Chapter 2 — ଜୀବକୋଷ ଓ ଏହାର ସଂଗଠନ (Cell and Its Structure)

Cells are too small to be seen with the naked eye, so the story of the cell begins with the invention of the microscope. In 1665 the English scientist Robert Hooke examined a thin slice of cork under a compound microscope he had built himself. He saw that the cork was made of countless tiny box like compartments, which reminded him of the small rooms of a monastery, and he named them cells (Latin cella, a small room). What Hooke saw were in fact only the dead cell walls of the cork, but the name stuck. A few years later, in 1674, the Dutch lens grinder Antonie van Leeuwenhoek used his simple microscope of a single powerful lens to observe living cells for the first time: bacteria, protozoa in pond water, red blood cells and sperm, which he called animalcules.

For the next 150 years cells were studied piecemeal. In 1831 Robert Brown discovered the nucleus in the cells of orchid leaves. In 1839 the Czech scientist J. E. Purkinje gave the name protoplasm to the living jelly like substance inside the cell. The decisive step came in 1838 and 1839, when two Germans, the botanist Matthias Schleiden and the zoologist Theodor Schwann, brought their observations together. Schleiden found that all plants are made of cells, and Schwann found the same for animals. Together they proposed the cell theory: (1) all living organisms are composed of cells and the products of cells, and (2) the cell is the basic structural and functional unit of life.

Schwann had believed that new cells arise spontaneously from a fluid. This was corrected in 1855 by Rudolf Virchow, who stated Omnis cellula e cellula, every cell arises from a pre-existing cell. This became the third principle of the cell theory. The modern cell theory therefore states:

• All living things are made up of one or more cells.
• The cell is the basic unit of structure and function in all organisms; the activities of an organism are the sum of the activities of its cells.
• All cells arise from pre-existing cells by division.
• Cells contain hereditary information (DNA) which is passed from cell to cell during division.

The cell theory has only one apparent exception: viruses, which are not made of cells and show the properties of life only inside a host cell. The invention of the electron microscope in the 1930s, with a magnification of several lakh times compared to about 2,000 times for a light microscope, revealed the fine structure of the organelles described in this chapter and confirmed the theory in every detail.

Organisms differ greatly in the number of cells they contain. In unicellular organisms such as bacteria, Amoeba, Paramecium, Chlamydomonas and yeast, the single cell is the whole organism and performs every function of life: it feeds, breathes, excretes, moves, grows and reproduces. In multicellular organisms such as fungi, plants and animals, the body is made of many cells, from a few hundred in a tiny worm to about 37 trillion in an adult human. In these organisms cells become specialised for particular tasks, a principle called division of labour, and groups of similar cells form tissues, tissues form organs, and organs form organ systems. A muscle cell contracts, a nerve cell conducts impulses, a red blood cell carries oxygen; each depends on the others, and none can live alone for long.

The shape of a cell is related to its function. Red blood cells are biconcave discs, which gives a large surface for oxygen exchange and lets them squeeze through capillaries. Nerve cells are extremely long with branching processes, so that they can carry messages across the body; a single neuron may run from the spinal cord to the toe. Muscle cells are spindle shaped or cylindrical so that they can shorten. White blood cells and Amoeba constantly change shape as they crawl and engulf particles. Plant cells with rigid walls are polygonal, and guard cells of stomata are bean shaped so that they can open and close the pore. Sperm cells have a tail for swimming, and egg cells are large and round to hold stored food.

The size of cells is measured in micrometres (1 µm = one thousandth of a millimetre). Most cells are between 1 and 100 µm and are invisible to the eye. Bacteria are about 1 to 5 µm, human red blood cells about 7 µm, a typical plant cell 10 to 100 µm. The smallest cells known are the bacteria called mycoplasma, only 0.1 to 0.5 µm across. At the other extreme, the egg of an ostrich, about 15 cm long, is a single cell and the largest cell known; a nerve cell of the giraffe may be more than a metre in length; and the fibre of ramie, a plant, is a single cell up to 55 cm long. Cell size is limited by the need for oxygen and nutrients to diffuse to the interior: as a cell grows, its volume increases faster than its surface, so very large cells would starve at the centre. This is why organisms grow by increasing the number of cells rather than their size, and why active cells tend to be small.

Whatever their size and shape, all cells have the same basic plan: a plasma membrane enclosing cytoplasm in which the genetic material lies, either free in prokaryotes or inside a nucleus in eukaryotes. The rest of the chapter examines these parts one by one.

On the basis of the organisation of the nucleus, all cells fall into two great types. A prokaryotic cell (Greek pro, before, and karyon, nucleus) has no true nucleus. Its genetic material is a single, circular molecule of DNA lying in a region of the cytoplasm called the nucleoid, with no nuclear membrane around it and no nucleolus. It also lacks all membrane bound organelles: there are no mitochondria, no plastids, no endoplasmic reticulum, no Golgi apparatus and no lysosomes. The only organelles present are ribosomes, and these are of the smaller 70S type. Prokaryotic cells are small, usually 1 to 10 µm, and are enclosed by a cell wall of peptidoglycan. Many also carry small extra rings of DNA called plasmids. In the photosynthetic cyanobacteria, chlorophyll lies in folds of the membrane rather than in chloroplasts. Bacteria and cyanobacteria (blue-green algae) are prokaryotes, and they make up Kingdom Monera.

A eukaryotic cell (Greek eu, true) has a true nucleus: the DNA is organised into several linear chromosomes, associated with proteins called histones, and enclosed in a double nuclear membrane with pores; one or more nucleoli are present. The cytoplasm contains a whole set of membrane bound organelles, each performing a specific function: mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles and, in plants, plastids. Ribosomes are larger (80S). Eukaryotic cells are generally larger, 10 to 100 µm, and have a cytoskeleton of protein fibres that maintains shape and moves organelles about. The cell wall, when present, is of cellulose (plants) or chitin (fungi). Protists, fungi, plants and animals are all eukaryotes.

The division between prokaryotes and eukaryotes is the deepest in the living world. Prokaryotes appeared about 3.5 billion years ago and eukaryotes about 1.5 to 2 billion years ago. It is widely accepted that mitochondria and chloroplasts were once free living bacteria that were engulfed by an early eukaryotic cell and stayed on as partners; this idea, the endosymbiotic theory, is supported by the fact that both organelles have their own circular DNA and 70S ribosomes like bacteria and divide independently within the cell.

Every cell is bounded by a thin, delicate, living covering called the plasma membrane or cell membrane. It is only about 7 to 10 nanometres thick and cannot be resolved by the light microscope; under the electron microscope it appears as three layers, a dark, a light and a dark band. According to the fluid mosaic model proposed by Singer and Nicolson in 1972, the membrane consists of a double layer (bilayer) of phospholipid molecules in which protein molecules are embedded or float like icebergs in a sea. Each phospholipid has a water loving (hydrophilic) head that faces outward towards the watery surroundings and two water hating (hydrophobic) tails that point inward, away from water. The lipids are free to move sideways, so the membrane is fluid and flexible, and the proteins scattered through it give it the appearance of a mosaic. Some proteins act as channels or carriers for substances, some as enzymes, and some as receptors that recognise hormones and other signals. Carbohydrate chains attached to the outer surface help cells recognise one another.

The most important property of the plasma membrane is that it is selectively permeable (semi-permeable): it allows some substances to pass through freely, lets others through only with help, and stops the rest. Water, oxygen and carbon dioxide diffuse across easily; glucose and amino acids enter through carrier proteins; large molecules such as proteins cannot cross at all. In this way the membrane maintains the internal composition of the cell, which is quite different from its surroundings, and controls the traffic of materials in and out.

Functions of the plasma membrane may be summarised as follows:

• It gives the cell a definite shape and holds the cytoplasm and organelles within.
• It regulates the entry and exit of substances by diffusion, osmosis, facilitated transport and active transport.
• It protects the cell contents from the surroundings.
• In animal cells it can fold inward to engulf food or fluid, a process called endocytosis (phagocytosis for solids, as when Amoeba takes in food or a white blood cell engulfs a bacterium; pinocytosis for liquids), and can fuse with vesicles to expel material by exocytosis.
• Its receptors receive chemical messages and allow cells to communicate.
• In nerve and muscle cells it conducts electrical impulses.

The plasma membrane is a living structure, constantly repaired and renewed, and it can be damaged by heat, strong chemicals and lack of oxygen. When the membrane breaks, the cell dies. In plant cells the plasma membrane lies just inside the cell wall and, when the cell loses water, it shrinks away from the wall, showing that the two are separate structures.

Substances move into and out of the cell across the plasma membrane by several processes. The simplest is diffusion, the movement of molecules of a substance from a region of its higher concentration to a region of its lower concentration until the concentration is equal everywhere. Diffusion is driven by the random motion of molecules and needs no energy from the cell. It is how oxygen enters a cell from the surrounding fluid, where its concentration is higher, and how carbon dioxide produced by respiration leaves. Gases, small uncharged molecules and fat soluble substances diffuse directly through the lipid bilayer; glucose and ions pass through protein channels or are carried by carrier proteins, still down the concentration gradient, which is called facilitated diffusion.

Osmosis is a special case of diffusion: the movement of water molecules through a selectively permeable membrane from a solution of lower solute concentration (more water, a dilute solution) to a solution of higher solute concentration (less water, a concentrated solution). The membrane lets water through but not the dissolved substance, so water flows towards the side with more solute. Three situations arise when a cell is placed in a solution:

• In a hypotonic solution (less concentrated than the cell, for example pure water), water enters the cell by osmosis; the cell swells. An animal cell such as a red blood cell may burst (haemolysis); a plant cell becomes firm or turgid because the cell wall resists expansion.
• In an isotonic solution (same concentration as the cell), water enters and leaves at equal rates and there is no net change; the cell keeps its shape.
• In a hypertonic solution (more concentrated than the cell, for example strong salt or sugar solution), water leaves the cell; an animal cell shrinks and crumples, and in a plant cell the cytoplasm and plasma membrane pull away from the cell wall, a condition called plasmolysis. The cell becomes flaccid.

Osmosis explains many everyday events. Roots absorb water from the soil because the cell sap is more concentrated than soil water. A wilted plant recovers when watered as its cells regain turgor. Salt sprinkled on cut vegetables draws out water. Raisins swell in water and grapes shrivel in sugar syrup. Pickles and jams keep because the strong salt or sugar draws water out of any microbe that lands on them. Unicellular fresh water organisms constantly take in water by osmosis and pump it out with contractile vacuoles.

Some substances must be moved against their concentration gradient, from low to high concentration, for example minerals taken up by roots from dilute soil solution, or potassium ions concentrated inside nerve cells. This is active transport, carried out by carrier proteins in the membrane using energy from ATP. Unlike diffusion and osmosis, which are physical processes that occur in dead systems too, active transport is a property of living cells only and stops when respiration stops.

Plant cells, fungal cells and bacterial cells possess a rigid, non living outer covering outside the plasma membrane called the cell wall. Animal cells have none. In plants the wall is made chiefly of cellulose, a complex carbohydrate made of thousands of glucose units joined in long chains that bundle into strong fibres; the fibres are embedded in a matrix of other carbohydrates (hemicellulose and pectin). In fungi the wall is of chitin, and in bacteria of peptidoglycan. The cell wall is secreted by the living protoplast within, and it is what Robert Hooke actually saw in cork.

A young plant cell first lays down a thin, elastic primary wall that can stretch as the cell grows. Between the primary walls of two neighbouring cells lies the middle lamella, a layer of calcium and magnesium pectate that cements the cells together; when a fruit ripens, enzymes dissolve the middle lamella and the flesh becomes soft. When the cell has finished growing, many cells add a thick secondary wall inside the primary wall, strengthened with lignin, which makes wood hard, or with suberin, which makes cork waterproof. Fine cytoplasmic threads called plasmodesmata pass through minute pores in the walls and connect the cytoplasm of adjacent cells, so that water and food can move from cell to cell throughout the plant.

The cell wall performs several functions:

• It gives the cell a fixed shape and mechanical strength; the polygonal outline of plant cells is due to the wall.
• It protects the delicate protoplast from injury and from attack by microbes.
• It prevents the cell from bursting when it takes in water by osmosis; instead the cell becomes turgid, and the turgor of millions of cells is what keeps a herbaceous plant upright and its leaves spread. When water is lost, turgor falls and the plant wilts.
• It is fully permeable to water and dissolved substances, unlike the selectively permeable plasma membrane, so it does not control what enters the cell; that is the membrane's job.
• Thick lignified walls of xylem and sclerenchyma give wood and fibres their strength; the whole trunk of a tree is largely cell wall.

The cell wall also has practical importance for people. Cotton, jute, linen and coir are cellulose walls of fibre cells; paper is a mat of cellulose fibres; timber is lignified walls; and the dietary fibre that keeps our digestion healthy is cell wall material that human enzymes cannot digest. Cellulose is the most abundant organic substance on the Earth, and cattle, goats and termites can use it only with the help of bacteria and protozoa living in their gut that produce the enzyme cellulase.

The nucleus is the largest and most conspicuous organelle of the eukaryotic cell, usually spherical or oval and 5 to 10 µm across, and stained darkly by dyes such as iodine and methylene blue. It was discovered by Robert Brown in 1831. It is often called the control centre or brain of the cell because it contains the hereditary material that directs every activity of the cell. Most cells have one nucleus; Paramecium has two, skeletal muscle fibres and some fungi have many, and mature red blood cells of mammals and the sieve tube cells of plants have lost theirs.

The nucleus has four parts. The nuclear membrane (nuclear envelope) is a double membrane, each layer similar to the plasma membrane, with a narrow space between; the outer membrane is continuous with the endoplasmic reticulum. It is pierced by numerous nuclear pores through which RNA and proteins pass between the nucleus and the cytoplasm. Inside is the nucleoplasm (karyolymph), a clear jelly containing nucleotides, enzymes and the other components. Suspended in it is the chromatin, a network of long, thin, thread like fibres of DNA (deoxyribonucleic acid) wound around histone proteins. When the cell prepares to divide, the chromatin threads coil and condense into short, thick, rod shaped bodies called chromosomes, which can be seen under a light microscope. Each species has a fixed number of chromosomes: 46 in humans, 24 in rice, 20 in maize, 8 in the fruit fly and 78 in the dog. Chromosomes carry genes, segments of DNA each of which carries the information for making one protein and thus controls one character; the gene is the unit of heredity. The nucleolus is a dense, round body, one or more per nucleus, not bounded by a membrane; it manufactures the ribosomal RNA and assembles the ribosome subunits.

The functions of the nucleus follow from its contents:

• It controls all the metabolic activities of the cell by directing the synthesis of proteins and enzymes according to the instructions in DNA.
• It stores hereditary information and passes it on to daughter cells during cell division; the exact duplication of chromosomes ensures that each daughter receives a full set.
• It transmits characters from parents to offspring through the gametes.
• It initiates and regulates cell division and hence growth and repair.
• The nucleolus makes ribosomes.

The importance of the nucleus is shown by removing it: an Amoeba from which the nucleus is taken out stops feeding and dividing and dies within a few days, but if the nucleus is put back it recovers. A red blood cell, which loses its nucleus at maturity, cannot repair itself and lives only about 120 days. In prokaryotes, as we saw, there is no nucleus, only a nucleoid region of naked DNA.

The cytoplasm is the jelly like, semi transparent substance that fills the cell between the plasma membrane and the nucleus. Together with the nucleus it makes up the protoplasm, the living matter of the cell, which Huxley called the physical basis of life. The fluid part of the cytoplasm, the cytosol, is about 70 to 90 per cent water with dissolved salts, sugars, amino acids, proteins and enzymes; it is the site of many chemical reactions, including the first stage of respiration (glycolysis). Suspended in it are the organelles, each a small living structure with a definite job, and non living inclusions such as starch grains, oil droplets and crystals. In plant cells the cytoplasm shows a slow streaming movement called cyclosis, easily seen in the cells of a Hydrilla leaf, which carries materials around the cell.

The endoplasmic reticulum (ER) is a network of membranous tubes and flattened sacs (cisternae) that runs through the cytoplasm from the nuclear membrane, with which it is continuous, almost to the plasma membrane. It was discovered with the electron microscope by Porter in 1945. There are two kinds. Rough endoplasmic reticulum (RER) has ribosomes attached to its outer surface, which give it a rough, granular appearance; the proteins made by these ribosomes pass into the ER channels, where they are folded and sent on to the Golgi apparatus. Cells that secrete a lot of protein, such as the enzyme producing cells of the pancreas and antibody producing plasma cells, are packed with RER. Smooth endoplasmic reticulum (SER) has no ribosomes; it synthesises lipids, steroids and the phospholipids of membranes, stores calcium in muscle cells, and in liver cells it detoxifies drugs and poisons. The ER as a whole acts as the transport system of the cell, carrying substances from one region to another, and provides an internal framework that supports the cytoplasm and gives the cell mechanical strength. Its membranes also serve as the site for the biogenesis of new membranes.

Ribosomes are minute, dense, spherical granules, about 20 to 25 nm across, first described by George Palade in 1955. They are not bounded by a membrane and are found in all cells, prokaryotic and eukaryotic, either attached to the RER or lying free in the cytoplasm; free ribosomes make proteins for use inside the cell, attached ribosomes make proteins for export. Each ribosome is made of ribosomal RNA and protein in two subunits, one large and one small, which fit together around a strand of messenger RNA. Eukaryotic ribosomes are 80S (subunits 60S and 40S) and prokaryotic ribosomes are 70S (50S and 30S), where S is the Svedberg unit of sedimentation. Ribosomes are the protein factories of the cell: they read the message copied from a gene and join amino acids in the correct order to build the protein. Several ribosomes often work along one messenger RNA at once, forming a chain called a polyribosome or polysome.

The Golgi apparatus (Golgi body or Golgi complex) was discovered by the Italian scientist Camillo Golgi in 1898 in nerve cells stained with silver salts. It consists of a stack of four to eight flattened, curved, membrane bound sacs called cisternae, piled like a stack of plates, surrounded by small round vesicles. The stack has two faces: the convex forming (cis) face, which receives vesicles budded off from the endoplasmic reticulum, and the concave maturing (trans) face, from which finished vesicles pinch off. In plant cells the Golgi apparatus occurs as many small scattered units called dictyosomes. It is absent in prokaryotes and in mammalian red blood cells.

The Golgi apparatus is the packaging and dispatch centre of the cell. Proteins and lipids made on the ER are delivered to it, and inside the cisternae they are modified, for example by attaching sugar chains to form glycoproteins, sorted according to their destination, and packed into vesicles. Some vesicles fuse with the plasma membrane and release their contents outside by exocytosis; this is how enzymes, hormones such as insulin, mucus and antibodies are secreted. Other vesicles become lysosomes. In plant cells the Golgi apparatus manufactures the pectin and hemicellulose of the cell wall and, during cell division, its vesicles gather at the middle of the dividing cell to form the cell plate that becomes the new wall. It also forms the acrosome at the tip of a sperm cell, which carries enzymes to penetrate the egg.

Lysosomes (Greek lysis, breaking down, and soma, body) were discovered by Christian de Duve in 1955. They are small, spherical, single membrane bound sacs, 0.2 to 0.8 µm across, formed by the Golgi apparatus and filled with about forty powerful hydrolytic enzymes that can digest proteins, carbohydrates, fats and nucleic acids. The enzymes work best in acidic conditions, and the interior of the lysosome is kept acidic. Lysosomes are abundant in animal cells, especially white blood cells and liver cells, and less common in plant cells, where the vacuole performs part of their role.

Lysosomes are the digestive system of the cell. They fuse with food vacuoles formed by endocytosis and digest the contents, so that a white blood cell can destroy the bacteria it engulfs. They break down worn out organelles and recycle their materials, a process called autophagy. During starvation they digest stored food to keep the cell alive. In the developing embryo they dissolve the webbing between the fingers, and in a tadpole they digest the tail during metamorphosis. When a cell is damaged or dies, the lysosome membranes rupture and the released enzymes digest the whole cell; because of this capacity for self destruction lysosomes are called the suicide bags of the cell. Defective lysosomal enzymes cause inherited storage diseases in which undigested material accumulates.

Mitochondria (singular mitochondrion; Greek mitos, thread, and chondrion, granule) are rod shaped, oval or spherical organelles, 0.5 to 1 µm wide and 2 to 10 µm long, found in the cytoplasm of all eukaryotic cells. They were first observed by Kolliker in 1880 in insect muscle and named by Benda in 1898. Their number varies with the energy needs of the cell: a few in a resting cell, hundreds in a liver cell, and thousands in a muscle cell or the tail of a sperm. Under the light microscope they appear as tiny granules or threads, visible after staining with Janus green B, which they turn blue green because of their oxidising enzymes.

The electron microscope shows that a mitochondrion is bounded by two membranes. The outer membrane is smooth and freely permeable to small molecules. The inner membrane is folded inward into shelf like or finger like projections called cristae, which greatly increase its surface area; on the cristae are the enzyme complexes and the electron transport chain that carry out the final stages of respiration. The space enclosed by the inner membrane is filled with a dense fluid, the matrix, which contains the enzymes of the Krebs cycle, ribosomes of the 70S type, and a small circular molecule of mitochondrial DNA. Because they have their own DNA and ribosomes, mitochondria can make some of their own proteins and can divide by fission independently of the nucleus; they are called semi autonomous organelles, and mitochondrial DNA is inherited only from the mother through the egg.

The function of the mitochondrion is aerobic respiration: the complete oxidation of food, chiefly glucose, using oxygen, to release energy. Glucose is first split in the cytoplasm to pyruvic acid; the pyruvic acid enters the mitochondrion, where it is broken down in the matrix to carbon dioxide and water, and the energy released is captured in the bonds of ATP (adenosine triphosphate) on the cristae. One molecule of glucose yields up to 38 molecules of ATP, of which 36 are made in the mitochondrion. ATP is the universal energy currency of the cell: it is carried to wherever energy is needed, for muscle contraction, active transport, protein synthesis, nerve impulses, cell division, and gives up its energy by losing one phosphate group to become ADP, which returns to the mitochondrion to be recharged. Because they generate this energy, mitochondria are called the powerhouses of the cell. They also store calcium and take part in the synthesis of certain fats and amino acids. Their absence in prokaryotes, which respire on the plasma membrane, and their bacteria like DNA and ribosomes are the basis of the endosymbiotic theory, which holds that mitochondria descend from free living aerobic bacteria that entered an ancestral cell about two billion years ago.

Plastids are large, double membrane bound organelles found only in plant cells and in photosynthetic protists such as Euglena and algae; they are absent in animal cells, fungi and prokaryotes. The name was given by Haeckel in 1866. All plastids develop from small colourless bodies called proplastids in the young cells of the growing points, and they can change from one type to another. Like mitochondria, plastids have their own circular DNA and 70S ribosomes, divide by fission and are thought to descend from cyanobacteria taken in by an early eukaryotic cell. On the basis of the pigments they contain, plastids are of three kinds.

Chloroplasts (Greek chloros, green) contain the green pigment chlorophyll along with yellow and orange carotenoids. They occur in the green parts of the plant, above all in the mesophyll cells of the leaf, where each cell may hold 20 to 100 chloroplasts. In higher plants they are lens shaped or oval, 4 to 10 µm long; in algae they may be spiral (Spirogyra), cup shaped (Chlamydomonas) or star shaped. A chloroplast has a smooth outer membrane and an inner membrane enclosing a colourless fluid, the stroma, which contains the enzymes of the dark reaction, DNA, ribosomes and starch grains. Suspended in the stroma is a third system of membranes: flattened sacs called thylakoids stacked in piles called grana (singular granum), connected by tubes called stroma lamellae. Chlorophyll is embedded in the thylakoid membranes. Chloroplasts are the site of photosynthesis: the light reaction on the grana traps solar energy and splits water, releasing oxygen, and the dark reaction in the stroma uses that energy to fix carbon dioxide into glucose. They are called the kitchens of the cell, and they are the source of almost all the food and oxygen on Earth.

Chromoplasts (Greek chroma, colour) contain carotenoid pigments, yellow xanthophyll and orange or red carotene, but no chlorophyll. They give colour to petals (marigold, sunflower), fruits (tomato, chilli, orange) and roots (carrot). Their function is to attract insects for pollination and animals for seed dispersal. Chloroplasts often change into chromoplasts as fruits ripen, which is why a green tomato turns red and a green mango turns yellow, and why leaves turn yellow and orange in autumn when chlorophyll breaks down and the carotenoids are revealed.

Leucoplasts (Greek leukos, white) are colourless plastids found in the cells of roots, tubers, seeds and other parts not exposed to light. They store food: amyloplasts store starch (potato tuber, rice grain, wheat endosperm), elaioplasts store oil (mustard, groundnut, castor seed) and aleuroplasts or proteinoplasts store protein (pulses). A leucoplast exposed to light develops chlorophyll and becomes a chloroplast, as when a potato left in sunlight turns green.

Vacuoles are fluid filled spaces in the cytoplasm bounded by a single membrane called the tonoplast. In a mature plant cell there is usually one enormous central vacuole that occupies 50 to 90 per cent of the cell volume and pushes the cytoplasm and nucleus into a thin layer against the wall; young plant cells have many small vacuoles that merge as the cell grows. The vacuole is filled with cell sap, a watery solution of sugars, salts, amino acids, organic acids, pigments and waste products. Its functions are several. It stores water and dissolved food and wastes. Its high solute concentration draws water in by osmosis and keeps the cell turgid, providing support to soft plant parts; when the vacuole loses water the plant wilts. The water soluble pigments called anthocyanins dissolved in the sap give the red, blue and purple colours of beetroot, rose petals, brinjal skin and red cabbage. In many plants the vacuole stores poisonous or bitter substances (tannins, alkaloids) that discourage grazing animals, and in some it digests materials like a lysosome. Animal cells have only small, temporary vacuoles: food vacuoles in Amoeba and white blood cells, in which engulfed food is digested, and contractile vacuoles in fresh water protozoa, which collect excess water entering by osmosis and pump it out, preventing the cell from bursting.

The centrosome is a small region of clear cytoplasm near the nucleus of animal cells, containing two tiny cylinders set at right angles to each other called centrioles. Each centriole is made of nine triplets of microtubules arranged in a ring. Just before cell division the centrioles duplicate, and the two pairs move to opposite poles of the cell, where they organise the spindle fibres that pull the chromosomes apart. Centrioles also give rise to the basal bodies of cilia and flagella. Centrosomes are present in animal cells and some lower plants but are absent from the cells of higher plants, which form their spindle without them.

Other structures deserve brief mention. The cytoskeleton is a network of protein fibres, microtubules and microfilaments, that gives the cell its shape, anchors the organelles, moves them around, and forms the tracks along which vesicles travel. Cilia and flagella are hair like projections from the cell surface with a core of microtubules in a 9 + 2 pattern; cilia are short and numerous (Paramecium, the lining of the windpipe where they sweep out dust), flagella are long and few (sperm, Euglena). Peroxisomes are small enzyme sacs that break down hydrogen peroxide; glyoxysomes in germinating oil seeds convert fat to sugar. Finally, cells contain non living inclusions: starch grains, glycogen granules, oil droplets and crystals of calcium oxalate, such as the needle like raphides that make raw yam and colocasia itch.

Plant and animal cells are both eukaryotic and share the same fundamental parts: plasma membrane, cytoplasm, nucleus, mitochondria, endoplasmic reticulum, ribosomes, Golgi apparatus and cytoskeleton. Yet the two are easy to tell apart under a microscope, because each has structures the other lacks, reflecting their different ways of life. A plant is fixed in one place, makes its own food and supports itself by turgor and cell walls; an animal moves, eats and supports itself with a skeleton.

A typical plant cell is enclosed in a rigid cell wall of cellulose outside the plasma membrane, which gives it a fixed, usually polygonal shape. It contains plastids, above all chloroplasts in the green parts, which carry out photosynthesis. A mature plant cell has a single large central vacuole filled with cell sap, so that the cytoplasm is a thin film against the wall and the nucleus is pushed to one side. It stores carbohydrate as starch. Higher plant cells have no centrioles, and lysosomes are few or absent; the vacuole and small vesicles perform digestion. The Golgi apparatus is present as scattered dictyosomes. Neighbouring cells are connected through plasmodesmata.

A typical animal cell has no cell wall; it is bounded only by the flexible plasma membrane, so it is rounded or irregular and can change shape, as Amoeba and white blood cells do. It has no plastids and cannot make its own food. Vacuoles are absent or small and temporary. The nucleus usually lies near the centre. It stores carbohydrate as glycogen, not starch. A centrosome with two centrioles lies beside the nucleus and forms the spindle during division, and lysosomes are numerous. Animal cells divide by pinching in the middle (cleavage furrow), whereas plant cells divide by laying down a cell plate.

These differences are the standard examination question on this chapter and are best learnt together with the labelled diagrams of the two cells. Students should be able to draw both cells, marking in the plant cell the cell wall, plasma membrane, cytoplasm, nucleus with nucleolus, chloroplasts, central vacuole, mitochondria, ER, ribosomes and Golgi body, and in the animal cell the plasma membrane, cytoplasm, nucleus, mitochondria, ER, ribosomes, Golgi body, lysosomes and centrosome.

Virchow's rule that every cell comes from a cell is fulfilled by cell division, in which one parent cell gives rise to two or more daughter cells. Division has two parts: division of the nucleus (karyokinesis) followed by division of the cytoplasm (cytokinesis). Before a cell divides it passes through interphase, a long period of growth during which every chromosome makes an exact copy of its DNA, so that each chromosome comes to consist of two identical chromatids joined at a point called the centromere. Eukaryotic cells divide in two ways.

Mitosis (equational division) occurs in the body (somatic) cells and produces two daughter cells each with exactly the same number of chromosomes as the parent (2n → 2n). It is a continuous process but is described in four stages. In prophase the chromatin condenses into visible chromosomes, each of two chromatids; the nucleolus and nuclear membrane disappear; the centrioles (in animal cells) move to opposite poles and spindle fibres form between them. In metaphase the chromosomes line up at the equator of the spindle, attached to the fibres by their centromeres; this is the stage at which chromosomes are best seen and counted. In anaphase the centromeres split, and the two chromatids of each chromosome, now called daughter chromosomes, are pulled to opposite poles by the shortening spindle fibres. In telophase the chromosomes reach the poles and uncoil, the nuclear membrane and nucleolus re-form, and two nuclei result. Cytokinesis follows: in animal cells the membrane pinches inward as a cleavage furrow; in plant cells a cell plate forms at the middle from Golgi vesicles and grows outward to become the new wall. Mitosis is the means of growth of the organism from a single fertilised egg, of replacement of worn out cells (skin, blood, gut lining), of repair of wounds, and of asexual reproduction in organisms such as Amoeba, yeast and Hydra. Because it copies chromosomes exactly, it keeps every body cell genetically identical.

Meiosis (reduction division) occurs only in the reproductive cells of the gonads (testis, ovary, anther, ovule) and produces gametes, sperm and eggs, pollen and egg cells. One diploid cell divides twice in succession to give four daughter cells, each with half the chromosome number of the parent (2n → n). In the first division (meiosis I) the homologous chromosomes, one from each parent, pair up, exchange segments (crossing over) and are then separated into two cells, halving the number; in the second division (meiosis II), which is like a mitosis, the chromatids separate. Meiosis is essential to sexual reproduction: if gametes carried the full number, the chromosome count would double every generation. When a haploid sperm (23 chromosomes in man) fuses with a haploid egg (23), the zygote regains the diploid number (46). Crossing over and the random assortment of chromosomes also shuffle the genes, so that offspring differ from their parents and from one another, which is the source of the variation on which evolution works.