The content of the lysosome is acidic. The enzymes have to be kept apart from the rest of the cell or they would destroy it. These enzymes breakdown every major macromolecule of cell, e.g. proteins, nucleic acids, lipids and carbohydrates.


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BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

Functions of Lysosomes The functions of lysosomes includes phagocytosis, autophagy, autolysis, exocytosis (fig: 1.17). When the material taken in by endocytosis is large, such as a food particle or another cell, the process is called phagocytosis (Gk. Phagein, to eat, and kytos, cell). When the endocytic vesicle fuses with a lysosome digestion occurs. This is how mammalian white blood cells engulf and destroy bacteria and other cells. The process by which unwanted structures within the cell are engulfed and digested within the lysosomes is called autophagy and the structure in which it occurs is called an autophagic vacuole or secondary lysosomes. This is the part of the normal turnover of cytoplasmic organelles, old ones being replaced by new ones e.g. digestion of mitochondria.

Fig: 1.17 Lysosomes: Formation and Functions


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

21

Autolysis is the self digestion of a cell by releasing the contents of lysosomes within the cell. In such circumstances lysosomes have been named as suicidal bags. Autolysis is a normal event in some differential process and may occur throughout a tissue. e.g. reabsorption of tadpole tail during metamorphosis. Autolysis also occurs in muscles, which are not exercised. Sometimes the enzymes of lysosomes are released from the cell by exocytosis. This occurs during the replacement of cartilage by bone during development. Similarly remodelling of bone that can occur in response to injury, new stress and so on. Sperm contains special lysosome called the acrosome. This releases its enzymes to digest path through the layers of cells surrounding the egg just before fertilization. The importance of lysosomes to cell function and human health is made strikingly clear by the serious hereditary disorders called lysosomal storage diseases. These are a group of disorders characterised by deficiency of a specific single lysosomal enzyme, resulting in an accumulation of abnormal metabolic products. Most of these diseases are fatal in early child hood. There are many lysosomal diseases but we will discuss here only Taysachs disease, Niemann-pick disease and Pompe disease. Tay-Sachs disease is a most common form of gangliosidosis. It is caused by a deficiency of hexosaminidase A, with consequent accumulation of GM2 ganglioside, especially in neurons. It is characterised by central nervous system degeneration severe mental and motor deterioration, blindness, and death before four years of age. Niemann-Pick disease is most often caused by a deficiency of sphingomyelinase, with consequent sphingomyelin accumulation in phagocytes. It is characterised by anaemia, fever, neuralgic deterioration and death occurs by three years of age. Pompe disease is caused by a deficiency of alpha-1,4-glucosidase (a lysosomal enzyme) with the consequent accumulation of glycogen, especially in the liver, heart and skeletal muscle. It is characterised by muscle hypotonia and splenomegaly. Death occurs from cardiorespiratory failure before three years of age. Q. Why are lysosomes sometimes referred to as the ‘‘self destruct system’’ of the cell?


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BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

Mitochondrion Mitochondrion (plural: mitochondria) is present in all eukaryotic cells. The number of mitochondria per cell varies form 500 to 1000 in vertebrates and depends on the type of the organism and nature of cell. The shape of mitochondria may be spiral, spherical, elongated, cup shaped even branched and are usually larger in active cells than Fig: 1.18 Mitochondrion in less active ones. Their length ranges form 1.5 to 10 Âľm and width from 0.25-1.00Âľm, but their diameter does not exceed 1 Âľm. Mitochondria are able to change shape and some are able to move to area in the cell where a lot of activities are taking place. Mitochondria are semiautonomous, colourless organelle. It is a double membrane structure, the outer membrane and the inner membrane. The two membranes are separated by a narrow space the intermembranal space, which is homogeneous. Mitochondrial membranes are composed of lipids and proteins. The outer membrane is smooth and somewhat like a sieve. Outer membrane is rich in lipids but poor in proteins. The inner membrane is folded inwards. The folds are called cristae (singular: crista). The region of the mitochondrion enclosed by membrane is called matrix. Mitochondrial matrix is a jelly like material that contains DNA, RNA, ribosomes and enzymes. In 1960, E. Rocker discovered that inner mtiochondrial membrane is provided with lollipop like structures called mitochondrial particles or elementary particles or oxysomes. The particles function in ATP synthesis, so it is called ATP-ase complex. Enzymes, coenzymes, organic and inorganic salts present in the mitochondrial matrix help in several processes like Krebs cycle, aerobic respiration, fatty acid metabolism.

Science Titbits Mitochondria divide and in this way their number doubles before cell division. Lysosomes regulate the number of mitochondria. Excess of mitochondria are digested by lysosomes. Because mitochondria are contained within ova (egg cells) but not within the heads of sperm cells, all the mitochondria in a fertilized egg are derived from mother.


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

23

Chloroplast Chloroplasts are green in colour and are found in green parts of the plants. Chloroplasts vary greatly in shape and size. These may be spheroid, ovoid or discoid. Generally their number ranges between 20-40 per cell. Their average size is 4 to 10 micrometre in diameter and 1-3 micrometre in thickness. A chloroplast consists of three parts i.e. envelope, matrix and thylakoids. Each chloroplast is bounded by a smooth double membrane (envelope). Between the two intermembrane space is 25-75 angstrom (Ă…) wide. The outer membrane is smooth and permeable to small molecules. The inner membrane is semipermeable and rich in protein. The matrix is called stroma. It is the colourless proteinaceous ground substance that fills the chloroplast. It contains, proteins, lipids, small (70S) ribosomes, DNA, RNAs, ions and enzymes. The chloroplast is semiautonomous because its DNA can replicate and transcribe to form RNA. With the help of ribosomes chlorplast is able to synthesize most of its enzymes.

Fig: 1.19 Chloroplast

The stroma contains a system of chlorophyll bearing double membrane lamellae that form flattened sac-like structures called thylakoids. There are two types of thylakoids: smaller thylakoids and the larger thylakoids. Smaller thylakoids are disc like and are piled over one another like piles of coins and each stack of grana lamellae is called granum. Thus each granum (plural: grana) is formed of a column of grana lamellae or


24

BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

thylakoids. There may be as many as 40-60 grana per chloroplast. Larger thylakoids connect the grana and are called intergranal lamellae or stromal lamellae. These may form an interconnectd network or may be simple parallel sacs running lengthwise. Thylakoids membranes possess photosynthetic pigments.

Science Titbits

New chloroplasts arise either by the division of pre-existing chloroplasts or by the division of their precursor proplastids. The functions of chloroplasts are photosynthesis, oxygen supply, storage of starch, fixa­ Centrioles tion of carbon dioxide and to Centrioles are non-membranous provide greenary. cell organelles found mainly in animal cells. Centrioles commonly occur in pairs and a pair of centrioles is called diplosome. These occur at right angle to each other near one pole of the nucleus. These lie in a distinctly, staining region of the cytoplasm known as centrosphere. The centrioles and centrosphere are together referred to as centrosome. A centriole looks like rounded cylinder about 0.15-0.25 µm in diameter and 0.3-2 µm in length. Just before the cell division, the centriole duplicates and the pairs migrate to the opposite sides of the nucleus. The spindle fibres are then formed between the two pairs of centrioles. Each centriole is formed of nine groups of triplet microtubules or fibres. Each microtubule is formed of three subtubules or subfibres.

Fig: 1.20 Centrioles

The functions of centroles are: (a) The centrioles play an important role in cell division by forming spindle and providing a mechanism for the alignment and dragging of chromatids. (b) Centrioles give rise to basal bodies or kinetosome of cilia and flagella. (c) The centrioles are present in the spermatid. The distal centriole forms the axial filament of the flagellum of sperm.


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

25

Cytoskeleton The cytoskeleton (Gk: Kytos, cell and skeleton, dried body) is a network of interconnected filaments and tubules that extends from the nucleus to the plasma membrane in eukaryotic cell. The cytoskeleton maintains cell shape and causes the cell and its organelles to move. There are three types of cytoskeleton elements of proteinous fibres. Microfilaments: Presently microfilaments are known as Fig: 1.21 Cytoskeleton actin filaments. These are extremely thin fibres about 7 nm in diameter that occur in bundles or mesh like networks. The actin filament contains two chains of globular actin monomers twisted about one another in a helical manner. Threads of tropomyosin wind about an actin filament and troponin occur at intervals along the thread. Actin filaments play a structural role when they form a dense complex web just under the plasma membrane to which they are anchored by special proteins. In plant cells, they apparently form the tracks along which chloroplasts circulate in a particular direction. Microtubules (Gk: micros, small, little and L. tubus, pipe): These are small hollow cylinders about 25nm in diameter and 0.2-25Âľm in length. Its basic protein subunit is tubulin, which occurs as alpha and beta tubule. The two form tubulin dimer. Microtubules have 13 rows of tubulin dimer surrounding what appear in electron micrograph to be an empty central case. Microtubules radiate from centrosome, helping to maintain the shape of the cell and acting as tracks along which organelles can move. Intermediate Filaments: The protein fibres are wrapped around one another. They are 8 to 10 nanometers in diameter, intermediate in size between actin filaments and microtubules, this is why they are called intermediate filaments. The basic protein subunit of the filament is vimentin. Some intermediate filaments support nuclear envelope, and others support plasma membrane. Q. How cytoskeleton is important to eukaryotic cells?


26

BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

Cilia and Flagella Cilia (L. cilium, eyelash, hair) and flagella (L. flagella, whip) are hair like projection on the surface of the cells. These are cytoplasmic processes and create water currents, food currents act as sensory organs and perform several other functions of the cell. There is no clear morphological or physiological difference

Fig: 1.22 Structure of a Eukaryotic Flagellum or Cilium


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

27

between cilia and flagella except that of size. Flagella are few in number i.e. 1, 2 or 4 and longer, exhibit undulating motion and beat independently. Cilia are numerous and relatively short and beat perpendicularly in metachronous (cilia of a row beating one after the other) or in synchronous rhythm (all cilia of a row beating simultaneously). Cilia and flagella originate from their basal bodies embedded in the cytoplasm. Each cilium and flagellum consists of a longitudinal axoneme enclosed in a spiral sheath of cytoplasm and a plasma membrane continuous with the cell membrane. Axoneme is made up of a bundle of eleven longitudinal fibrils or microtubules. Of them 9 are peripheral microfibrils and the two are central microfibrils. These are arranged in 9 + 2 pattern. The central fibrils are enclosed in a central sheath. The nine peripheral microfibrils form a ring around the central sheath. Each peripheral microfibrils is composed of two subfibres forming a doublet. The inner subfiber of the doublet is complete and the outer subfiber is C-shaped. Each inner subfibre has two arms composed of Critical Thinking dynein protein and a radial spoke extends from it What will happen if all to the central sheath. Each cilium and a flagellum dynein of a cilium is has a basal body lying in the cytoplasm at its base. removed? Basal bodies have the same circular arrangement of microtubule triplets as centrioles. Mechanism of Movement: Figure 1.23 shows the position of two microtubule doublets in a flagellum that is stationary (left) and in the process of bending (right). Bending involves protein knobs attached to each microtubule doublet --- the dynein arms. Using energy from ATP, the dynein arms grab an adjacent doublet and exert a sliding force as they start to “walk” along it. The doublets are held together by cross-links (not illustrated), if they were not held in place, the walking action would make one doublet slide past the other. Instead, the microtubules (and consequently the flagellum or cilium) bend. In 1955 Bradford suggested that the movement of cilia is due to sliding of double fibrils in two groups one after the other. Five out of nine double fibrils contract simultaneously. As a result cilium bends or shortens. It is called effective stroke. Four out of nine double fibrils contract and cilium becomes straight. It is called recovery stroke. A flagellum causes movement by the passage of rapid successive waves of bending from the attached to the free end. Flagellar movement of human sperms propel them forward within the fluid medium of the female reproductive tract.


28

BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

Fig: 1.23 The Mechanism of Microtubules Bending in Cilia and Flagella

Power stroke or Effective stroke

or Recovery stroke

Fig: 1.24 Characteristic Movement Patterns of Cilia and Flagella. (a) Cilia usually “row” along, providing a force of movement parallel to the plasma membrane, just as oars provide movement parallel to sides of a rowboat. (b) Flagella often move in a wave like motion with a continuous bending that starts at the base and move up to the tip. This motion provides a force of movements perpendicular to the plasma membrane. In this way a flagellum attached to a sperm can move the sperm straight ahead.


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

29

Nucleus The largest and most easily seen of all the organelleles within a eukaryotic cell is the nucleus. A cell may be mononucleate, binucleate or multinucleate. In animal cells the nucleus is typically located in the central region. It controls all the activities of the cell. Nucleus consists of nuclear membrance, nucleoplasm, nucleolus and Fig: 1.25 Nucleus chromosomes. A double membrane called nuclear envelope or nuclear membrane, bounds the surface of the nucleus. The two membranes are separated by a fluid-filled perinuclear space. These are connected at nuclear pores. It consists of protein and lipid bilayer. The outer membrane is covered with ribosomes and is connected with the membranes of ER. The perinuclear space is continuous with the lumen of ER. The inner membrane is smooth. The nuclear membrane is perforated by numerous pores. The pores regulate the nucleo-cytoplasmic exchange of materials.

Science Titbits The structures and molecules which can cross the nuclear envelops are ions, micromolecules, macromolecules, tRNA, mRNA, ribosomal RNA, proteins, nucleotides, and some hormones for regulation of DNA. Nucleoplasm is the transparent semifluid ground substance formed of a mixture of proteins, enzymes (DNA and RNA polymerase), phosphorus, nucleotide, some nucleic acids and metal ions (Mg) for the synthesis of DNA and RNAs. It also contains histone and non-histone protein. Whereas cytoplasm contains amino acids, carbohydrates, proteins, enzymes, vitamins, nucleotides, tRNA etc. Proteins occur as colloidal particles. The dark staining region in the nucleus is called nucleolus, (plural: nucleoli). A cell may have one or more nucleoli. Nucleolus appears during


30

BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

interphase and disappears during cell division. The nucleolus is associated with a specific region of a particular chromosome. Nucleolus consists of a ribosomal ribonucleic acid and some ribosomal proteins. The highly stained thread like chromatin (chroma colour, teino strech) fibres form a chromatin network in the nucleoplasm. During cell division chromatin fibre condenses and coils up into structures called chromosomes (Gk: chroma, colour, soma, body), which are thick enough to be seen with a light microscope. Chromosomes are separate thread like structures, which stain heavily during cell division so they are visible only during cell division. At other times they lose their ability to stain. Each chromosome is bounded by delicate membrane. The centromere is a constriction functionally related to the movement of chromosomes during cell division. Each centromere had two plaques of protein called kinetochores that are oriented on the opposite sides of the constriction. Each kinetochore forms the site of attachment for a single microtubule during cell division. Each specie has a characteristic number of chromosome e.g. human 46, frog 26, chimpanzee 48 etc. Individual chromosmes can be identified by their size and shape. Chromosomes contain DNA, which is the hereditary material.

Fig: 1.26 Chromosome

Skills: Analyzing, Interpreting and Communication 1. Compare and contrast the structure and function of mitochondria with those of chloroplasts. 2. Compare in tabular form, the functions of organelles with the processes occurring in animals and plants. 3. List the structure and molecules, which can cross the nuclear envelope. Q. What are the differences between microfilaments and microtubules? Q. What is the difference between: (a) cytoplasm and nucleoplasm (b) chromatin and chromosomes. (d) nucleolus and nucleosome (e) centromere and kinetochores.


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

31

1.4 PROKARYOTIC AND EUKARYOTIC CELLS Two kinds of structurally different cells have evolved overtime. Prokaryotic cells i.e. bacteria and cyanobacteria, whereas all other forms of life are composed of eukaryotic cells. A prokaryotic cell (Greek pro: before and karyon: kernel) lacks a nucleus, its DNA is coiled into a nucleoid (nucleus like) region. Structures Missing in a Prokaryotic Cell The structures that are present in eukaryotic cells but are missing in prokaryotic cells are: nucleus, endoplasmic reticulum, Golgi complex, mitochondrion, lysosomes, microvilli, nucleolus, chloroplast , microtubule, 80S ribosomes (larger), flagella lacking 9+2 arrangement of microtubules, cellulose is absent in cell wall. Chemical Composition of Prokaryotic Cell Wall The cell wall is composed of an inner layer of peptidoglycan (see section 6.3) and an outer membrane that varies in thickness and chemical composition depending upon the type of bacteria. Peptidoglycan is also known as murein. This substance is found only in prokaryotes. Pattern of Cell Division in Prokaryotic and Eukaryotic Cells In eukaryotes somatic cells divide by mitosis and gametes are produced by the cell division called meiosis. Whereas prokaryotic cells divide by binary fission. In binary fission the bacterium cell divides to form two identical sister cells. During this process, the single circular chromosome duplicates itself. Along with DNA duplication the cytoplasm divides into two halves, each having its own nuclear material. Then wall is formed and two daughter cells are produced (see fig. 6.11). Under ideal conditions bacterial cell divides every 20-30 minutes. A Model Prokaryotic Cell — Bacteria The bacterial cell is surrounded by cell wall. The cell wall gives shape and protection to cell. Bacterial cell wall has no cellulose. The cell membrane or plasma membrane lies beneath the cell wall. Bacterial cell membrane differs from eukaryotic membrane in lacking sterols such as cholesterol. Plasma membrane contains enzymes for respiratory metabolism. A slimy capsule, secreted in the cell, envelops a bacterium. Many bacteria have fine thread like outgrowth called flagella (singular: flagellum). The flagella are anchored in the cell membrane and project out through the cell


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BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

Fig: 1.27 A Bacterium

wall. The flagellum is a structure for movement. Pili (singular: pilus) are hollow protein filaments that are anchored in the membrane and project through the cell wall. They can be observed only by electron microscope and are found only on certain species of Gram negative bacteria. Pili are used to transfer genetic material during conjugation. The other function of pili is attachment on the surface of tissues of an infected person. Pili are also known as fimbriae. The glycocalyx (slime layer) is a polysaccharide coating that is secreted by many bacteria. In some bacteria there is an infolding of the cell membrane into the cytoplasm. This is called mesosome. Mesosomes are in the form of vesicles, tubules or lamellae. The functions of mesosomes are: (a) increase membrane surface area, allowing the cell greater activity in respiration and active transport (b) the formation of new cross wall occurs with the help of mesosomes during cell division (c) photosynthesis. The cytoplasm is dense. Small vacuoles and granules of stored food e.g. glycogen, proteins, fats, are present in bacteria. About 90% of the cell is water. Ribosomes are large in number and occur free in the cytoplasm. Bacterial ribosomes are the site of protein synthesis as in eukaryotic cells.


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Plasmids are small circular pieces of double stranded DNA. They replicate when the cell replicates. Plasmids can replicate independent of the chromosomes. They often contain drug resistant, heavy metals, disease and insect resistant genes on them. Transposons are pieces of DNA that move readily from one site to another, either within or between the DNAs of bacteria, plasmids and becteriophages.

Science Titbits Plasmids are important vectors in modern genetic engineering techniques. Plasmids also occur in lower eukaryotes e.g. yeast. Several different types of plasmids can exist in one cell. Transmissible plasmids can be transferred from cell to cell by conjugation. Nontrasmissible plasmids are small, they are frequently present in many copies per cell. The cytoplasm contains several different types of granules that serve storage areas for nutrients and stain characteristically with certain dyes. The nuclear region of bacteria is not separated from the cytoplasm by nuclear membrane. It is seen in the electron microscope as an area lighter than the cytoplasmic contents called nucleoid. Bacteria are haploid organisms. The nucleoid contains a single chromosome. The DNA that is tightly folded so as to fit inside the cell component. The nucleoid contains no nuclear membrane, nucleolus, no mitotic spindle and no histones, so there is little resemblance to the eukaryotic nucleus. The bacterial DNA has no introns (see glossary)whereas it is present in eukaryotic DNA.

SECTION I : MULTIPLE CHOICE QUESTIONS Select the correct answer 1. Which of the following is the major advantage of using a light microscope instead of an electron microscope? A)

superior resolving power

B)

constant depth of focus

C)

observation of living matter

D)

use of very thin sections


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BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

2. Which technique provided the primary evidence for the fluid mosaic model of the cell membrane? A) cell fractionation

B)

chemical analysis of membrane proteins

C) freeze-fracture

D)

chemical analysis, microscopic staining

3. Some cellular organelles are bound by a single membrane, while other organelles have two membranes (envelopes) around them. Which one of the folowing is correct? single membrane

two membranes

A. peroxysomes lysosome

nucleus

chloroplast

B. chloroplast

lysosome

nucleus

peroxysomes

C. nucleus

chloroplast

lysosome

peroxysomes

D. nucleus

lysosome

chloroplast

peroxysomes

4. Which of the following cell structures contains the highest concentration of RNA? A)

centriole

B)

lysosome

C)

chromosome

D)

nucleolus

5. A tadpoleÂ’s tail is grdually broken down during metaporphosis into an adult frog. Which organelle increases in number in the cells of the tail at this time? A)

centriole

B)

endoplasmic reticulum

C)

Golgi complex

D)

lysosomes

6. The diagram shows the fluid-mosaic model of membrane structure? What are X, Y and Z? X

Y

Z Z X

Y


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS A

lipid

carbohydrate

protein

B.

lipid

protein

carbohydrate

C.

protein

carbohydrate

lipid

D.

protein

lipid

carbohydrate

35

7. Which of the following organelles always contains DNA? A)

centriole

B)

Golgi complex

C)

lysosome

D)

mitochondria

8. Which distinguishes a prokaryotic cell from a eukaryotic cell? A)

prokaryotic cell have a cell wall and a nucleus

B)

prokaryotic cells have no membrane bound organelles

C)

prokaryotic cells have a centriole

D)

prokaryotic cells have no ribosomes

9. The elasticity of the plasma membrane demonstrates that it is made up in part of: A)

lipids

B)

nucleic acids

C)

carbohydrates

D)

proteins

10. The name of the structure where photosynthesis takes place begins with the letter? A)

C-Z

B)

L-Z

C)

F-K

D)

O-R

11. Filaments present in flagella and cilia are A)

microfibrils

B)

microtubules

C)

microfilaments

D)

microvilli

1) Why are the following scientists famous for? : (a) Rudolf Virchow,


36

BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

SECTION II : SHORT QUESTIONS (b) Robert Hooke, (c) Sanger and Nicholson, (d)

De Duve

2) Write the difference between: (a)resolution and magnification, (b) stage micrometre and ocular micrometre, (c) plant cell wall and bacterial cell wall, (d) cytoplasm of eukaryotic and prokaryotic cell, (e) rough ER and smooth ER, (f) centrifugation and differential centrifugation, (g) primary lysosomes and secondary lysosomes. 3) Name three organelles revealed by an electron microscope. 4) What holds the ribosomes together in a polysome? 5) Why lysososmes are called suicide bags? 6) How and where lysosomes are formed in the cell? 7) Why ER is present in all eukaryotic cells but not in prokaryotic cells? 8) Name the structures and organelles which are common in plant cell, animal cell and a prokarytic cell. 9) How is a chloroplast similar to a bacterium? 10)What would happen if there are no lysosomes in human cells? 11) Draw a labelled diagram of a section through: (a) mitochondrion (b) bacterium (c) chloroplast. 12. What are lysosomal storage diseases? Give examples. 13. Name the organelles of cells that are highly speciallized to do (a) protein synthesis of (b) actively transport substances into the cell (c) synthesis of lipids (d) phagocytize foreign substances. 14. Why does a eukaryotic cell need both membranous organelles and cytoskeleton? 15. What are the applications of chromatography and electrophoresis?

1. Explain briefly: (a) Fractionation, (b) Microdissection, (c) Tissue culture


BIOLOGY XI: CHAPTER 1 , CELL STRUCTURE AND FUNCTIONS

37

SECTION III : EXTENSIVE QUESTIONS (d) Differential staining, (e) Centrifugation, (f) Chromatography, (g) Electrophoresis, (h) Spectrophotometry 2) How the size of a cell is measured under microscope? 3) Describe the structure and roles of plant cell wall. 4) What are the functions of the plasma membrane proteins? 5) What is the chemical nature of cytoplasm? Explain the metabolic roles of cytoplasm. 6) Discuss the structure and functions of: (a) Endoplasmic reticulum, (b) Mitochondrion, (c) Chloroplast 7) What are the types of cytoskeleton elements of proteinous fibres? Write the structure composition and functions of each. 8) Explain the structure of cilia and flagella and the mechanism of their movement. 9) Compare and contrast the structure and functions of mitochondria and chloroplasts. 10)Describe the structure of typical prokaryotic cell?

ANSWER MCQS 1. C

2. D 3. A 4. D 5. D 6. D

7. D 8. B

9. D 10. A 11. B

SUPPLEMENTARY READING MATERIAL 1. Campbell N.A. Mhchell, L.G. & Reece J.B., Biology Concepts and connections, 2nd edition Benjamin/Cummings Company California, 2003

USEFUL WEBSITES 2. Madar, S.S., Biology, 6th edition, WCB, McGraw-Hill, USA, 1998 1) en.wikipedia.org/wiki/Biological_tissue


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