Chapter 1 — Cell its Structure and Functions

Cells are so small that nobody could see one until the microscope was invented. In 1665 the English scientist Robert Hooke examined a thin slice of cork (the bark of a tree) under a simple microscope that he had built himself. He saw that the cork was made of many tiny box-like compartments separated by walls, rather like the small rooms in which monks lived. He called these compartments cells, from the Latin word cellula meaning a small room. What Hooke actually saw were the dead, empty cell walls of cork, but the name has stayed with us.

A few years later, in 1674, the Dutch cloth merchant Anton van Leeuwenhoek, who ground his own lenses, observed living cells for the first time: he saw tiny organisms swimming in pond water, bacteria from tooth scrapings, red blood cells and sperm cells. He called the moving organisms animalcules. This was the first proof that there are living beings too small for the eye to see.

Over the next century and a half better microscopes revealed more and more. In 1831 the Scottish botanist Robert Brown discovered a dense round body inside the cells of orchids and named it the nucleus. In 1839 the Czech scientist Purkinje gave the name protoplasm to the living jelly-like material inside the cell. These observations prepared the ground for the cell theory.

The importance of these discoveries lies in a simple idea: living things are not a continuous mass of matter; they are built up of units. Once scientists knew that, they could ask what each unit does and how units of different kinds work together to make a leaf, a muscle or a brain. The compound microscope you use in your laboratory magnifies objects about 100 to 400 times, and the electron microscope invented in the twentieth century magnifies up to a few lakh times, which is how the fine structure of organelles described later in this chapter was worked out.

A useful timeline to remember: Hooke, 1665, dead cork cells and the word cell; Leeuwenhoek, 1674, living cells and bacteria; Brown, 1831, nucleus; Purkinje, 1839, protoplasm; Schleiden and Schwann, 1838 to 1839, cell theory; Virchow, 1855, cells come from cells.

Between 1838 and 1839 two German scientists put together the observations of many years into a general statement about life. Matthias Schleiden, a botanist, studied many plants and concluded in 1838 that every plant is made up of cells. Theodor Schwann, a zoologist, studied animal tissues and in 1839 reported that animals too are made of cells, and that the cells of animals have a thin outer layer, which we now call the plasma membrane. Together they proposed the cell theory.

The cell theory as they stated it has two parts: (1) all plants and animals are composed of cells, and (2) the cell is the basic unit of life. In 1855 Rudolf Virchow added the third part, which is often quoted in Latin as omnis cellula e cellula, meaning all cells arise from pre-existing cells. Before Virchow many people believed that cells could form spontaneously from non-living fluids; Virchow showed that new cells are produced only by the division of existing cells.

The modern cell theory therefore states:

• All living organisms are made up of one or more cells.
• The cell is the structural and functional unit of life.
• All cells arise from pre-existing cells by cell division.

Why is the cell called the structural unit? Because just as a house is built of bricks, an organism is built of cells; if you take an organism apart you reach cells, and below the cell level you find only molecules, which are not alive. Why is it called the functional unit? Because every activity of the organism, such as breathing, digestion or movement, is really the sum of activities of cells: gas exchange happens across the membranes of individual cells, digestive enzymes are made by individual gland cells, and contraction is done by individual muscle cells.

The cell theory has a few exceptions and refinements that you will meet in higher classes, but its power lies in its universality. A cell of a mango tree, of a frog and of a human being share the same basic plan: a membrane, cytoplasm and genetic material. This shared plan is strong evidence that all life on Earth is related.

The best way to believe the cell theory is to see cells yourself. Two easy specimens are the thin skin of an onion and the cells lining the inside of your cheek.

Onion peel mount. Take a piece of onion bulb and, with forceps, peel off the thin transparent membrane from the inner concave surface of a fleshy scale leaf. Place it flat in a drop of water on a clean glass slide. Add a drop of dilute iodine solution or safranin to stain it; without a stain the transparent cells are hard to see. Gently lower a cover slip with a needle so that no air bubbles are trapped, remove the extra liquid with blotting paper, and observe first under the low-power objective and then under high power. You will see rectangular, brick-like cells fitted closely together in rows. Each cell has a distinct boundary, the cell wall, and a small round dark body, the nucleus, which takes the stain strongly. The cytoplasm appears as a lightly stained region, and in many cells a large clear space, the vacuole, occupies most of the interior.

Cheek cell mount. Rinse your mouth, then gently scrape the inside of your cheek with the blunt end of a clean toothpick or spatula. Spread the scraping in a drop of water on a slide, add a drop of methylene blue, place a cover slip and observe. Cheek cells are flat, irregular in outline and lie separately or in small groups. Each has a stained nucleus near the centre and a thin boundary, the plasma membrane, but no thick wall and no large vacuole.

Comparing the two. The onion cells are regular and box-shaped because the rigid cell wall holds them in position; the cheek cells are irregular and soft because animal cells have only a flexible membrane. Both have a nucleus and cytoplasm, which shows the common plan of all cells. This simple activity therefore demonstrates in one sitting the two great classes of cells you will study: plant cells and animal cells.

Precautions: the peel should be a single layer, the stain should not be too dark, the cover slip must be placed at an angle to avoid bubbles, and the specimen must not dry out while you observe it.

Cells are not all alike. They differ in shape, in size and in number, and each difference is related to the work the cell does or to the kind of organism it belongs to.

Shape. Cells may be spherical, oval, rectangular, spindle-shaped, disc-shaped, polygonal, columnar or branched. The shape usually fits the function. A nerve cell is long with many branches so that it can carry messages over long distances; the longest nerve cells in the human leg are about a metre long. A red blood cell is a biconcave disc, which gives it a large surface for exchange of gases and lets it squeeze through narrow capillaries. Muscle cells are long and spindle-shaped so that they can contract along their length. White blood cells can change their shape to engulf germs, and the amoeba constantly changes shape to move and feed. Plant cells with rigid walls are usually polygonal or rectangular. Cells that have no fixed shape, like amoeba and white blood cells, are described as amoeboid.

Size. Most cells are microscopic, between 1 and 100 micrometres (a micrometre, written um, is one thousandth of a millimetre). Bacteria are among the smallest cells, about 0.5 to 5 um. Human red blood cells are about 7 um across. But some cells are visible to the naked eye: the egg of a hen is a single cell, and an ostrich egg, about 15 cm long, is the largest known cell. The size of a cell does not depend on the size of the organism; an elephant does not have bigger cells than a mouse, it has more of them.

Number. Organisms made of a single cell are unicellular, such as amoeba, paramecium, euglena, chlamydomonas, yeast and bacteria. Organisms made of many cells are multicellular, such as fungi, most plants and all animals. A human adult contains roughly 37 trillion cells. In a multicellular organism the cells are of many types and each type is specialised for a particular task; in a unicellular organism the single cell performs every function of life by itself.

In a multicellular body cells of similar structure and function group together to form a tissue, tissues form organs, organs form organ systems, and organ systems make up the organism. This ladder of organisation, cell to tissue to organ to system to organism, is the framework for the chapters that follow.

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 seen clearly under the ordinary light microscope, but the electron microscope shows it as a double line. It is made of a double layer of lipid molecules with protein molecules embedded in it; the proteins float in the lipid layer like boats on a pond, which is why it is described as flexible and fluid.

The most important property of the plasma membrane is that it is selectively permeable (also called semi-permeable). It allows some substances to pass in and out of the cell freely, lets others pass only slowly or with help, and blocks some completely. In this way it acts as the gatekeeper of the cell, keeping the inside chemically different from the outside.

Substances cross the membrane in several ways. Gases such as oxygen and carbon dioxide and small molecules cross by diffusion, the movement of particles from a region of higher concentration to a region of lower concentration. Water crosses by osmosis, which is the diffusion of water through a selectively permeable membrane from a dilute solution to a concentrated solution. Some substances are moved by the membrane against the concentration gradient using energy; this is active transport. These processes are the subject of a later chapter, but the membrane is where they all happen.

The plasma membrane can also take in large particles by folding around them. An amoeba engulfs a food particle by extending pseudopodia and pinching off a food vacuole; this is endocytosis. White blood cells swallow bacteria in the same way. The reverse process, in which the cell pushes material out by fusing a vacuole with the membrane, is exocytosis.

Functions of the plasma membrane in brief: it gives the cell a definite boundary and shape (in animal cells it is the only boundary), it regulates the entry and exit of materials, it maintains the cell's internal environment, it allows exchange of gases and nutrients, and it helps cells to recognise and communicate with one another. If the membrane is damaged the cell contents leak out and the cell dies, which is how many disinfectants kill bacteria.

Plant cells, fungal cells and bacterial cells have an extra covering outside the plasma membrane called the cell wall. Animal cells never have a cell wall. In plants the wall is made chiefly of cellulose, a complex carbohydrate made of long chains of glucose units that are laid down as fibres, rather like the fibres in cotton (which is nearly pure cellulose). Fungal walls are made of chitin and bacterial walls of peptidoglycan, but in this class you need to remember the plant cell wall.

The cell wall is non-living; it is secreted by the living protoplasm inside and once formed it does not grow or respond like the membrane does. It is rigid and gives the plant cell its fixed shape, which is why onion peel cells look like neat bricks. It is also freely permeable, which means that water and dissolved substances pass through it without hindrance; the selection of what enters the cell is done by the plasma membrane underneath, not by the wall.

The wall serves several purposes. It gives mechanical strength and support, which is what allows a plant to stand upright without a skeleton. It protects the cell from injury and from attack by microbes. Most importantly, it prevents the cell from bursting. When a plant cell is placed in water, water enters by osmosis and the cell swells; the rigid wall resists further expansion so the cell becomes firm or turgid but does not burst. An animal cell in the same situation, having no wall, would swell and burst. When a plant cell loses water the cytoplasm shrinks away from the wall; this is called plasmolysis and it is why a wilted plant recovers when watered.

Between the walls of neighbouring cells is a cementing layer called the middle lamella, made of calcium pectate, that glues the cells together. Very thin strands of cytoplasm called plasmodesmata pass through tiny pores in the walls and connect the protoplasm of one cell with the next, so that plant cells can exchange materials directly.

Young cells have a thin primary wall; in many mature cells a thicker secondary wall is deposited inside it, often strengthened with lignin, as in wood. Cork cells have walls coated with suberin, which makes them waterproof; this is what Hooke saw in 1665.

The nucleus is the most prominent structure inside a cell, usually round or oval and lying near the centre of animal cells and to one side in mature plant cells (pushed there by the large vacuole). Robert Brown named it in 1831. It is often called the control centre or the brain of the cell because it directs all the cell's activities and carries the hereditary information.

Structure: the nucleus is bounded by a double-layered nuclear membrane (nuclear envelope) with many tiny nuclear pores through which substances such as RNA move between nucleus and cytoplasm. Inside is a jelly-like nucleoplasm. In it lies the chromatin, a network of thread-like material made of DNA (deoxyribonucleic acid) and protein. When the cell prepares to divide the chromatin threads coil and thicken into rod-shaped chromosomes, which can be seen under the microscope. Every species has a fixed number of chromosomes: a human body cell has 46 (23 pairs), an onion has 16, a dog has 78. There is also one or more dense round bodies called the nucleolus, which manufactures the ribosomes.

The DNA in the chromosomes is arranged in units called genes, each of which carries the instructions for one character, such as the colour of a flower or the shape of a nose. DNA is the hereditary material: it is copied before the cell divides so that each daughter cell receives a full set of instructions, and it is passed from parents to offspring through the gametes.

Functions of the nucleus:

• It controls all metabolic activities of the cell by directing the synthesis of proteins and enzymes.
• It carries the genetic information and transmits it to daughter cells and to the next generation.
• It is necessary for cell division; a cell from which the nucleus is removed cannot divide and soon dies.
• The nucleolus makes ribosomal RNA and assembles ribosomes.

Cells whose nucleus has a nuclear membrane are called eukaryotic (true nucleus), and include all plants, animals, fungi and protists. Bacteria and blue-green algae have no nuclear membrane; their DNA lies naked in the cytoplasm as a nucleoid, and they are called prokaryotic. Mature human red blood cells lose their nucleus, which is why they cannot divide and live only about 120 days; sieve tube cells of plants are another example of living cells without a nucleus.

The whole living content of the cell, that is the plasma membrane, the cytoplasm and the nucleus together, is called the protoplasm, a name given by Purkinje in 1839. Huxley called protoplasm the physical basis of life, because it is the only material known to be alive. The protoplasm outside the nucleus is the cytoplasm; the protoplasm inside the nucleus is the nucleoplasm.

The cytoplasm is a jelly-like, semi-fluid, translucent substance made of about 70 to 90 percent water in which proteins, carbohydrates, lipids, salts and many other molecules are dissolved or suspended. Its transparent fluid part is called the cytosol or hyaloplasm. Suspended in it are the various cell organelles, each bounded by its own membrane, along with non-living inclusions such as starch grains, oil droplets and crystals. Because the cytoplasm is semi-fluid it can flow; in many plant cells you can watch it streaming round and round the cell under the microscope, carrying the chloroplasts with it. This movement is called cyclosis or cytoplasmic streaming and helps distribute materials within the cell.

Functions of cytoplasm: it is the site of most of the cell's chemical reactions, such as the first stage of respiration (glycolysis) and the synthesis of many substances; it holds the organelles in place and provides a medium through which materials move between them; it stores food and waste; and its streaming helps transport.

Why do we need organelles at all? A cell carries out hundreds of different reactions, many of which would interfere with each other if they happened in the same place. Organelles are membrane-bound compartments, each with its own chemical environment, in which particular tasks are done: energy release in mitochondria, photosynthesis in chloroplasts, digestion in lysosomes, and so on. This division of labour among organelles is like the division of a factory into different departments, and it is the main reason eukaryotic cells can be larger and more complex than prokaryotic cells, which have no membrane-bound organelles.

The organelles you must know are the endoplasmic reticulum, ribosomes, Golgi apparatus, lysosomes, mitochondria, plastids, vacuoles and centrosome. The next sections take them one by one.

The endoplasmic reticulum (ER) is a network of membrane-bound tubes and flattened sacs that spreads through the cytoplasm, connecting the nuclear membrane with the plasma membrane. The word reticulum means network. The ER divides the cytoplasm into compartments and provides a large surface on which reactions can take place; it also acts as the cell's internal transport system, carrying materials from one part of the cell to another, especially between the nucleus and the cytoplasm.

There are two kinds of ER. Rough endoplasmic reticulum (RER) has ribosomes attached to its outer surface, which give it a rough, dotted look under the electron microscope. The proteins made by these ribosomes pass into the channels of the RER and are carried to the Golgi body for packaging. Cells that make and export a lot of protein, such as the enzyme-secreting cells of the pancreas or antibody-producing cells, have a great deal of RER. Smooth endoplasmic reticulum (SER) has no ribosomes. It manufactures lipids and steroid hormones, which are needed for building new cell membranes, and in liver cells it detoxifies poisons and drugs. This is why the liver of a person who takes alcohol regularly develops more SER.

Ribosomes are the smallest organelles, about 20 nanometres across, and are not bounded by a membrane. Each is made of two unequal subunits composed of ribosomal RNA and protein, manufactured in the nucleolus. Ribosomes are found attached to the RER and also lying free in the cytoplasm; free ribosomes make proteins for use inside the cell while attached ribosomes make proteins for export. They are called the protein factories of the cell because on them amino acids are joined together in the order dictated by the messenger RNA that comes from the nucleus. Since enzymes are proteins and every reaction of the cell needs an enzyme, ribosomes are indispensable; even prokaryotic cells, which lack all other organelles, have ribosomes (slightly smaller ones).

Together the ER and the ribosomes form the manufacturing wing of the cell: the ribosomes make the products, the ER channels carry them, and the Golgi body, described next, finishes and dispatches them.

The Golgi apparatus (Golgi body or Golgi complex) was discovered by the Italian scientist Camillo Golgi in 1898, who saw it in nerve cells stained with silver. It consists of a stack of flattened, membrane-bound sacs called cisternae, arranged roughly parallel like a pile of plates, together with small vesicles budding from their edges. In plant cells the units are scattered and are called dictyosomes. The Golgi body is usually located near the nucleus and works closely with the ER.

Its job is packaging and dispatch. Proteins and lipids made in the ER arrive in vesicles at one face of the Golgi stack, are modified as they pass through the cisternae (for example sugar groups are added to make glycoproteins), and are then sorted and packed into new vesicles that bud off from the other face. Some vesicles fuse with the plasma membrane and release their contents outside the cell; this is how gland cells secrete enzymes, mucus and hormones. Other vesicles remain in the cell as lysosomes. The Golgi apparatus also makes some polysaccharides and, in plant cells, supplies the material for building the cell wall and the cell plate during division. For these reasons it is called the packaging and secretory organelle of the cell.

Lysosomes were discovered by de Duve in 1955. They are small, spherical, single-membrane sacs formed by the Golgi body and filled with powerful digestive enzymes (hydrolytic enzymes) made on the RER. They are abundant in animal cells, especially white blood cells, and less common in plant cells. Their functions:

• They digest food particles and worn-out organelles inside the cell (intracellular digestion).
• They destroy bacteria and viruses that have been engulfed by the cell; white blood cells rely on them.
• They remove damaged or old cell parts so that the cell can recycle them.
• When a cell is damaged or dying the lysosomes burst and their enzymes digest the entire cell. Because of this they are called the suicide bags of the cell.

The lysosome membrane normally protects the rest of the cell from its own enzymes. The disappearance of the tadpole's tail during metamorphosis into a frog is carried out by lysosomes, and so is the removal of the webbing between the fingers of a human embryo.

Mitochondria (singular mitochondrion) are rod-shaped or oval organelles, about 1 to 10 micrometres long, scattered through the cytoplasm of all eukaryotic cells. They were first observed by Kolliker in 1880 and named by Benda in 1898. A single cell may contain a few hundred to several thousand of them; cells that need a great deal of energy, such as muscle cells, liver cells and sperm cells, have the most.

Structure. Each mitochondrion has a double membrane. The outer membrane is smooth and porous. The inner membrane is folded inwards into many finger-like or shelf-like projections called cristae, which greatly increase the surface area available for chemical reactions. The inner membrane encloses a jelly-like fluid called the matrix, which contains enzymes, ribosomes and a small circular molecule of DNA. Because mitochondria have their own DNA and ribosomes they can make some of their own proteins and can divide independently to produce new mitochondria; they are therefore called semi-autonomous organelles.

Function. Mitochondria are the sites of aerobic respiration. The glucose that has been partly broken down in the cytoplasm enters the matrix, where it is completely oxidised using oxygen into carbon dioxide and water, and the energy released is trapped in the chemical bonds of ATP (adenosine triphosphate). ATP is the energy currency of the cell: whenever a cell needs energy, for contraction, for active transport, for building molecules, for dividing, it breaks a phosphate bond of ATP. Because they release the energy the cell runs on, mitochondria are called the powerhouses of the cell.

The overall reaction of aerobic respiration is glucose plus oxygen giving carbon dioxide, water and energy: C₆H₁₂O₆ + 6O₂ gives 6CO₂ + 6H₂O + energy (about 38 ATP per glucose molecule). Notice that this is the reverse of photosynthesis; the mitochondrion and the chloroplast are partners in the flow of energy through living things.

Mitochondria also take part in the synthesis of some fats and in regulating cell death. They are inherited only from the mother, because the egg cell contributes the cytoplasm of the fertilised egg. Mitochondrial DNA is therefore used to trace maternal ancestry.

Plastids are organelles found only in plant cells and in some algae; animal cells never have them. Like mitochondria they are bounded by a double membrane and contain their own DNA and ribosomes, so they too are semi-autonomous. They are of three kinds according to the pigment they contain and the work they do.

Chloroplasts are green plastids containing the pigment chlorophyll. They are found in the leaves and green stems of plants and in algae. A chloroplast is usually disc-shaped and about 4 to 6 micrometres across; a single leaf mesophyll cell may hold 20 to 100 of them. Inside the double membrane is a fluid called the stroma, and within the stroma lie stacks of flattened membrane discs called thylakoids; each stack is a granum (plural grana), and the grana are joined by membrane strands. The chlorophyll sits in the thylakoid membranes, where it traps light energy, and the sugar-making reactions take place in the stroma. Chloroplasts are the sites of photosynthesis, in which carbon dioxide and water are combined using light energy to make glucose and oxygen: 6CO₂ + 6H₂O + light gives C₆H₁₂O₆ + 6O₂. For this reason chloroplasts are called the kitchens of the cell. Every food chain on Earth begins in a chloroplast.

Chromoplasts are coloured plastids containing yellow, orange or red pigments such as carotene and xanthophyll. They give colour to petals, ripe fruits such as tomato, chilli and papaya, and roots such as carrot. Their bright colours attract insects for pollination and animals for seed dispersal. When a green tomato ripens and turns red its chloroplasts change into chromoplasts.

Leucoplasts are colourless plastids found in cells that are not exposed to light, such as roots, tubers and seeds. They store food: amyloplasts store starch (potato tuber, rice grain), elaioplasts store oil (groundnut, castor seed) and aleuroplasts store protein (pulses). A potato that is left in the light turns green because its leucoplasts change into chloroplasts, which shows that the three kinds of plastid can change into one another.

Vacuoles are fluid-filled sacs in the cytoplasm bounded by a single membrane called the tonoplast. In a mature plant cell there is usually one very large central vacuole that may occupy 50 to 90 percent of the cell's volume and pushes the cytoplasm and nucleus to a thin layer against the wall. The fluid inside is the cell sap, a solution of water, sugars, salts, amino acids, pigments and waste products. Animal cells have vacuoles too, but they are small, temporary and many.

Functions of the plant vacuole: it stores water and dissolved substances; it holds waste products that the plant cannot excrete; it contains pigments (anthocyanins) that colour beetroot, blue and purple flowers and red cabbage; and above all it maintains turgidity. When the vacuole is full of water it presses the cytoplasm against the wall and the cell is firm, which keeps leaves spread out and soft stems upright. Loss of water from the vacuole causes wilting. In unicellular freshwater organisms such as amoeba and paramecium a contractile vacuole collects excess water that has entered by osmosis and pumps it out at intervals, preventing the cell from bursting; a food vacuole is a temporary sac in which engulfed food is digested.

The centrosome is a small region of cytoplasm near the nucleus found in animal cells and in the cells of some lower plants; it is absent from the cells of higher plants. It contains two tiny cylindrical bodies called centrioles placed at right angles to each other, each made of nine triplets of microtubules. During cell division the centrioles move to opposite poles of the cell and organise the spindle fibres that pull the chromosomes apart; they thus ensure that each daughter cell receives a complete set of chromosomes. Centrioles also give rise to the basal bodies of cilia and flagella, the hair-like structures that some cells use to move.

Two other cell components deserve a mention. Peroxisomes are small sacs of enzymes that break down hydrogen peroxide, a toxic by-product of metabolism, into water and oxygen. The cytoskeleton is a network of protein fibres in the cytoplasm that gives the cell its shape, anchors the organelles and enables movement.

On the basis of the organisation of the nucleus, all cells are divided into two great groups. The word karyon means nucleus in Greek; pro means before and eu means true.

Prokaryotic cells are the cells of bacteria and blue-green algae (cyanobacteria). They are small, usually 1 to 10 micrometres, and simple. They have no nuclear membrane; the genetic material is a single circular DNA molecule lying naked in the cytoplasm in a region called the nucleoid. There is no nucleolus. Prokaryotic cells have no membrane-bound organelles: no mitochondria, no plastids, no ER, no Golgi body, no lysosomes. Respiration and, in cyanobacteria, photosynthesis take place on folds of the plasma membrane itself. Ribosomes are present but are smaller (70S). Most prokaryotes have a cell wall, but it is made of peptidoglycan, not cellulose. Some carry small extra rings of DNA called plasmids. They divide by simple binary fission.

Eukaryotic cells are the cells of protists, fungi, plants and animals. They are larger, 10 to 100 micrometres, and have a true nucleus bounded by a nuclear membrane with a nucleolus and several linear chromosomes made of DNA and protein. The cytoplasm contains membrane-bound organelles, each doing a specialised job. Ribosomes are larger (80S). The cell wall, where present, is of cellulose (plants) or chitin (fungi). Eukaryotic cells divide by mitosis and meiosis.

The prokaryotes were the first cells on Earth, appearing about 3.5 billion years ago; eukaryotes appeared much later. One widely accepted idea is that mitochondria and chloroplasts were once free-living prokaryotes that came to live inside a larger cell, which explains why they have their own circular DNA and 70S ribosomes like bacteria.

Plant cells and animal cells are both eukaryotic and share the same basic plan: plasma membrane, cytoplasm, nucleus, mitochondria, ER, ribosomes and Golgi body. But the two kinds of organism live differently, and their cells reflect this. Plants make their own food, do not move about, and support themselves without a skeleton; animals eat other organisms, move, and have skeletons or muscles. The differences between their cells follow from these facts.

The cell wall is the most obvious difference. It explains the fixed shape of plant cells, their ability to become turgid without bursting, and the fact that a tree can stand without bones. Plastids exist only in plants because only plants (and algae) photosynthesise; the chloroplast is the reason plants are green and are the producers of every ecosystem. The large vacuole is a cheap way of making a cell big and firm: a plant fills its cells with water rather than with expensive cytoplasm. Animal cells, which must be flexible and mobile, cannot afford a wall or a huge vacuole. Centrioles organise the spindle in animal cells; plant cells manage the spindle without them.

When you draw the two cells for the examination, show the plant cell as a rectangle with a double boundary (wall and membrane), a large central vacuole, chloroplasts and a nucleus at the side; show the animal cell as an irregular rounded outline with a single membrane, a central nucleus, small vacuoles, mitochondria, ER, Golgi body, lysosomes and a centrosome. Label at least six parts of each. Remember that both cells have mitochondria; students often forget to draw them in the plant cell.