Chapter 2: Cell — The Unit of Life
In the winter of 1831, a Scottish botanist named Robert Brown sat in a small room in London with a pile of orchid leaves and a lens. Brown was not looking for anything hidden. He wanted to know how orchids were pollinated, and to find out he was peeling the thin outer skin off leaf after leaf and examining it.
The skin, magnified, was a sheet of tiny compartments packed edge to edge. That much was already known. What caught Brown’s attention was something inside them. In one compartment after another, near the middle, lay a single rounded, slightly cloudy body. It was not a speck of dirt, because it was not scattered at random. It was in every compartment, one to each, in orchid after orchid.
Brown described what he had seen to the Linnean Society and gave the body a plain Latin name meaning ‘little kernel’ — the nucleus. He could not say what it did. He could only say that it was always there.
That was enough to change the question. For over a century after the first microscopes, these compartments had been thought of as empty boxes, and a plant or an animal as a heap of such boxes. Brown’s little kernel was the first clear sign that each box has an inside, that the inside is organised, and that something in there is running the compartment’s life.
Diagram
Fig. 2.0-1 — Cells Of An Orchid Leaf Epidermis As Brown Saw Them Surface view line drawing of a sheet of thin-walled epidermal cells packed edge to edge, each cell showing a single rounded nucleus placed near its centre; label cell boundary, cell contents, nucleus.
2.1 What Is A Cell?
Biology is the study of living organisms, and the range of those organisms is enormous. A blue whale is the largest animal alive today. Some living things are so small that a single drop of pond water may hold hundreds of them.
Yet if you could examine any of them closely enough, you would find the same thing. Every living body is built out of small units, and each unit is separated from its neighbours by a boundary. These units are called cells — the small living units from which every plant and animal body is built.
A cell is not simply a building block, like a brick in a wall. A brick does nothing. A cell takes in food, uses energy, gets rid of wastes, responds to its surroundings and, in most cases, divides to make more cells.
That is why a cell is called both the structural unit and the functional unit of a living body. The body is made of cells, and everything the body does is done by its cells.
Where Cells Sit In The Body
Cells are rarely scattered about on their own inside a large organism. Cells of one kind, doing one kind of work, are grouped together into a tissue.
Several tissues working together form an organ, such as the skin, the brain or a bone. Several organs working together form an organ system, such as the digestive system.
This gives a ladder of levels, set out in FC 2.1. Each step up is built from the step below it, and the cell is the lowest step at which life itself can be said to be running.
Flowchart
FC 2.1 — Levels Of Organisation In A Living Body Cell → tissue → organ → organ system → organism
The same ladder holds for a plant. The leaf, the flower, the root and even the wood of a tree are built of very large numbers of cells, arranged into the plant’s own kinds of tissues and organs.
Every Cell Has Its Own Life
A cell is alive in its own right. It has a working life of its own, and like any living thing it eventually ages and dies. In a large body, old and worn-out cells are steadily removed and replaced by new cells formed from cells that already exist.
Human red blood cells are a clear example. Each one lives for about 120 days and is then broken down, while fresh red blood cells are made continuously to take its place. The body stays the same while its cells are constantly being renewed.
Life Begins As A Single Cell
Most organisms, including human beings, begin life as one single cell called the zygote — the cell formed when an egg and a sperm join. Every one of the many kinds of cells in the adult body traces back, by repeated division, to that one cell.
Cells are so small that they cannot be seen with the unaided eye. That is why cells went unnoticed for so long, and why the study of the cell could not begin until instruments were invented that magnified what the eye alone could not separate.
2.2 The Microscope And The Discovery Of The Cell
Cells are far too small for the unaided eye. So the discovery of the cell had to wait for an instrument, and the story of the cell begins as the story of the microscope.
A microscope works by bending light. Glass lenses collect the light coming from a small object and spread it out, so that the object appears far larger than it really is.
2.2.1 The Simple Microscope
The first useful microscopes were built by a Dutchman, Antony van Leeuwenhoek (1632–1723). He was an ordinary public official, and grinding lenses was his hobby rather than his profession.
Over his lifetime he is said to have made about 400 microscopes. Every one of them used a simple microscope design: a single biconvex lens, thicker in the middle than at the edges, held in a small metal plate.
One of these instruments is shown in Fig. 2.2-1. The object was fixed on a needle-like screw point on one side of the lens, and the eye was brought right up against the lens on the other side.
The design looks crude, but his lenses were superb. The best of them magnified an object up to about 200 times, which was enough to reveal a world nobody had suspected.
Diagram
Fig. 2.2-1 — Leeuwenhoek’s Simple Microscope Front and back views of Leeuwenhoek’s hand-held simple microscope drawn side by side; label single biconvex lens, needle-like screw point for mounting the object, adjusting screw, metal plate.
2.2.2 The Compound Microscope And Hooke’s Cork
A single lens can only be pushed so far. Robert Hooke (1635–1703), an English scientist, took the next step by using two lenses in one tube, one at each end.
The lens near the object magnifies it once. The second lens, at the eye end, magnifies that already enlarged image a second time. Because the two magnifications multiply, such instruments — called compound microscopes — reach far higher powers than any single lens.
In Hooke’s microscope, shown in Fig. 2.2-2 A, the object was placed on a stage below the tube. Light came from an oil flame and was thrown up onto the object by a concave mirror, because a small object seen at high magnification looks hopelessly dim without extra light.
In 1665 Hooke cut a very thin slice of cork and put it under this microscope. What he saw is drawn in Fig. 2.2-2 B: rows of tiny box-like compartments, piled together with walls between them.
The pattern reminded him of the rows of small rooms, or cells, in which monks lived in a monastery. He therefore said the cork was made of cells — and the name has stayed ever since.
One point about Hooke’s observation is easy to miss. Cork is dead tissue, so the cells he saw were dead and empty. What he was actually looking at were the walls; the living contents had long gone.
Diagram
Fig. 2.2-2 — Hooke’s Microscope And His Drawing Of Cork Two-part line drawing. A: Hooke’s compound microscope with the object on the stage below the tube, an oil lamp beside it and a water-filled flask focussing the light; label eyepiece, tube, objective lens, stage, oil lamp, flask. B: Hooke’s drawing of a thin slice of cork showing rows of empty box-like compartments; label cell wall, empty space.
The Modern Compound Microscope
The compound microscope you will use in the laboratory is Hooke’s instrument greatly improved, and it is worth knowing its parts before you handle it. Fig. 2.2-3 names them.
The object rests on the stage, held by clips, and is lit from below. The nose piece carries two objective lenses, one of low power and one of high power, so that you can move from a wide view to a close one by turning it. The coarse adjustment brings the image roughly into view and the fine adjustment sharpens it.
Diagram
Fig. 2.2-3 — An Ordinary Compound Microscope Labelled line drawing of a modern compound microscope in side view; label eyepiece, body tube, arm, nose piece, low power objective lens, high power objective lens, stage, stage clips, coarse adjustment, fine adjustment, light source, base. Labels on straight, non-crossing lead lines.
2.2.3 The Electron Microscope
Even the best light microscope has a limit. An ordinary compound microscope magnifies an object up to about 2,000 times, and beyond that the image only grows blurred.
The electron microscope removed that limit. It gives magnifications of over 200,000 times, and it is this instrument that revealed the fine structures packed inside a cell.
The difference lies in what is being focussed. A compound microscope uses light, which is bent by glass lenses. An electron microscope uses beams of electrons instead, and these are bent by magnets rather than by glass. Table 2.1 sets the two side by side.
Diagram
Fig. 2.2-4 — An Electron Microscope Line drawing of an electron microscope in a laboratory, showing the tall column, the specimen chamber, the control console and the viewing screen; label electron column, specimen chamber, viewing screen, controls.
2.3 The Cell Theory
Once microscopes were good enough, cells turned up everywhere anyone looked. The next question was how far the pattern went. Was every living thing built this way, or only some?
2.3.1 How The Cell Theory Was Built
The answer was assembled by three people over twenty years, each adding one piece.
In 1838 Matthias Schleiden, a German botanist, announced that every plant is made up of a large number of cells. He added an important point: each of these cells carries out the processes of life for itself.
A year later Theodor Schwann, a German zoologist, examined animal tissues and reported the same thing. He declared that all animals and plants are composed of cells, and that these cells serve as the units of structure and function.
Between them, Schleiden had covered plants and Schwann had covered animals. Their joint statement, made in 1839, is what we call the cell theory.
One question was still open. If bodies are made of cells, where does a new cell come from? Many people at the time assumed cells simply formed out of non-living material.
In 1858 Rudolf Virchow settled it. He stated that every cell arises from a cell that already exists — a new cell is always produced by the division of an older one.
Virchow’s third point also explains why the supply of cells never runs out. A cell divides into two, each of those grows and divides again, and the chain has no gap in it anywhere. FC 2.2 shows the cycle.
Flowchart
FC 2.2 — The Never-Ending Chain Of Cells Existing cell → grows to full size → divides → two new cells → each grows to full size → each divides (returns to the start of the cycle)
2.3.2 What The Cell Theory Means
A theory stated in three lines can sound empty until you test it on something real. Take two living things — a mango tree, Mangifera indica, and a frog — and look at what the theory claims about each.
The Cell As A Structural Unit
Cut a piece from anywhere in the frog — skin, gut, bone, muscle — and put it under a microscope. It will show a cellular structure.
Do the same with any part of the mango tree, from the root tip to the flower. Once again you find cells. There is no part of either body that is built out of something else.
The Cell As A Functional Unit
Whatever the whole body does, some cell is doing it. The frog moves because its muscle cells contract and pull on its bones. Its food is digested by enzymes that the cells of the gut wall secrete.
The digested food is then absorbed by the cells lining the gut, and is used up inside cells for the chemical activities that keep the animal alive.
The mango tree tells the same story from the plant side. Its food is made by photosynthesis in the cells of its leaves, and the water for that food-making is drawn out of the soil by the cells of its roots.
Cells Die And Are Replaced
Both bodies are made of many millions of cells, and many of these are dying at any moment. Each one that dies is replaced by a new cell formed when a younger cell divides.
This is Virchow’s point at work in a living body. Because cells are always being formed from cells that already exist, the chain of replacement never breaks while the organism lives.
Life Starts As A Single Cell
The frog began as a fertilised egg. That egg was one cell, produced by the cells of the ovary of the mother frog, and every cell in the adult frog descends from it.
The mango tree began the same way. Inside its seed was an embryo, formed after pollination and fertilisation in the ovary of a mango flower — and that embryo, too, started as a single cell.
2.4 Cells — How Many?
Organisms differ enormously in size, but the size of a cell hardly varies. So a bigger body is not made of bigger cells; it is made of more of them.
That single idea sorts living things into three groups by the number of cells they are built from. Fig. 2.4-1 shows one member of each group, and Table 2.2 sets them side by side.
Diagram
Fig. 2.4-1 — Single-Celled, Few-Celled And Many-Celled Organisms Three line drawings in a row, drawn at their own convenient scales. A: a yeast cell, single and rounded, with one bud. B: a short filament of Spirogyra showing about eight cells joined end to end, and beside it a spherical Volvox colony. C: a leafy plant with a circular magnified inset showing a block of packed cells. Label single-celled, few-celled, many-celled.
Single-Celled Organisms
Many small plants and animals consist of just one cell. That one cell is the whole organism, so it must carry out every life process by itself.
Bacteria, yeast and amoeba belong here. There is no division of labour in such a body, because there is no second cell to share the work with.
Few-Celled Organisms
Some very small plants and animals are built from a relatively small number of cells — a few hundred or a few thousand.
Spirogyra, the slippery green filament found in ponds, is a chain of such cells joined end to end. Volvox, a hollow green ball that rolls slowly through pond water, is another.
Many-Celled Organisms
Most of the plants and animals you see around you, including yourself, are made of millions and billions of cells.
Human beings and mango trees are examples. In bodies of this size the cells are no longer alike; different groups of them take up different jobs.
The Numbers In A Human Body
Some of these numbers have actually been estimated, and they are worth pausing over. An average-sized adult human body is made of roughly 37.2 trillion cells, and about 100 billion of these are the nerve cells of the brain alone.
Blood cells are so numerous that they are not counted for the whole body at all. They are counted in a fixed small volume — one cubic millimetre, a cube each of whose sides is a single millimetre.
In that tiny volume of blood there are about 4 to 6 million red blood cells and about 7 thousand white blood cells. Multiply that by the volume of blood in a body and the totals become very large indeed.
2.5 Cells — How Small?
Cells are small enough that a microscope is needed for almost every one of them. To say how small, we need a unit smaller than the millimetre.
That unit is the micrometre (µm) — one thousandth of a millimetre, and one millionth of a metre. A thousand micrometres laid end to end would stretch across a single millimetre division on your ruler. Fig. 2.5-1 compares a few cells on this scale.
Diagram
Fig. 2.5-1 — Comparing The Sizes Of Some Cells Line drawing comparing, side by side against a common scale bar marked in micrometres, a rod-shaped bacterium, a human red blood cell seen in surface view, and a cheek-lining cell; label bacterium, red blood cell, scale bar in micrometres.
The Smallest Cells
The smallest cells belong to bacteria. Their length ranges from about 0.3 µm to 5.0 µm, so several thousand of them could sit along one millimetre.
Among the cells of the human body, the red blood cells are among the smallest, at about 7 µm across. Their small size is not an accident, as you will see when their shape is discussed.
The Longest Cells
Length is a different question from size. The longest cells in the body are the nerve cells, and some of them are astonishingly long.
Picture a single nerve cell that begins at the tip of your finger and runs, unbroken, all the way to the spinal cord inside your backbone. One cell spans that whole distance. Fig. 2.5-2 shows this.
Diagram
Fig. 2.5-2 — A Single Nerve Cell Spanning Arm To Spinal Cord Outline line drawing of a human arm and upper back, with one nerve cell drawn along its length: its receptive endings at the fingertip, a long thread-like fibre running up the arm, and its ending at the spinal cord; label fingertip endings, nerve fibre, spinal cord.
The Largest Cells
The largest single cells of the living world are the eggs of birds — but only a part of what you crack into a pan is the cell.
The cell is the central yellow sphere, the yolk. The white, or albumen, and the shell around it are extra layers, added on later as the egg travels down the reproductive tract of the bird.
By this measure, the egg of an ostrich, before development inside it begins, is the largest single cell alive today.
2.6 Smallness Of Cells: A Greater Efficiency
A cell could, in principle, have been the size of a pea. It is worth asking why cells are instead held down to a few micrometres. Being small turns out to be useful for two main reasons.
Reason One: Messages Travel Faster
A cell is not a still bag of chemicals. Substances made in one part of it are constantly needed in another part, and the parts must work in step.
In a small cell, the distance between any two regions is short, so materials pass from one region to another quickly and the cell functions as a single working unit. In a very large cell, one end would be slow to learn what the other end was doing.
Reason Two: A Large Surface For Its Volume
The second reason is the more important one, and it needs a little arithmetic.
Everything a cell takes in or throws out must cross its surface. The cell membrane is the only way in and the only way out. But everything that is taken in has to supply the whole volume inside.
So what matters is not the surface alone and not the volume alone, but the surface area to volume ratio — how much surface the cell has for each unit of its bulk. The higher that ratio, the better supplied the cell is.
Why Cutting Something Up Increases Its Surface
Here is the surprise. Cutting an object into smaller pieces does not change how much material there is, but it sharply increases the total surface. Fig. 2.6-1 shows the demonstration, and the working is set out below it.
Diagram
Fig. 2.6-1 — A Cube Cut Into Eight Smaller Cubes Three-part line drawing. Left: a single cube of side 2 mm. Middle: the same cube with cut lines dividing it into eight equal smaller cubes. Right: one of the small cubes drawn separately, of side 1 mm. Label side 2 mm, side 1 mm, cut lines.
What The Extra Surface Is For
Substances cross the cell surface mainly by diffusion — the spreading of a substance from a region where it is more concentrated to a region where it is less concentrated, without the cell having to spend anything on moving it.
The larger the surface relative to the volume, the more diffusion the cell can manage in a given time. Fig. 2.6-2 shows what is crossing.
Diagram
Fig. 2.6-2 — Substances Crossing The Surface Of A Cell Simple rounded cell drawn in outline with straight arrows crossing its boundary; inward arrows labelled oxygen, nutrients, water; outward arrows labelled carbon dioxide, wastes, water; caption arrow labelled other substances.
A large surface area relative to volume therefore ensures:
- nutrients diffuse into the cell fast enough to feed it;
- metabolic wastes diffuse out of the interior before they build up;
- respiratory gases exchange freely — oxygen in, carbon dioxide out;
- any damage to the cell can be easily repaired, since a small cell has less to rebuild.
2.7 Cell Shapes Suit Cell Functions
Cells vary greatly in shape. Some are disc-like, some polygonal, rectangular or cuboid, others thread-like, branched, or of no regular shape at all. Fig. 2.7-1 shows a selection.
The variety is not decoration. In a many-celled body, different cells take up different jobs, and the shape of a cell is usually a direct answer to the job it has to do. Once you know the work, the shape stops looking arbitrary.
Diagram
Fig. 2.7-1 — Some Different Kinds Of Cell Shapes Panel of separate labelled line drawings, each drawn at its own convenient scale: epithelial cells in a sheet, human blood cells (red blood cells and white blood cells together), cartilage cells, connective tissue cells, striated muscle fibre, nerve cell, bone cell, smooth muscle cells, Chlamydomonas, a plant cell from a leaf, guard cells of a stomatal pore, and Amoeba. Label each drawing with the name of the cell.
Cells That Must Travel
Human red blood cells are circular and biconcave — flattened discs, dished inwards on both faces. They spend their working life being pushed through capillaries narrower than they are, and this flexible disc shape lets them bend and squeeze through while carrying oxygen.
White blood cells solve a harder problem. They must leave the blood vessels altogether to reach an infection in the surrounding tissue.
They do this by being amoeboid — that is, by moving as an amoeba does, pushing out temporary extensions of the cell called pseudopodia and flowing into them. Having no fixed shape, a white blood cell can squeeze out through the wall of a capillary.
Cells That Must Reach Or Pull
Nerve cells are long. Their work is to carry impulses between distant parts of the body and the brain, in both directions, and a long cell carries a message the whole way without handing it over.
Muscle cells are long as well, but for a different reason. They are contractile — able to shorten — and a long cell that shortens can pull a bone or squeeze the wall of a tube. A short cell could do neither.
Cells That Must Open And Shut
The underside of a leaf is dotted with tiny pores, called stomatal pores, through which gases pass in and out of the leaf. Each pore is bounded by a pair of guard cells.
These guard cells are bean-shaped. Because the two curved cells face each other, a change in the water content of the pair alters their curvature, and the gap between them widens or closes. A pore that can be shut is how a leaf controls what it loses and what it takes in.
2.8 The Structure Of A Cell
Cells differ in shape and size, and they differ in what they do. Yet if you strip away those differences, the same basic plan appears in almost all of them. Biologists describe this shared plan under the name ‘generalised cell’.
A generalised cell has three essential parts: the cell membrane, the cytoplasm and the nucleus. Fig. 2.8-1 shows this plan drawn twice — once for an animal cell and once for a plant cell.
Diagram
Fig. 2.8-1 — A Generalised Animal Cell And A Generalised Plant Cell Two labelled line drawings side by side, as seen under a compound microscope. A — animal cell: label cell membrane, cytoplasm, nucleus, nuclear membrane, nuclear pore, nucleolus, chromatin fibres, mitochondrion, rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi body, lysosome, centrioles. B — plant cell: label cell wall, cell membrane, cytoplasm, nucleus, vacuole, chloroplast, mitochondrion, rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi body, and portions of the cell walls of the surrounding cells with the thin middle layer between them. Labels on straight, non-crossing lead lines.
The Little Organs Of A Cell
Look inside either drawing and you find distinct bodies, each with a definite shape, a definite structure and a definite job. These are the cell organelles, a name that simply means ‘little organs’.
The comparison is exact. An organelle does for a cell what an organ does for a whole plant or animal — it is a part set aside to perform one particular function. Cell organelles are living parts of the cell.
2.8.1 Cell Boundaries
Every cell has to keep its contents in one place, keep unwanted things out, and still allow food, gases and wastes to pass. That work is done at its boundary.
A boundary of this kind does three things at once: it separates the inside of the cell from the outside, it regulates what moves in and out, and it helps the cell hold its shape.
2.8.1.1 The Cell Membrane
Every cell, plant or animal, is bounded by a cell membrane, also called the plasma membrane. It is a living part of the cell, made of lipoproteins — that is, of fats combined with proteins.
The membrane carries fine pores through which substances may enter or leave. But it does not let everything through. It is selectively permeable: it allows certain substances to pass and prevents others.
That selectivity is the whole point of a boundary. A membrane that let everything through would be no better than no membrane at all, and one that let nothing through would starve the cell.
2.8.1.2 The Cell Wall
Plant cells have a second boundary outside the first. Surrounding the cell membrane is a cell wall, made mainly of cellulose, a tough carbohydrate. Unlike the membrane, the cell wall is non-living.
The wall gives the plant cell its shape and a certain rigidity — which is why plant cells have the neat, straight-sided outlines you see in Fig. 2.8-1 B. It does this without interfering in any way with the work of the cell membrane inside it.
The cell wall is freely permeable. Substances in solution pass in and out through it without hindrance, so the selecting is still done entirely by the cell membrane beneath.
Look again at the plant cell in Fig. 2.8-1 B and you will see portions of the walls of six neighbouring cells around it. Between the wall of one cell and the wall of the next lies a thin middle layer, drawn as a thick grey line, which cements the two adjacent cells together.
2.8.2 Cytoplasm And Its Organelles
Fill the space between the cell membrane and the nucleus and you have the cytoplasm — a semi-liquid substance that occupies most of the cell. Under a compound microscope it looks colourless, partly transparent and somewhat watery.
Two things are true of living cytoplasm. A great many of the cell’s chemical reactions take place in it, and it is always in a state of some movement, never completely still.
Embedded in this cytoplasm are the organelles. We take them one at a time.
2.8.2.1 Endoplasmic Reticulum
The endoplasmic reticulum, usually shortened to ER, is so fine that a compound microscope cannot show it at all. Its existence was revealed only by the electron microscope.
It is an irregular network of membrane-bounded tubules and flattened sacs, spread through the whole cytoplasm of the cell. At its outer end the network connects with the cell membrane, and at its inner end with the membrane around the nucleus.
That double connection matters. It means the ER forms a continuous system of channels running from the boundary of the cell right up to the nucleus, and materials can be distributed from one part of the cell to another along it.
The ER also gives the cell a supporting framework, holding the cytoplasm’s contents in position rather than letting them drift.
Parts of the network carry small granules on their outer surface and parts do not. Where the granules are attached, the ER appears rough; where they are absent, it appears smooth.
2.8.2.2 Ribosomes
Those granules are the ribosomes. They are numerous, extremely small, and made mainly of RNA together with protein. Unlike most organelles they have no membrane around them at all.
Some ribosomes lie scattered freely in the cytoplasm and others are attached to the membranes of the endoplasmic reticulum. Wherever they are, they do the same work: they are the factories at which the cell’s proteins are put together.
2.8.2.3 Mitochondria
The mitochondria — a single one is a mitochondrion — are spherical or rod-shaped bodies. Each is a minute double-walled bag: there is an outer wall, and inside it a second wall thrown into finger-like folds that project inwards. These folds are called cristae, and Fig. 2.8-2 shows them.
Diagram
Fig. 2.8-2 — Structure Of A Mitochondrion Cut-open view of a single rod-shaped mitochondrion showing the smooth outer wall, the inner wall thrown into finger-like folds projecting into the interior, and the fluid-filled space within; label outer wall, inner wall, cristae, inner space.
Mitochondria are the sites where cell respiration takes place. In respiration the cell breaks down food to release the energy stored in it.
That released energy is not used up on the spot. It is stored in an energy-rich compound called ATP — adenosine triphosphate — which the cell can then spend on any activity that needs energy, anywhere in the cell and so, in the end, anywhere in the body.
Because they supply the cell’s energy in this usable form, mitochondria are called the power houses of the cell. They also carry their own DNA, which can be copied, so a mitochondrion can multiply within the cell.
2.8.2.4 Golgi Apparatus
The Golgi apparatus appears as granules, filaments or rods, and is supposed to have originated from the endoplasmic reticulum. It is generally found lying near the nucleus.
Its structure is a set of small groups of hollow tubular structures with membranous walls, called cisternae, associated with very small vesicles and vacuoles of different shapes.
In plant cells this apparatus is more scattered, and takes a name of its own — the dictyosomes. Dictyosomes contain smaller and more widely spread groups of cisternae and vesicles than the compact Golgi complex of an animal cell.
Both do the same work. They are concerned with the secretions of the cell — enzymes, hormones and so on — and with packaging those secretions and transporting them to different sites within the cell.
Together with the ER and the ribosomes, the Golgi apparatus completes a production line. FC 2.3 sets out the sequence, and the trace below follows one protein along it.
Flowchart
FC 2.3 — The Route Of A Secretion Within A Cell Ribosome on rough ER (protein assembled) → channel of the endoplasmic reticulum (protein transported) → Golgi apparatus (protein packaged into a vesicle) → vesicle carried through the cytoplasm → cell membrane (secretion released) or → stored within the cell as a lysosome
2.8.2.5 Lysosomes
Lysosomes are small membranous sacs of different shapes, budded off from the Golgi bodies. Each is filled with digestive enzymes, and this single fact explains everything they do.
Their enzymes destroy and digest foreign substances that get into the cell, such as germs taken in by a white blood cell.
They also digest the cell’s own stored food during unfavourable conditions, when no food is reaching the cell from outside. A cell can live for a while on what it has put by.
Finally, when a cell is old, injured or damaged beyond repair, its own lysosomes break open and rapidly destroy it from within. It is this last property that has earned lysosomes the name suicide bags.
2.8.2.6 Centrosome And Centrioles
A centrosome is found only in an animal cell. It is a small region of the cytoplasm lying close to the nucleus.
Within it lie two centrioles, short cylindrical bundles of microtubules set at right angles to each other. Because of that arrangement, they always appear in the microscope as two short rods lying across one another.
The centrosome does its work during cell division, in both mitosis and meiosis. The fine spindle fibres that pull the chromosomes apart develop from it, and radiating microtubules form a star-shaped aster around each centriole pair.
Plant cells have neither a centrosome nor centrioles. They divide perfectly well without them.
2.8.2.7 Plastids
Plastids are found only in plant cells. They are special organelles of various shapes — oval, spherical or disc-shaped — and each is bounded by a double membrane.
Plastids are classified by the colour they impart into three kinds: leucoplasts, chromoplasts and chloroplasts. Fig. 2.8-3 shows all three.
Diagram
Fig. 2.8-3 — The Three Kinds Of Plastids Three separate line drawings in a row: A — a colourless rounded leucoplast containing starch grains; B — an irregular chromoplast shown with coloured pigment; C — a disc-shaped chloroplast cut open to show the stacked disc-like thylakoids inside. Label leucoplast, stored starch, chromoplast, pigment, chloroplast, thylakoids.
Leucoplasts (leuco: white) are colourless plastids. They have no pigment at all, and their work is to store starch. The cells of a potato are full of them.
Chromoplasts (chromo: colour) are the variously coloured plastids — yellow, orange and red. They occur mostly in the petals of flowers and in fruits. The colouring substances, or pigments, associated with them are xanthophyll, which is yellow, and carotene, which is orange-red.
Not every plant colour comes from a plastid, and this is a common source of confusion. The pigments called anthocyanins are not associated with plastids at all. They lie dissolved in the cell sap inside vacuoles, and they give red, purple and blue colours to plant parts, especially to fruits and vegetables.
Chloroplasts (chloro: green) are the green plastids, and the green comes from the pigment chlorophyll they contain. They are abundant in the parts of a plant exposed to light, above all in the leaves.
Chloroplasts contain orange and yellow pigments as well, but you never see them, because the large quantity of chlorophyll masks them completely.
The function of the chloroplast is to trap solar energy and to absorb carbon dioxide, so that starch and sugar can be manufactured during photosynthesis. Like mitochondria, chloroplasts contain DNA and have the capacity to divide.
2.8.2.8 Cell Inclusions: Granules And Vacuoles
Not everything in the cytoplasm is an organelle. Certain non-living substances lie suspended in it, and these are called cell inclusions. They are not alive, but they matter.
Granules are the many small particles, crystals or droplets scattered through the cytoplasm. They are believed to hold food materials — starch in plant cells, glycogen in animal cells, and fat droplets in both.
Vacuoles are clear spaces in the cytoplasm, filled with water and with various substances in solution. Each is bounded by its own covering membrane, the tonoplast.
In plant cells the vacuoles are usually large — often one big vacuole fills most of the cell — and the liquid inside is called cell sap. An animal cell has no such prominent vacuole; its vacuoles, if any, are small and temporary.
A large vacuole does more than store. As it fills with water it presses outwards against the cell wall, and this pressure is what keeps a plant cell firm. It is also where the anthocyanin pigments are held.
2.8.3 The Nucleus
Of all the parts of a cell, the nucleus is the one the cell cannot do without. Three statements sum up why it matters.
- It regulates and coordinates the various life processes of the cell.
- It plays an essential part in cell division.
- It contains the factors, called genes, which determine heredity.
2.8.3.1 Structure Of The Nucleus
The nucleus is a small spherical mass, generally lying somewhat towards the centre of the cytoplasm. Fig. 2.8-4 A shows its structure.
Diagram
Fig. 2.8-4 — Structure Of The Nucleus, And Chromosomes In Cell Division Two labelled line drawings. A: a nucleus in section — label nuclear membrane, nuclear pore, nucleoplasm, nucleolus, chromatin network. B: an early stage of cell division, diagrammatic — a cell showing two pairs of thick ribbon-like chromosomes condensed from the chromatin network, with centrioles moving apart and an aster at each; label chromosomes, nucleus, cytoplasm, centrioles moving apart, aster.
It is bounded by a delicate nuclear membrane, which carries pores through which substances can pass between the nucleus and the cytoplasm. Within the membrane lies a relatively dense fluid, the nucleoplasm.
Suspended in the nucleoplasm is a network of fine thread-like structures called chromatin fibres. For most of the cell’s life this is all you can see of them — a tangled network.
When the cell begins to divide, the fibres coil up, becoming short, thick and ribbon-like. In this condensed form they are called chromosomes, and they can then be counted and studied, as in Fig. 2.8-4 B.
Each nucleus also contains at least one nucleolus, a small round body inside it. Some cells have more than one, and the number in a given kind of cell is fixed. The nucleolus takes part in protein synthesis by producing the ribosomes.
Not every cell has a nucleus of this kind. In bacteria there is no nuclear membrane at all: the nuclear material lies free in a central region of the cytoplasm called the nucleoid. You will meet this difference again in section 2.11.
2.8.3.2 Chromosomes, Genes And DNA
The number of chromosomes is definite in each species. Every human body cell has 46 chromosomes, which is 23 pairs. Some other familiar chromosome numbers are:
- Ascaris (round worm) — 2
- Garden pea — 14
- Onion — 16
- Maize — 20
- Honey-bee — 32
- Lion — 38
- Mouse — 40
- Wheat — 42
- Humans — 46
- Potato — 48
- Chimpanzee — 48
- Monkey — 54
- Dog — 78
- Sugarcane — 80
- Crayfish — 200
- Some insects — more than 1000
Chromosomes carry the genetic characters from the parents to the offspring. They are handed on when the egg of the female and the sperm of the male unite.
Chromosomes are made of chromatin, and chromatin is composed of the units of heredity, called genes. Each gene is made of a complex chemical substance, DNA — deoxyribonucleic acid.
Notice what follows from that. It is the genes, and not the number of chromosomes, that decide what a species is like. The lion, the tiger and the house cat all have 38 chromosomes, yet they are utterly different animals, because the genes carried on those chromosomes are different.
2.8.4 Parts Of A Cell At A Glance
Table 2.3 gathers together the parts described above, with their main characteristics and their chief functions. Use it to check yourself, not to learn from — each entry will make sense only if you can see why the structure suits the function beside it.
2.9 Plant Cells Compared With Animal Cells
Start with what is the same, because most of it is. Plant cells and animal cells share the same basic structure: both have a cell membrane, cytoplasm and a nucleus, and both contain endoplasmic reticulum, Golgi apparatus, mitochondria and ribosomes.
The differences are real but few, and they follow from one fact about how plants live. A plant does not move about to find food; it stands still, makes its own food in the light, and holds itself upright without a skeleton. Each difference below is a consequence of that.
The Four Structural Differences
A plant cell has a definite cell wall of cellulose outside its membrane; an animal cell has none. The wall is what holds a plant up, and an animal that must bend and move is better off without one.
A plant cell contains plastids; an animal cell does not. Plastids are the organelles of food manufacture and storage, and only a plant makes its own food.
A plant cell has prominent vacuoles, often a single large one, concerned with excretion or secretion. An animal cell’s vacuoles, if it has any, are small and temporary.
An animal cell has a centrosome; a plant cell does not. Plant cells organise their cell division without one.
The Three Quantitative Differences
The remaining differences are matters of degree rather than of presence and absence.
Plant cells are usually larger, and their rigid walls give them distinct, straight outlines. Animal cells are usually smaller, with less distinct boundaries.
The cytoplasm of an animal cell is denser and more granular; that of a plant cell is not so dense.
Most striking is where the cytoplasm sits. In a plant cell the large central vacuole occupies the middle, so the cytoplasm is pushed out into a thin lining against the wall. In an animal cell the cytoplasm fills almost the entire cell. Fig. 2.9-1 shows the contrast.
Diagram
Fig. 2.9-1 — Where The Cytoplasm Lies In The Two Cells Two simple outline drawings side by side. A — plant cell: a rectangular cell with a large central vacuole occupying most of the interior, the cytoplasm shown as a thin shaded lining against the wall, with the nucleus pressed to one side. B — animal cell: a rounded cell with cytoplasm shaded throughout and the nucleus near the centre. Label cell wall, large central vacuole, thin lining of cytoplasm, nucleus pushed to one side, cytoplasm filling the cell.
2.10 Protoplasm
Biologists have long needed a single word for the living material itself — not for a particular organelle, but for everything in a cell that is alive. That word is protoplasm.
The arithmetic of it is simple:
\[\text{Cytoplasm} + \text{Nucleus} = \text{Protoplasm}\]
Fig. 2.10-1 shows what that adds up to inside a cell.
Diagram
Fig. 2.10-1 — Protoplasm In A Cell Outline drawing of a single generalised cell with the cytoplasm and the nucleus together shown in one shade and the cell wall left unshaded; label cell wall (non-living), cell membrane, cytoplasm, nucleus, and a bracket across the shaded region marked protoplasm.
To the eye, protoplasm is not impressive. It has been described as a translucent fluid, somewhat colourless, greyish or brownish. Nothing about its appearance suggests that it is the only material in the universe that is alive.
What Protoplasm Is Made Of
The chemical composition of protoplasm is very complex, and it varies a little from one cell to another. What does not vary are the elements involved.
Carbon, hydrogen, oxygen, nitrogen, sulphur, iron and phosphorus are found in the protoplasm of all cells alike. These elements are not present as free elements, but combined into specific compounds — water, proteins, carbohydrates, fats and mineral salts.
Why It Cannot Be Analysed Exactly
There is a genuine difficulty here, and it is worth understanding rather than memorising. It is impossible to make an accurate chemical analysis of protoplasm.
The reason is that protoplasm has to be removed from the organism before it can be analysed — and the moment it is removed, it stops being protoplasm. What the chemist tests is no longer living material, so the analysis describes the remains rather than the thing itself.
A Fuller Definition Of The Cell
We can now describe a cell more completely than we could at the start of the chapter. Section 2.1 defined it by what it does; this definition adds what it is made of.
That last clause is deliberate. Some cells enclose their nuclear material in a membrane and some do not, and that single difference divides all living things into two great groups.
2.11 Prokaryotic And Eukaryotic Cells
The difference hinted at in the last section is the deepest division in the living world. It turns on one question: is the cell’s genetic material shut inside a membrane, or is it not?
The two words for it come from Greek. Karyon means nucleus. Pro means early or primitive, and eu means true or complete.
Prokaryotic cells are cells with only a primitive nucleus — no nuclear membrane, the nuclear material lying free in the region called the nucleoid. Bacteria are prokaryotic. Fig. 2.11-1 shows a bacterial cell.
Eukaryotic cells are cells with a perfectly formed nucleus, bounded by a nuclear membrane. All plants and animals are built of eukaryotic cells.
Diagram
Fig. 2.11-1 — A Bacterial Cell Highly diagrammatic line drawing of a rod-shaped bacterial cell in cut-away view; label cell wall, capsule, cell membrane, mesosome, cytoplasm, ribosomes, bacterial chromosome (nucleoid), plasmid, glycogen granules, flagellum. Labels on straight, non-crossing lead lines.
What Else Follows From The Missing Membrane
A prokaryotic cell differs from a eukaryotic one in more than the nucleus, and the other differences fit the same pattern of simplicity.
Its genetic material is a single length of DNA, not divided into separate chromosomes and not wound around proteins. A eukaryotic cell carries several lengths of genetic material — the chromosomes — with the DNA wound around certain proteins.
Its ribosomes are smaller than those of a eukaryotic cell. And apart from those ribosomes, a prokaryotic cell has no organelles at all: no mitochondria, no endoplasmic reticulum, no chloroplasts, no Golgi apparatus. The reactions those organelles would carry out take place in the cytoplasm or at the cell membrane instead.
Table 2.5 sets the two kinds of cell side by side.
Why This Difference Comes First
This nuclear difference is the first basis on which organisms are classified. Before living things are sorted into plants and animals, or into any smaller group, they are sorted into prokaryotes and eukaryotes.
The reason is historical. The prokaryotes were the earliest form of life to appear on the earth, and the eukaryotes are the later, more complex type. Every division we make afterwards is a division within one of these two.
2.12 Every Activity Of An Organism Is Cellular Activity
The cell theory says that the cell is the unit of function. That is easy to state and easy to forget. This section makes it concrete by taking the ordinary activities of a living body, one at a time, and asking which cells are doing them.
Growth And Repair
All organisms grow. Growth happens partly because cells increase in size, but mainly because they increase in number — more cells means more body substance.
Repair works the same way. When an injury heals, or when a lizard regenerates the tail it has shed, the lost part is rebuilt by cells dividing.
Movement
Movement of any kind comes from the contractility of cells or of their parts — their ability to shorten and pull.
- Animals walk, run, jump, swim and fly using muscles, which are contractile cells, and these move the bones. Bone itself is formed of cells and of the substances those cells secrete.
- Even movements you never think about are muscular. Blood flows along blood vessels, and food is pushed along the gut, because muscle cells in their walls contract.
- The feathers that let a bird fly are themselves products of cells.
- Plants move too. Touch a leaf of the sensitive plant, Mimosa pudica, and it droops within seconds, then slowly recovers. Both the drooping and the recovery are caused by the cells at the base of the leaves.
- Roots bend towards water and downwards with gravity; stems bend towards light and away from gravity. These bending movements are the work of the cells on the two sides of the growing organ.
- The closing of stomata at night and the opening of the petals of a flower both follow from changes in the water content of the cells concerned.
Feeding And Nutrition
Feeding looks like one activity but is a chain of cellular ones. Sensory cells on the tongue taste the food. Muscle cells of the jaws and the tongue chew it and swallow it.
The cells of the digestive glands then secrete enzymes to digest it, and the cells lining the intestine absorb what has been digested.
Whatever is surplus is stored — as fat in the fat cells, and as glycogen in the cells of the liver.
Transport Within The Body
Blood circulates, and other fluids are moved about, because muscle cells in the heart and elsewhere contract and set up the necessary force.
The blood carries the respiratory gases: blood cells transport oxygen from the lungs to the rest of the body.
Defence Against Disease
The body protects itself from disease-causing germs through certain cells — the white blood cells you met in section 2.7.
Some of them devour the germs outright. Others release antibodies and antitoxins, substances that kill the germs or neutralise the poisons they produce.
Sensation, Memory And Thought
You see, hear, smell, taste and feel touch, pain, heat and cold through sensory cells. Every sensation begins with a cell responding to something.
Memory, and the capacity to solve a problem, are likewise the outcome of the activity of cells — in this case the nerve cells of the brain.
Keeping The Body Steady
Body heat is maintained by cellular activity, and this control of body temperature is called thermoregulation.
When you are too hot, the cells of the sweat glands in the skin release sweat, and its evaporation cools you. A cell secreting a few drops of liquid is how the whole body loses heat.
What Plant Cells Do For The Plant
In plants, water and nutrients are absorbed from the soil by root cells, and the cells of the stem conduct that water — and the food made in the leaves — to every other part of the plant.
Light is trapped by the leaf cells that contain chloroplasts, and food is manufactured there.
Even the attracting of insects is cellular. The colour that draws an insect to a flower lies in the cells of the petals, and the nectar that rewards it is produced by nectar-secreting cells.
Producing The Next Generation
All organisms produce young ones — fertilised eggs in birds, babies in mammals, seeds in plants — and in every case this happens through the activity of two special cells, the egg and the sperm.
These cells also decide what the young one will be like. A hen’s egg produces a hen, and the seed of a particular plant produces that same kind of plant. This passing on of parental features is called inheritance, and it depends entirely on what the egg and the sperm carry with them.
The General Rule
Table 2.6 gathers a few of these activities together, with the cells responsible for each.
There is not a single activity in the body of an organism that is not carried out by its cells. What differs from one activity to another is only which cells do it — because cells, of course, are specialised for particular functions.
2.13 Summary
- All living organisms, from the microscopic to the enormous, are built of the same kind of building block — the cell.
- The cell is the basic structural and functional unit of all living beings: the smallest part capable of independent existence and of performing the essential functions of life.
- Cells are microscopic, so they could not be discovered until magnifying instruments were built.
- Antony van Leeuwenhoek (1632–1723) made the first useful microscopes, each with a single biconvex lens, magnifying up to about 200 times.
- Robert Hooke (1635–1703) used two lenses in one tube to make a compound microscope, in which the second lens magnifies the image formed by the first.
- Hooke examined a thin slice of cork, saw box-like compartments, and named them cells after the small rooms of monks in a monastery. These cork cells were dead, so what he saw were their empty walls.
- A compound microscope uses light bent by glass lenses and magnifies up to about 2,000 times; an electron microscope uses beams of electrons bent by magnets and magnifies over 200,000 times.
- The cell theory was proposed by Schleiden (1838, plants) and Schwann (1839, animals), and completed by Virchow (1858).
- The cell theory states three points: the cell is the smallest unit of structure of all living things; the cell is the unit of function of all living things; all cells arise from pre-existing cells.
- Old cells die continuously and are replaced by new cells formed by the division of existing cells, so the chain of cells never breaks. Human red blood cells, for example, live about 120 days.
- Most organisms, including human beings, begin life as a single cell, the zygote.
- By number of cells, organisms are single-celled (bacteria, yeast, amoeba), few-celled (Spirogyra, Volvox) or many-celled (human beings, mango trees).
- An average adult human body has about 37.2 trillion cells, about 100 billion nerve cells in the brain, and about 4 to 6 million red blood cells and 7 thousand white blood cells in each cubic millimetre of blood.
- Bacteria are the smallest cells (about 0.3–5.0 µm) and human red blood cells measure about 7 µm; nerve cells are the longest cells, and the yolk of an ostrich egg is the largest single cell alive today.
- Cells stay small for two reasons: different regions of a small cell communicate rapidly, and a small cell has a large surface area for its volume.
- Cutting a 2 mm cube into eight 1 mm cubes doubles the total surface area from 24 mm² to 48 mm² while the volume stays at 8 mm³ — which is why many small cells serve a body better than few large ones.
- A large surface area to volume ratio allows greater diffusion of nutrients in, wastes out and respiratory gases both ways, and makes damage easier to repair.
- The shape of a cell suits its work: red blood cells are biconcave discs, white blood cells are amoeboid and move on pseudopodia, nerve cells are long, muscle cells are long and contractile, and guard cells are bean-shaped so as to open and close a stomatal pore.
- A generalised cell has three essential parts — cell membrane, cytoplasm and nucleus — and the distinct working bodies within it are called cell organelles.
- The cell membrane is living, made of lipoproteins, has fine pores and is selectively permeable.
- Plant cells have an additional cell wall outside the membrane, made of cellulose, non-living, rigid and freely permeable; a thin middle layer cements adjacent cell walls together.
- Cytoplasm is the semi-liquid material between the membrane and the nucleus; most of the cell’s chemical reactions occur in it and it is always in some movement.
- The endoplasmic reticulum is a network of membrane-bounded tubules and sacs, connected to the cell membrane outside and the nuclear membrane inside; it forms a supporting framework and a pathway for distributing materials, and is rough where ribosomes are attached and smooth where they are not.
- Ribosomes are very small granules of RNA and protein, free in the cytoplasm or attached to the ER, and are the sites of protein synthesis.
- Mitochondria are double-walled bodies whose inner wall is folded into cristae; cell respiration occurs in them, releasing energy that is stored as ATP, and they contain their own DNA.
- The Golgi apparatus, called dictyosomes in plant cells, is made of cisternae with vesicles and vacuoles, and handles the secretion, packaging and transport of enzymes and hormones.
- Lysosomes are enzyme-filled sacs budded off from the Golgi bodies; they digest foreign substances and stored food, and destroy old or injured cells, which is why they are called suicide bags.
- A centrosome, containing two centrioles at right angles, occurs only in animal cells and forms the spindle fibres during cell division.
- Plastids occur only in plant cells and are of three kinds — leucoplasts (colourless, store starch), chromoplasts (yellow, orange and red, with xanthophyll and carotene) and chloroplasts (green, with chlorophyll, trapping solar energy for photosynthesis).
- Anthocyanins are not plastid pigments; they lie dissolved in the cell sap of vacuoles and give red, purple and blue colours.
- Granules and vacuoles are non-living cell inclusions. Granules store starch, glycogen and fat; vacuoles hold cell sap, are bounded by a tonoplast, and are large in plant cells and small in animal cells.
- The nucleus regulates and coordinates the life processes of the cell, is essential in cell division, and carries the genes that determine heredity.
- The nucleus consists of a nuclear membrane with pores, dense nucleoplasm, at least one nucleolus that produces ribosomes, and a network of chromatin fibres.
- Chromatin fibres condense during cell division into thick ribbon-like chromosomes; their number is definite for each species, and every human body cell has 46, that is 23 pairs.
- Chromatin is composed of genes, which are made of DNA; it is the genes, not the number of chromosomes, that determine the characteristics of a species.
- Plant and animal cells share membrane, cytoplasm, nucleus, ER, Golgi apparatus, mitochondria and ribosomes; they differ in the cell wall, plastids and large vacuoles (plant only) and the centrosome (animal only), and in size, density and arrangement of cytoplasm.
- Protoplasm is the total living substance of a cell — cytoplasm plus nucleus. It contains carbon, hydrogen, oxygen, nitrogen, sulphur, iron and phosphorus, combined as water, proteins, carbohydrates, fats and mineral salts, and cannot be analysed exactly because it ceases to be protoplasm once removed from the organism.
- Prokaryotic cells have no nuclear membrane and no membrane-bounded organelles; their single length of DNA lies free as a nucleoid, and their ribosomes are small. Examples are bacteria and blue green algae.
- Eukaryotic cells have a well defined nucleus, several chromosomes and many organelles. Examples are Euglena, amoeba, and all plants and animals. This nuclear difference is the first basis of the classification of organisms.
- Every activity of an organism — growth, repair, movement, nutrition, circulation, transport of gases, defence, sensation, thermoregulation and reproduction — is carried out by cells, each kind specialised for its own function.
Flowchart
FC 2.4 — The Parts Of A Cell At A Glance Cell → cell membrane (selectively permeable boundary; cell wall additionally in plant cells) → cytoplasm → organelles: endoplasmic reticulum, ribosomes, mitochondria, Golgi apparatus, lysosomes, centrosome (animal cells), plastids (plant cells); and inclusions: granules, vacuoles → nucleus → nuclear membrane, nucleoplasm, nucleolus, chromatin fibres → chromosomes → genes → DNA
2.14 Key Terms
- Anthocyanins — red, purple and blue pigments that are not held in plastids but lie dissolved in the cell sap of vacuoles.
- ATP (adenosine triphosphate) — the energy-rich compound in which a cell stores the energy released during respiration.
- Carotene — the orange-red pigment of chromoplasts, also present in chloroplasts but masked by chlorophyll.
- Cell — the basic structural and functional unit of all living beings, and the smallest part able to exist independently and perform the essential functions of life.
- Cell membrane — the living, selectively permeable boundary of every cell, made of lipoproteins.
- Cell organelles — the distinct living parts within a cell, each with a definite shape, structure and function.
- Cell sap — the liquid filling the vacuole of a plant cell.
- Cell theory — the statement that the cell is the unit of structure and of function of all living things, and that all cells arise from pre-existing cells.
- Cell wall — the non-living, freely permeable outer covering of a plant cell, made mainly of cellulose.
- Cellulose — the tough carbohydrate of which plant cell walls are mainly built.
- Centrioles — two short cylinders of microtubules, set at right angles, lying within the centrosome of an animal cell.
- Centrosome — the region near the nucleus of an animal cell from which the spindle fibres develop during cell division.
- Chlorophyll — the green pigment of chloroplasts that traps solar energy for photosynthesis.
- Chloroplasts — green plastids containing chlorophyll, in which food is manufactured.
- Chromatin fibres — the network of fine DNA-containing threads in a nucleus that is not dividing.
- Chromoplasts — coloured plastids, yellow to red, found mainly in petals and fruits.
- Chromosomes — the thick, ribbon-like bodies into which chromatin fibres condense during cell division, carrying the genes.
- Cisternae — the hollow membranous sacs that make up the Golgi apparatus.
- Compound microscope — a microscope using two lenses in a tube, the second magnifying the image made by the first.
- Cristae — the finger-like inward folds of the inner wall of a mitochondrion.
- Cytoplasm — the semi-liquid material filling the cell between the cell membrane and the nucleus.
- Dictyosomes — the more scattered form the Golgi apparatus takes in plant cells.
- Diffusion — the spreading of a substance from where it is more concentrated to where it is less concentrated.
- DNA (deoxyribonucleic acid) — the complex chemical substance of which genes are made.
- Electron microscope — a microscope that uses beams of electrons bent by magnets, magnifying over 200,000 times.
- Endoplasmic reticulum — a network of membrane-bounded tubules and sacs that supports the cell and distributes materials through it.
- Eukaryotic — describing a cell with a true nucleus enclosed in a nuclear membrane.
- Genes — the units of heredity, made of DNA, carried on the chromosomes.
- Granules — non-living particles in the cytoplasm that store starch, glycogen or fat.
- Guard cells — the pair of bean-shaped cells that open and close a stomatal pore.
- Leucoplasts — colourless plastids that store starch.
- Lysosomes — enzyme-filled sacs budded off from the Golgi bodies, which digest foreign matter, stored food, and worn-out cells.
- Micrometre (µm) — one thousandth of a millimetre; the unit in which cells are measured.
- Mitochondria — double-walled organelles with folded inner walls, where respiration releases energy and stores it as ATP.
- Nuclear membrane — the pored membrane enclosing the nucleus.
- Nucleoid — the region of a prokaryotic cell in which the DNA lies free, without any membrane around it.
- Nucleolus — a small round body within the nucleus that produces ribosomes.
- Nucleoplasm — the dense fluid filling the nucleus.
- Nucleus — the organelle that regulates the cell’s activities, takes part in cell division and carries the genes.
- Plastids — organelles found only in plant cells, of three kinds — leucoplasts, chromoplasts and chloroplasts.
- Prokaryotic — describing a cell with no nuclear membrane and no membrane-bounded organelles, as in bacteria.
- Protoplasm — the total living substance of a cell, that is, the cytoplasm together with the nucleus.
- Pseudopodia — temporary extensions of a cell into which it flows in order to move.
- Ribosomes — very small granules of RNA and protein at which proteins are assembled.
- Selectively permeable — allowing certain substances to pass through while preventing others.
- Simple microscope — a microscope using a single biconvex lens.
- Surface area to volume ratio — the amount of surface a body has for each unit of its volume; it rises as the body gets smaller.
- Tonoplast — the membrane bounding a vacuole.
- Vacuoles — non-living clear spaces in the cytoplasm filled with water and dissolved substances.
- Xanthophyll — the yellow pigment of chromoplasts.
- Zygote — the single cell formed by the union of a male and a female gamete, from which an organism develops.
2.15 Points To Ponder
An ostrich egg yolk is a single cell many centimetres across, yet you have learnt that cells stay small because a large surface area to volume ratio is an advantage. How can such a cell survive at that size, and what is different about the way it lives?
Mitochondria and chloroplasts both carry their own DNA and can divide within the cell, which no other organelle does. What might that suggest about where these two organelles came from?
A prokaryotic cell carries out respiration, protein synthesis and secretion without a single membrane-bounded organelle. If the work gets done either way, what does a eukaryotic cell gain by dividing its interior into compartments?
Every cell in your body carries the same 46 chromosomes and therefore the same genes. Why, then, does a nerve cell look and behave nothing like a fat cell?
Lysosomes hold enzymes powerful enough to destroy the cell that made them. Why is it useful for a cell to keep such a dangerous set of chemicals, and what must it be doing to stay safe from them?
Hooke saw only empty walls and still gave us the word ‘cell’, which we now use for something that is alive and full. Should a scientific term be corrected when the thing it names turns out to be quite different from what was first seen?
2.16 Objective Questions
- Which of these parts of a plant cell is non-living? (1 mark)
- Cell membrane
- Cell wall
- Nucleus
- Mitochondrion
- The organelle in which energy is released and stored as ATP is the: (1 mark)
- Ribosome
- Lysosome
- Mitochondrion
- Golgi apparatus
- The largest single cell of the living world today is: (1 mark)
- A nerve cell
- A human red blood cell
- The yolk of an ostrich egg
- A bacterium
- Chromatin fibres become short, thick and ribbon-like during: (1 mark)
- Protein synthesis
- Cell division
- Respiration
- Diffusion
- Which one of these pigments is not held in a plastid? (1 mark)
- Chlorophyll
- Carotene
- Xanthophyll
- Anthocyanin
- For questions a. to c., choose from the options below:
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
- Assertion (A): Cells generally remain very small in size. Reason (R): A small cell has a large surface area for every unit of its volume. (1 mark)
- Assertion (A): A plant cell divides without a centrosome. Reason (R): Spindle fibres are not formed during the division of a plant cell. (1 mark)
- Assertion (A): Lysosomes are called suicide bags. Reason (R): They contain digestive enzymes that can destroy the cell’s own worn-out parts. (1 mark)
- Match each item in Column I with the correct item in Column II. (5 marks)
| Column I | Column II |
|---|---|
| a. Nucleolus | i. Packaging and transport of secretions |
| b. Golgi apparatus | ii. Traps solar energy |
| c. Chloroplast | iii. Produces ribosomes |
| d. Tonoplast | iv. Forms the spindle fibres |
| e. Centrosome | v. Bounds a vacuole |
- Fill in the blanks: (4 marks)
- The cytoplasm together with the nucleus makes up the ______ of a cell.
- The finger-like inward folds of the inner wall of a mitochondrion are called ______.
- The liquid held in the vacuole of a plant cell is called ______.
- In a prokaryotic cell the DNA lies free in a region called the ______.
- State whether each statement is true or false: (4 marks)
- The cell wall is selectively permeable.
- Ribosomes may lie free in the cytoplasm or be attached to the endoplasmic reticulum.
- An animal cell contains plastids.
- Every human body cell contains 23 chromosomes.
- Name the following: (5 marks)
- The kind of microscope built from a single biconvex lens.
- The scientist who stated that all cells arise from pre-existing cells.
- The plastid that has no pigment and stores starch.
- The structure inside the nucleus that produces ribosomes.
- The unit in which the size of a cell is usually measured.
- Pick the odd one out and give a reason for your choice: (3 marks)
- Mitochondrion, ribosome, cell wall, lysosome
- Chlorophyll, xanthophyll, carotene, cellulose
- Bacteria, amoeba, Euglena, human beings
2.17 Check Your Concepts
Define a cell. (2 marks)
Name two organelles found only in a plant cell and one found only in an animal cell. (2 marks)
State two functions of the nucleus. (2 marks)
Differentiate between cytoplasm and protoplasm, stating the basis of difference. (2 marks)
State the three points of the cell theory. (3 marks)
Give a reason for each: (3 marks)
- Lysosomes are described as suicide bags.
- The cell wall does not interfere with the selective working of the cell membrane.
- Mitochondria are called the power houses of the cell.
Why did the discovery of the cell have to wait for the invention of the microscope? (3 marks)
Differentiate between a prokaryotic and a eukaryotic cell, stating four bases of difference. (4 marks)
Explain, with one example each, how the shape of a red blood cell, a nerve cell and a guard cell suits the work it does. (4 marks)
What would happen to a cell if each of the following occurred? Give a reason in each case. (4 marks)
- Its nucleus were removed.
- All its lysosomes broke open at once.
- Its cell membrane became freely permeable.
- A plant cell lost the water from its vacuole.
Describe the structure of a mitochondrion and explain how that structure suits its function. (5 marks)
Cells remain small rather than growing large. Explain the two reasons for this, using the surface area to volume ratio in your answer. (5 marks)
Describe the three kinds of plastids, stating for each its colour, the pigment or material it holds, and where in a plant you would expect to find it. (5 marks)
2.18 Figure-Based Questions
- Study Fig. 2.8-1 B. (4 marks)
- Identify the type of cell shown, and give two reasons visible in the drawing for your answer.
- Name the part that lies immediately outside the cell membrane, and state one function of it.
- Study Fig. 2.8-2. (3 marks)
- Name the organelle shown.
- Name the folds of its inner wall.
- State the process that takes place in this organelle.
- Study Fig. 2.8-4 A. (3 marks)
- Name the structure shown.
- Name the network of threads seen inside it, and state what these threads become during cell division.
- State one function of the small round body within it.
- Study Fig. 2.11-1. (3 marks)
- Identify the type of cell shown and give one reason for your answer.
- Name any two organelles that are absent from this cell but present in the cell of a monkey.
- Study Fig. 2.6-1. (3 marks)
- Calculate the total surface area of the single large cube and of the eight smaller cubes together.
- State what happens to the total volume, and explain what this demonstration tells you about the size of cells.
- Redraw the generalised animal cell shown in Fig. 2.8-1 A and label any six parts. Labels must sit on straight, non-crossing lead lines. (5 marks)