Chapter 1: Cell — The Structural and Functional Unit of Life
Robert Hooke was an English scientist who liked to point his microscope at ordinary things — a fly’s eye, the tip of a needle, a flake of stone. One day he cut a very thin slice from a piece of cork, the light bark used to stopper bottles, and held it under the lens.
What he saw did not look like solid material at all. The cork was divided into rows of tiny boxes, packed side by side like the rooms of a building, each shut off from its neighbours by a thin wall. There were so many of them in so little cork that he worked out that a cube of cork an inch across would hold more than a thousand million.
Hooke needed a name for the little boxes. They reminded him of the small bare rooms in which monks lived, which were called cells, and that is the word he wrote down in his book Micrographia in 1665. It has been the word ever since.
He did not know what he had really found. The boxes looked empty because the cork was dead. What he was looking at were the leftover walls of cells whose living contents had long since gone. Better microscopes and two centuries of patient work filled those boxes in — and showed that every living thing you have ever seen is built of them. Fig. 1.0-1 shows Hooke’s instrument and the view that gave the cell its name.
Diagram
Fig. 1.0-1 — Hooke’s Microscope And His View Of Cork Two-part line drawing. (A) Hooke’s compound microscope of 1665, drawn in side view; label eyepiece lens, body tube, objective lens, specimen holder, focusing screw, oil lamp and water-filled glass globe used as a condenser. (B) The appearance of a thin slice of cork under that microscope; label thin dividing wall, empty box-like compartment (cell). Labels on straight, non-crossing lead lines.
1.1 Cells — A Basic Study In Biology
Every organism you can name is built of cells — a blade of grass, a housefly, the person reading this line. A cell is far too small to be seen with the unaided eye, and yet each one is busy. Each carries out its own set of activities, and those activities together add up to the life of the whole individual.
From One Cell To Many
Most organisms begin life as a single cell. That cell divides into two, the two divide into four, and the doubling continues until a large mass of cells has been produced.
The cells in that mass do not all stay alike. As the body takes shape, groups of cells become specialised — they change their shape and their internal make-up to suit one particular job.
Some cells come to give support and hold the body together. Others produce secretions such as sweat, saliva or digestive juice. Still others carry messages, store food, or take part in the many other activities of living. A body works because its cells divide the labour between them. Fig. 1.1-1 follows a single cell through these stages.
Diagram
Fig. 1.1-1 — From One Cell To Specialised Cells Line drawing in three stages, left to right, joined by arrows. Stage 1: a single rounded cell. Stage 2: the same cell after repeated division, drawn as a ball of many similar cells. Stage 3: two specialised cells drawn separately — a supporting cell with thick rigid walls, and a gland cell releasing a droplet of secretion. Label single cell, repeated division, mass of similar cells, supporting cell, gland cell. Labels on straight, non-crossing lead lines.
Four Things True Of Every Cell
- State whether each of the following is true or false.
- All living beings are made up of cells. No organism has ever been found that is built of anything else. Whether a body holds one cell or many billions, cells are the material it is made of.
- The cell is the structural and functional unit of the body. It is the structural unit because the body is built out of cells, the way a wall is built out of bricks. It is the functional unit because every activity of the body — digestion, movement, growth, the sensing of a pinprick — is finally carried out inside cells.
- All cells are basically alike in chemical composition and in their metabolic processes. They are made of the same kinds of substances — water, proteins, carbohydrates, fats and nucleic acids. They also run the same basic chemical reactions, together called metabolism, which is the whole set of chemical changes by which a cell releases energy, builds materials and breaks them down. This is why something learnt from the cells of one organism usually holds good for the cells of another.
- All living beings arise from a pre-existing cell. A cell never assembles itself out of non-living matter. Every cell in your body was produced by the division of a cell that was already there.
1.2 Cells — How Many?
All living things are built of microscopic cells, but they are not built of the same number of them. Some organisms are formed of just one cell, such as bacteria and Amoeba. Large plants and animals are made of billions.
The rule behind this is simple: the larger an organism, the greater the number of cells in its body. A body does not grow by swelling its cells to a larger size, because a cell that grows too big cannot supply its own centre with materials. It grows by making more cells of much the same small size.
Diagram
Fig. 1.2-1 — Some Organisms Whose Whole Body Is One Cell Four separate outline line drawings, side by side, each drawn as a single cell with its boundary and nucleus shown: Amoeba with its irregular changing outline, Paramecium with its slipper shape, a rod-shaped bacterium, and Chlamydomonas with its two flagella. Label each organism by name, and label cell membrane and nucleus on the Paramecium drawing only. Labels on straight, non-crossing lead lines.
1.2.1 Unicellular Organisms
An organism whose entire body is a single cell is a unicellular organism. That one cell has to do everything — take in food, release energy, get rid of wastes, respond to its surroundings and reproduce. There is no other cell to share the work with.
Examples are Amoeba, Paramecium, bacteria and Chlamydomonas. Fig. 1.2-1 shows the first four of these; notice how different their shapes are, although each is one cell.
1.2.2 Multicellular Organisms
Most living organisms are made up of millions or billions of cells. These are multicellular organisms. Because there are so many cells, the work can be shared out, and each cell is free to become specialised for one job and to do it well.
Examples are Volvox, rose, mosquito, mango and humans.
Counting The Cells In A Human Body
An adult human of average size is made of a very large number of cells. The figures below are careful scientific estimates rather than exact counts. A trillion here means a million million.
About 200 different types of cell make up the body.
The whole body contains roughly 37.2 trillion cells (estimate published in 2013).
The brain contains about 86 billion nerve cells, of which roughly 16 billion lie in the cerebral cortex, its outer layer.
About 25 trillion of the total are red blood cells.
Read those numbers together and one thing stands out. Red blood cells alone account for the majority of the cells in your body, even though they are among the smallest cells you have.
1.3 Cells — How Small?
Cells are very small, and can be seen only with the help of a microscope. Your unaided eye cannot separate two points that lie closer together than about a tenth of a millimetre, and almost every cell in your body is smaller than that.
Cell sizes are usually given in micrometres. One micrometre (µm) is one-thousandth of a millimetre. A human red blood cell measures roughly 7 µm across, which is about a fourteenth of the smallest gap your eye can resolve.
Why Cells Stay Small
Everything a cell takes in and everything it gives out has to cross its surface. Food, oxygen and water enter through it; carbon dioxide and other wastes leave through it. The crossing happens mainly by diffusion, the spreading of a substance from where it is more concentrated to where it is less concentrated.
Now think about what happens as a cell grows. Its volume increases much faster than its surface area, so each unit of volume is served by less and less surface. A cell that grew too large would have a surface too small to feed its own interior.
Staying small keeps the surface area to volume ratio large. Fig. 1.3-1 shows why: the same amount of material divided into many small units has far more surface than a single large one. That is why bodies grow by adding cells rather than by enlarging them.
Diagram
Fig. 1.3-1 — Small Units Have More Surface Than One Large Unit Line drawing comparing two blocks of the same total volume. On the left, a single large cube; on the right, the same volume divided into eight small cubes arranged as a block of equal size. Label total volume the same in both, and mark the exposed surfaces by shading. Add the caption labels large cube — small surface for its volume, and eight small cubes — much greater total surface for the same volume.
The Smallest, The Longest And The Largest
Cells are not all of one size, and the extremes are worth knowing.
Smallest cells. Certain bacteria are the smallest cells of all. Among human cells, the red blood cells are among the smallest.
Longest cells. Nerve cells are the longest. A single nerve cell can stretch from the spinal cord all the way to a toe.
Largest cells. The largest cells are the eggs of birds — strictly, the central yellow sphere, the yolk. Before development begins in it, the ostrich egg is the largest single cell in the living world today.
The yolk is a single cell, but almost all of it is stored food for the developing chick. The living material — the cytoplasm and the nucleus — is only a small pale spot on the yolk’s surface. So the yolk is not really an exception to the rule about size: the active living part of it is small, as living parts always are.
The white of the egg and the shell are extra parts. They are added on to the actual egg as it passes down the reproductive tract, and they take no direct part in reproduction. Fig. 1.3-2 shows where each of these lies.
Diagram
Fig. 1.3-2 — Longitudinal Section Of A Bird’s Egg Longitudinal section of a bird’s egg drawn as a line diagram; label shell, shell membrane, albumen (egg white), yolk — the actual egg cell, and the small pale disc of living cytoplasm and nucleus on the yolk surface. Labels on straight, non-crossing lead lines.
1.4 Cell Shapes
Cells come in scores of different shapes, and the shapes are not accidental. In almost every case the shape of a cell is related to the work that cell has to do. If you know what a cell must achieve, you can often predict roughly what it will look like. Fig. 1.4-1 sets four familiar examples side by side.
Diagram
Fig. 1.4-1 — Four Cell Shapes And The Work They Suit Line drawings of four cells arranged in a row. (A) Human red blood cell, drawn twice — in surface view showing the circular outline with a pale centre, and in vertical section showing the biconcave shape. (B) White blood cell with an irregular outline and pseudopodia. (C) Nerve cell drawn as a cell body with a long fibre, the fibre shown broken to indicate great length. (D) A pair of bean-shaped guard cells enclosing an open stomatal pore, in surface view. Label each cell, and label pseudopodia, nerve fibre and stomatal pore. Labels on straight, non-crossing lead lines.
Red blood cells of humans are biconcave and disc-shaped, which means they are circular with a dip in the centre of both faces. The shape leaves them thin and flexible, so they bend easily and pass through the narrowest blood capillaries. The two dips also give the cell a large surface for its volume, which helps it take up and give out oxygen quickly.
White blood cells are amoeboid. They have no fixed shape and move the way Amoeba does, pushing out pseudopodia — temporary finger-like extensions of the cytoplasm — and flowing into them. Because they can change shape, they squeeze out between the cells of a capillary wall and reach a site of infection in the tissues.
Nerve cells are long. A single nerve cell carries an impulse — the signal that travels along a nerve — from a distant part of the body to the brain, or from the brain out to a muscle. A long cell carries that signal along its own length instead of handing it over again and again, and a message handed over fewer times arrives faster.
Guard cells are bean-shaped. A pair of them lies curved against each other around each stoma, one of the tiny pores in the surface of a leaf through which gases enter and leave. When the two cells swell they bow apart and the pore opens; when they lose water they straighten against each other and the pore closes.
1.5 Gross Structure Of A Cell — The Three Essential Parts
Look at any cell under a good microscope and three parts stand out at once: a boundary around the outside, a jelly-like material filling the inside, and a dense rounded body sitting in that jelly. These are the cell membrane, the cytoplasm and the nucleus, and no living cell of a plant or animal is without them.
A plant cell carries one extra layer outside the membrane, the cell wall, which an animal cell does not have. Fig. 1.5-1 sets a generalised animal cell beside a generalised plant cell so that you can see which parts are shared and which belong to only one of them. The many small bodies inside the cytoplasm are named in the figure and are taken up one by one in section 1.6.
Diagram
Fig. 1.5-1 — A Generalised Animal Cell And A Generalised Plant Cell Two large line drawings side by side, each a whole cell cut open to show its contents. (A) Animal cell; label cell membrane, cytoplasm, nucleus, nuclear membrane, nuclear pore, nucleolus, chromatin fibre, mitochondrion, rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi body, lysosome and centrioles. (B) Plant cell; label cell wall, cell membrane, cytoplasm, nucleus, large central vacuole, chloroplasts, mitochondrion, rough endoplasmic reticulum, smooth endoplasmic reticulum and Golgi body. Draw the parts found in only one of the two cells with a boxed label so that they stand out. Labels on straight, non-crossing lead lines.
1.5.1 The Cell Membrane
The cell membrane, also called the plasma membrane, encircles the cell and forms its outer boundary. In an animal cell it is the outermost layer of all.
It is a living membrane, very thin, and carries fine pores through which substances can pass. It is made up of lipoproteins — molecules that are part lipid, or fat, and part protein.
The membrane is flexible, and this flexibility passes to the cell as a whole. A cell can bend, stretch and change shape without its boundary tearing.
Its most important property, however, is that it is semi-permeable, also called selectively permeable or differentially permeable. It allows only certain substances to pass through and prevents others.
This is what makes a cell an independent unit. If everything in the surroundings could wash freely in and out, the cell could not hold on to what it makes or keep out what would harm it. The membrane decides the traffic, and so the cell keeps control of its own contents.
1.5.2 The Cell Wall In Plant Cells
A plant cell has an additional outermost layer lying outside the cell membrane. This is the cell wall, and it is non-living.
It is mainly made up of cellulose, a tough carbohydrate that plants build in large quantities. Because the wall is rigid, it gives the cell a definite shape and holds that shape against pressure from inside.
That rigidity matters to the whole plant. A plant has no bones, so the stiff walls of its cells are what hold a stem upright and keep a leaf spread flat to the light.
The cell wall is freely permeable. Substances in solution enter and leave through it without any hindrance, so it is no barrier at all. Table 1.2 sets the wall and the membrane side by side.
1.5.3 The Cytoplasm
Cytoplasm is the part of the cell that lies inside the cell membrane and outside the nucleus. It is a semi-liquid substance, thicker than water and rather like a jelly.
The cytoplasm is where most of the cell’s chemistry happens. A great many chemical reactions take place in it, each speeded up by an enzyme, a protein that makes a particular reaction run fast enough to be useful at body temperature.
Suspended in the cytoplasm are several organelles — small bodies, each concerned with one specific function. They are described one by one in section 1.6.
The part of the cytoplasm other than the organelles is called the cytosol. The cytosol is mainly the liquid medium itself, the fluid in which the organelles are held.
1.5.4 The Nucleus
The nucleus is a large, roughly spherical body lying nearly in the centre of the cytoplasm. In most cells it is the largest structure you can see, and it is denser than the material around it.
It is surrounded by a double-layered nuclear membrane carrying nuclear pores. The double membrane keeps the contents of the nucleus separate from the cytoplasm.
The pores matter as much as the membrane. The nucleus has to send its instructions out to the cytoplasm and take materials in, so a completely sealed nucleus would be useless. The pores let selected substances move both ways while the nucleus stays a distinct compartment. Fig. 1.5-2 shows this arrangement.
Diagram
Fig. 1.5-2 — Structure Of The Nucleus Enlarged line drawing of a single nucleus, cut open to show its interior; label nuclear membrane (double layered), nuclear pore, nucleoplasm, nucleolus and chromatin network. Labels on straight, non-crossing lead lines.
1.5.4.1 Nucleoplasm And Nucleolus
The ground substance filling the nucleus is a semi-solid material called nucleoplasm. Everything else in the nucleus is held within it.
Lying in the nucleoplasm are one or more round-shaped bodies called nucleoli (singular, nucleolus). A nucleolus forms and stores RNA, and it makes the ribosomes — the tiny bodies on which the cell builds its proteins, described in section 1.6.5.
1.5.4.2 Chromatin Fibres And Chromosomes
The nucleoplasm also contains a network of fine, dark-coloured threads called chromatin fibres. They stain darkly, which is how they were first noticed, and in a resting cell they lie tangled together as a loose network.
These fibres are made of DNA, the substance that carries the instructions for building and running the cell. Along their length lie the genes, the units that carry hereditary information from parents to offspring.
When a cell prepares to divide, the chromatin fibres coil up tightly and condense into short, thick bodies called chromosomes. Fig. 1.5-3 shows them at an early stage of division in an animal cell.
Why should they condense at all? A long, fine, tangled thread would break or be shared out unevenly when the cell splits in two. Coiled into short, thick rods, the same material can be moved about and divided neatly between the two new cells. The number of chromosomes is fixed for a species but varies from one organism to another.
Diagram
Fig. 1.5-3 — Chromosomes At An Early Stage Of Cell Division Diagrammatic line drawing of an animal cell at an early stage of division; show two pairs of short thick chromosomes inside the nucleus, condensed from the chromatin network, with the centrioles moving apart to opposite sides and radiating aster fibres around them. Label nucleus, cytoplasm, chromosomes, centrioles moving apart and aster. Labels on straight, non-crossing lead lines.
1.6 Finer Structure Of A Cell — The Organelles
A cell is not simply a bag of jelly with a nucleus floating in it. Scattered through the cytoplasm are a number of small bodies, each built differently from the others and each doing one particular job. These bodies are called organelles.
The comparison worth holding on to is with your own body. A body has organs — a heart, a stomach, a kidney — and each organ performs one or more special functions. A cell has organelles, and each organelle has a specific function within the cell. An organelle is to a cell what an organ is to a body.
1.6.1 Protoplasm And Protoplast
The living parts of a cell — the cytoplasm, the nucleus and the other living bodies within them — are together called protoplasm. Protoplasm is not one substance but the whole of the living material of a cell taken together.
The same living material is also called the protoplast. This second term is more appropriately used for a plant cell after its cell wall has been removed, when only the living contents are left.
The organelles are the same in plant and animal cells, with two exceptions. The cell wall and the chloroplasts are present in plant cells only, and the centrosome with its centrioles is present in animal cells only.
The various organelles are: nucleus, nucleolus, mitochondria, endoplasmic reticulum, ribosomes, Golgi bodies and lysosomes. You have already met the first two in section 1.5; the rest are taken up below.
Not everything inside a cell is alive. The non-living parts, that is the vacuoles and the granules, are called cell inclusions. FC 1.1 sets out how the parts of a cell divide between living and non-living.
Flowchart
FC 1.1 — How The Parts Of A Cell Are Divided Parts of a cell → branch 1: living parts, together called protoplasm → cytoplasm and nucleus → organelles held within them: nucleus → nucleolus → mitochondria → endoplasmic reticulum → ribosomes → Golgi bodies → lysosomes Parts of a cell → branch 2: non-living parts, called cell inclusions → vacuoles → granules
1.6.2 Endoplasmic Reticulum
The endoplasmic reticulum (ER) is an irregular network of tubules running through the cytoplasm. Each tubule is bounded by a single membrane.
The network is not shut off at either end. It is continuous with the plasma membrane on the outside and with the nuclear membrane on the inside, so it forms a system of channels linking the boundary of the cell to the boundary of the nucleus. Materials can be carried along these channels from one part of a cell to another without crossing the open cytoplasm.
The ER is of two kinds, shown in Fig. 1.6-1. Rough endoplasmic reticulum (RER) has ribosomes attached to its outer surface, which give it a granular appearance. Smooth endoplasmic reticulum (SER) has no ribosomes on it.
The ER has two chief functions. It acts as a supportive framework for the cell, a kind of internal scaffolding that keeps the contents of the cytoplasm in place. It also carries out the synthesis and transport of proteins and fats — proteins where the ribosomes sit on the rough form, fats on the smooth form.
Diagram
Fig. 1.6-1 — Rough And Smooth Endoplasmic Reticulum Line drawing of a portion of cytoplasm next to the nucleus, showing a network of flattened tubules. On one side draw rough endoplasmic reticulum with small dark ribosomes studding its outer surface and continuous with the nuclear membrane; on the other side draw smooth endoplasmic reticulum without ribosomes, continuous with the rough form. Label nuclear membrane, rough endoplasmic reticulum, ribosomes, smooth endoplasmic reticulum and cytoplasm. Labels on straight, non-crossing lead lines.
1.6.3 Mitochondria
Mitochondria are of various shapes, but they are usually sausage-like. A single one is called a mitochondrion.
A mitochondrion is double walled. The outer wall is smooth, while the inner wall is thrown into a series of folds called cristae, which project into the interior. Fig. 1.6-2 shows this arrangement in section.
Mitochondria have their own DNA, containing several genes, and they also contain their own ribosomes. They are the only organelles apart from the plastids that carry genetic material of their own.
The work of a mitochondrion is the release of energy. Glucose is first broken down in the cytoplasm to a substance called pyruvic acid. The mitochondrion takes that pyruvic acid and, using oxygen, breaks it down completely, releasing energy which is stored in the form of ATP — the chemical in which a cell keeps energy in a ready-to-use form.
For this reason the mitochondrion is called the seat of aerobic cellular respiration, that is, respiration that uses oxygen. It also carries out the synthesis of the respiratory enzymes that this breakdown needs.
Diagram
Fig. 1.6-2 — Structure Of A Mitochondrion Longitudinal section of a single sausage-shaped mitochondrion drawn as a line diagram; show the smooth outer membrane, the inner membrane folded inwards into finger-like cristae, and the matrix filling the interior. Label outer membrane, inner membrane, cristae and matrix. Labels on straight, non-crossing lead lines.
1.6.4 Golgi Apparatus
The Golgi apparatus is a set of stacks of flattened membrane sacs lying in the cytoplasm. Each stack is made of tubules called cisternae, together with vesicles and vacuoles. Fig. 1.6-3 shows the arrangement.
The same structures are known by two names. In animal cells they are called the Golgi apparatus or Golgi bodies; in plant cells they are called dictyosomes, and they lie scattered rather than gathered in one place.
The Golgi apparatus carries out the synthesis and secretion of enzymes, hormones and similar substances. Materials made elsewhere in the cell are collected here, finished off, packed into vesicles and sent out.
In animal cells it has one further job: it forms the acrosome of a sperm, the cap at the tip of the sperm that helps it enter the egg.
Diagram
Fig. 1.6-3 — Structure Of The Golgi Apparatus Line drawing of a stack of four or five curved, flattened membrane sacs lying one above another, with small rounded vesicles budding off from their edges and a larger vacuole nearby. Label cisternae (flattened sacs), vesicles and vacuole. Labels on straight, non-crossing lead lines.
1.6.5 Ribosomes
Ribosomes are small granules, either scattered freely in the cytoplasm or attached to the outer surface of the endoplasmic reticulum.
They are dense, spherical bodies built of RNA together with protein. Unlike the other organelles, a ribosome is not bounded by a membrane at all.
The function of ribosomes is protein synthesis — the building of proteins from their smaller units. A cell that makes and secretes a great deal of protein carries very large numbers of them.
1.6.6 Lysosomes
Lysosomes are membranous sacs budded off from the Golgi body. Each sac contains about 40 different types of digestive enzymes.
Keeping those enzymes shut inside a sac is essential. Enzymes powerful enough to digest the cell’s own material would wreck the cell within minutes if they were free in the cytoplasm, so the membrane around them is not a detail but the whole point.
Lysosomes carry out intracellular digestion, that is, digestion inside the cell. They break down food particles and worn-out parts, and they destroy foreign substances such as bacteria that have been taken into the cell.
They have one further role that sounds surprising. When a cell grows old or is injured, its lysosomes rapidly destroy its organelles, and the cell is broken down. Because of this, lysosomes are called ‘suicide bags’. The same ability is put to use during development, when lysosomes digest cartilage as bone forms in its place.
1.6.7 Centrosome
The centrosome is found in animal cells only. It is a region of cytoplasm lying close to the nucleus, and it surrounds one or two small rod-like bodies called centrioles. The centrioles themselves are surrounded by microtubules.
The centrosome initiates and regulates cell division. As division begins it forms the spindle fibres, the threads along which the chromosomes are moved, with the help of the star-shaped asters that radiate around each centriole. You saw the centrioles moving apart, with their asters, in Fig. 1.5-3.
1.6.8 Plastids
Plastids are found in plant cells only. Each is double membraned and holds a proteinaceous matrix, and like mitochondria they contain DNA of their own.
Plastids are of several kinds, which differ in colour and in what they do. FC 1.2 sets out the three.
Flowchart
FC 1.2 — The Three Kinds Of Plastid Plastids → branch 1: chloroplasts → green, contain chlorophyll → trap solar energy for photosynthesis Plastids → branch 2: leucoplasts → colourless, no pigment → store starch Plastids → branch 3: chromoplasts → variously coloured, contain xanthophyll and carotene → give colour to flowers and fruits
Chloroplasts are the most common kind. They are green because they contain the green pigment chlorophyll, and their work is to trap solar energy for photosynthesis. Inside a chloroplast lie disc-like structures called thylakoids, and it is in these that the chlorophyll is held. Fig. 1.6-4 shows a chloroplast in section.
Leucoplasts are colourless and contain no pigment at all. They store starch, and they are found in parts of a plant that are not exposed to light, such as roots and underground stems.
Chromoplasts are variously coloured. They contain the pigments xanthophyll, which is yellow, and carotene, which is orange-red, and they impart colour to flowers and fruits. That colour does real work for the plant: bright flowers attract pollinators, and brightly coloured ripe fruits attract the animals that eat them and carry the seeds away.
Diagram
Fig. 1.6-4 — Structure Of A Chloroplast Section through a single oval chloroplast drawn as a line diagram; show the double membrane, the matrix filling the interior, and disc-like thylakoids arranged in stacks within the matrix. Label outer membrane, inner membrane, matrix and thylakoids. Labels on straight, non-crossing lead lines.
1.6.9 Cell Inclusions — Vacuoles And Granules
The two parts described here are non-living, but they matter in their own way.
Vacuoles are clear spaces holding water, or other substances in solution. Each is covered by a membrane called the tonoplast. Plant cells have few vacuoles but large ones, often a single large central vacuole, while animal cells have several small ones.
A vacuole stores water and other substances, including food, pigments and waste products. Some hold pigments called anthocyanins, which give violet and blue colours to flowers and fruits.
A vacuole also does something mechanical. As it fills with water it presses outwards against the cell wall, and this pressure gives the plant cell its turgidity — the firmness that keeps a soft stem upright and a leaf spread flat.
Granules are small particles, crystals or droplets lying in the cytoplasm. They store starch in plant cells, glycogen in animal cells, and fat droplets in both, all of which serve as food for the cell.
1.7 Plant Cells Compared With Animal Cells
A plant cell and an animal cell are alike in most of what matters. Both have a cell membrane, cytoplasm and a nucleus, and both carry mitochondria, endoplasmic reticulum, ribosomes, Golgi bodies and lysosomes doing the same work in each.
The differences between them nearly all follow from two facts about plants. A plant makes its own food using sunlight, so its cells need plastids. A plant has no skeleton, so each of its cells builds a rigid wall around itself and the walls together hold the plant up.
One further difference is worth working out rather than memorising. A mature plant cell holds one large central vacuole, and as that vacuole fills with water it presses everything else outwards. The cytoplasm is squeezed into a thin lining against the wall instead of filling the cell. In an animal cell, with no such vacuole, the cytoplasm fills almost the whole of the space.
Table 1.3 gathers the differences together.
1.8 Microscopic Examination Of Onion Peel
Everything you have read so far in this chapter can be seen, and the easiest cells to see are those of an onion. The thin skin lining an onion scale is a peel of epidermis — the outermost layer of cells covering a plant part. This peel is only one cell thick and almost transparent, so light passes straight through it and every cell in it shows separately.
Preparing The Peel
Cut the onion bulb lengthwise into four quarters, as in Fig. 1.8-1 A. Ask your teacher to help with the cutting.
From one quarter, remove one thick fleshy scale leaf.
Hold the fleshy scale in your hand and tear it from the inner, concave side. A thin transparent strip — the peel — comes away, as in Fig. 1.8-1 B.
Using a pair of forceps, lift this strip off and put it in a watch glass containing water, as in Fig. 1.8-1 C. The water keeps the peel from drying and curling.
Diagram
Fig. 1.8-1 — Preparation Of An Onion Peel Three line drawings in a row, labelled A, B and C. (A) An onion bulb cut lengthwise into four quarters. (B) A hand holding a fleshy scale leaf and tearing a thin transparent strip from its inner concave side. (C) A pair of forceps lifting the strip and placing it in a watch glass of water. Label onion bulb, scale leaf, inner (concave) side, transparent peel, forceps and watch glass. Labels on straight, non-crossing lead lines.
Mounting And Examining It
Cut a square piece of the peel, roughly 5 mm by 5 mm, and mount it on a slide in a drop of water, as shown in Fig. 1.8-2.
Lower a coverslip on to it carefully, so that the tissue does not get wrinkled. A wrinkle folds cells over one another and nothing can be made out where that happens.
Examine the preparation under the low power of the microscope first, then under high power for finer detail.
You may also stain the material with iodine or eosin solution, which makes the nucleus more distinct. A stain is worth understanding rather than simply using. Under the microscope the parts of a living cell are almost equally colourless and equally transparent, so there is little to tell them apart. A stain is taken up more strongly by some parts than by others, and the difference in colour is what lets you pick the nucleus out from the cytoplasm around it.
Diagram
Fig. 1.8-2 — Preparation Of A Slide Of An Onion Peel Three line drawings in a row showing the mounting of the peel. (1) A dropper placing a drop of water on a clean glass slide. (2) A needle lowering a coverslip on to the peel at an angle. (3) The finished slide with the mounted peel under the coverslip. Label glass slide, drop of water, onion peel, needle and coverslip. Labels on straight, non-crossing lead lines.
What You See
The cells of the onion peel have a somewhat rectangular shape, as in Fig. 1.8-3. Each cell shows a prominent cell wall, a nucleus, and cytoplasm encircling one or two large vacuoles. The thick cell wall is seen best under high power.
Two things about the arrangement are worth noticing. The cells are firmly bound together, with no gaps between them, because the walls of neighbouring cells are cemented to one another; this is what makes the peel strong enough to lift with forceps in one piece.
The nucleus is not in the centre. It lies pushed towards one side of the cell, which is usually the case in almost all plant cells. You already know the reason: the large vacuole occupies the middle of the cell and presses the cytoplasm, with the nucleus in it, out against the wall.
Diagram
Fig. 1.8-3 — Onion Peel Cells Under Low Power Line drawing of a sheet of onion epidermal cells as seen under the low power of a microscope; show about twenty roughly rectangular cells fitted closely together with no gaps, each with a thick cell wall and a rounded nucleus lying to one side. Label cell wall, nucleus, cytoplasm and vacuole on one cell only. Labels on straight, non-crossing lead lines.
1.9 The Nucleus — Key To The Life Of A Cell
Every part of a cell contributes to the life of the cell as a whole. Even so, one part is the key performer, the master of the cell, and that part is the nucleus.
That is a large claim, and in biology a large claim has to be shown rather than asserted. It can be tested, and the test is simple enough to picture.
Amoeba suits the test particularly well. Its whole body is a single cell, so nothing has to be dissected away to reach that cell, and it is large enough to be operated on under a microscope with a very fine glass needle. FC 1.3 sets out the three parts of the experiment, and Fig. 1.9-1 shows what is seen.
Flowchart
FC 1.3 — An Experiment On Amoeba To Test The Role Of The Nucleus Part A (the normal amoeba) → amoeba with its nucleus → grows → divides into two amoebae → each grows and divides again → the line continues Part B (nucleus removed) → nucleus taken out with a fine needle → amoeba stops dividing → amoeba dies Part C (nucleus replaced) → nucleus taken from a donor amoeba → transplanted into an amoeba whose own nucleus was removed → recipient survives and divides → donor, now without its nucleus, dies
Diagram
Fig. 1.9-1 — An Experiment On Amoeba To Show That The Nucleus Is Essential For Normal Life Three rows of line drawings, labelled A, B and C, with arrows between the stages in each row. Row A: a normal amoeba with its nucleus, dividing into two amoebae, each of which grows and divides again. Row B: a fine glass needle drawing the nucleus out of an amoeba, the amoeba without a nucleus, and the same amoeba dying. Row C: a nucleus being taken from a donor amoeba and transplanted into an amoeba whose nucleus has been removed; the recipient then divides into two, while the donor dies. Label nucleus, glass needle, donor, recipient, and mark the cells that die. Labels on straight, non-crossing lead lines.
Reading The Result
Part B on its own is suggestive but not conclusive. Removing a nucleus means cutting into the cell, and someone could fairly object that the amoeba died of the injury rather than of the loss.
Part C answers that objection. The recipient amoeba was operated on in exactly the same way and was injured to the same extent, yet it recovered, survived and divided — the one difference being that a nucleus was put back into it. Meanwhile the donor, uninjured except for the loss of its nucleus, died.
So the conclusion follows safely: it is the absence of the nucleus, and not the operation, that kills the cell. A cell whose nucleus is removed dies soon.
Why The Nucleus Matters So Much
The reason lies in what the nucleus holds. Its chromosomes carry the genes, and those genes hold the instructions for building every protein the cell needs, including all its enzymes.
A cell is constantly wearing out and replacing its own materials. Cut off from the instructions, it cannot make the replacements. It carries on for a time on what it already has, and then it stops. This is why the nucleus is said to control all the activities of the cell.
Every activity of the body of an organism is carried out by cells, and each kind of cell is specialised for a particular function. Behind that specialisation stands the nucleus of each cell, holding the instructions that make one cell a guard cell and another a nerve cell.
1.10 Summary
All living beings are made up of cells. No organism has been found that is built of anything else.
The cell is the structural and functional unit of the body. Bodies are built out of cells, and every activity of a body is finally carried out inside cells.
All cells are basically alike in chemical composition and in their metabolic processes.
All living beings arise from a pre-existing cell.
Most organisms begin life as a single cell. That cell divides repeatedly, and the cells produced become specialised for particular jobs such as support and secretion.
The larger an organism, the greater the number of cells in its body. A body grows by adding cells rather than by enlarging them.
A unicellular organism has a body of a single cell that performs every life activity by itself. Examples are Amoeba, Paramecium, bacteria and Chlamydomonas.
A multicellular organism is made of millions or billions of cells, among which the work is shared. Examples are Volvox, rose, mosquito, mango and humans.
An adult human of average size has about 200 different types of cell, roughly 37.2 trillion cells in all, about 86 billion nerve cells in the brain of which some 16 billion lie in the cerebral cortex, and about 25 trillion red blood cells.
Cells are microscopic and can be seen only with the help of a microscope.
Cells stay small because a small cell has a large surface area compared with its volume, so substances diffuse in and out fast enough to serve every part of it.
The smallest cells are certain bacteria, and among human cells the red blood cells. The longest cells are nerve cells.
The largest cells are birds’ eggs, the actual cell being the central yellow yolk. The ostrich egg, before development begins in it, is the largest single cell in the living world today.
The white and the shell of an egg are extra parts, added on as the egg passes down the reproductive tract, and they take no direct part in reproduction.
The shape of a cell is related to the function it performs.
Red blood cells are biconcave and disc-shaped, for easy passage through blood capillaries and for the transport of oxygen.
White blood cells are amoeboid, moving by pseudopodia, and can squeeze out through capillary walls.
Nerve cells are long, and conduct impulses between distant parts of the body and the brain.
Guard cells of the stomatal pores are bean-shaped, and open and close the pore.
Every cell has three essential parts: the cell membrane, the cytoplasm and the nucleus.
The cell membrane, or plasma membrane, is a living, flexible membrane of lipoproteins with fine pores. It is semi-permeable, allowing only certain substances to pass through while preventing others.
A plant cell has an additional non-living cell wall outside the membrane, made mainly of cellulose. It is freely permeable and gives the cell a definite shape and rigidity.
Cytoplasm is the part of the cell inside the cell membrane and outside the nucleus. It is semi-liquid, holds the organelles, and is the site of many reactions catalysed by enzymes. The part other than the organelles is the cytosol.
The nucleus is a large spherical body lying nearly in the centre of the cytoplasm, surrounded by a double-layered nuclear membrane with nuclear pores.
The nucleoplasm is the semi-solid ground substance of the nucleus. It holds one or more nucleoli, which form and store RNA and produce ribosomes.
Chromatin fibres are a network of DNA threads in the nucleoplasm. During cell division they condense into short thick chromosomes, which carry the genes that pass hereditary information from parents to offspring.
Organelles are specialised living structures inside a cell, usually bounded by a membrane, each concerned with a definite function.
The living parts of a cell together form the protoplasm, also called the protoplast, the second term being used for a plant cell after removal of the cell wall. The non-living parts, the vacuoles and granules, are called cell inclusions.
The endoplasmic reticulum is an irregular network of single-membraned tubules, continuous with the plasma membrane outside and the nuclear membrane inside. Rough ER bears ribosomes, smooth ER does not. It gives a supportive framework and carries out the synthesis and transport of proteins and fats.
Mitochondria are usually sausage-like and double walled, with the inner wall thrown into folds called cristae, and they have their own DNA and ribosomes. They are the seat of aerobic cellular respiration, releasing energy from pyruvic acid in the form of ATP, and they synthesise respiratory enzymes.
The Golgi apparatus, called dictyosomes in plant cells, is made of stacks of flattened membrane sacs consisting of cisternae, vesicles and vacuoles. It synthesises and secretes enzymes and hormones, and in animal cells it forms the acrosome of the sperm.
Ribosomes are small dense granules of RNA and protein, either scattered in the cytoplasm or attached to the endoplasmic reticulum, and are not bounded by a membrane. They are the site of protein synthesis.
Lysosomes are membranous sacs budded off from the Golgi body and contain about 40 types of digestive enzymes. They carry out intracellular digestion, destroy foreign substances, rapidly destroy the organelles of old or injured cells — hence the name ‘suicide bags’ — and digest cartilage during the formation of bones.
The centrosome, found in animal cells only, is a region near the nucleus containing one or two centrioles surrounded by microtubules. It initiates and regulates cell division and forms the spindle fibres with the help of the asters.
Plastids are found in plant cells only. They are double membraned and contain DNA, and are of three kinds.
Chloroplasts are green, hold chlorophyll in disc-like thylakoids, and trap solar energy for photosynthesis. Leucoplasts are colourless, carry no pigment and store starch. Chromoplasts are variously coloured, contain xanthophyll and carotene, and give colour to flowers and fruits.
Vacuoles are clear spaces holding water or substances in solution, covered by a membrane called the tonoplast. They store water, food, pigments such as anthocyanins, and wastes, and give turgidity to plant cells by pressing against the cell wall. Plant cells have few but large vacuoles, animal cells small and temporary ones.
Granules are small particles, crystals or droplets that store starch in plant cells, glycogen in animal cells and fat droplets, all serving as food for the cell.
Plant cells differ from animal cells in being usually larger, in having a cell wall and plastids, in having less dense cytoplasm confined to a thin lining at the periphery, in having large prominent vacuoles, and in lacking a centrosome.
Onion peel makes an easy preparation for study. Its cells are somewhat rectangular and firmly bound together, each with a prominent cell wall, cytoplasm around one or two large vacuoles, and a nucleus pushed towards one side, as in almost all plant cells. Staining with iodine or eosin makes the nucleus more distinct.
An experiment on Amoeba shows that the nucleus is essential for life. An amoeba whose nucleus is removed dies; if a nucleus from another amoeba is transplanted into it, the recipient survives and divides while the donor dies.
Every activity of the body of an organism is carried out by cells, and each kind of cell is specialised for a particular function.
1.11 Key Terms
- Acrosome — The cap at the tip of a sperm, formed by the Golgi apparatus, which helps the sperm enter the egg.
- Amoeboid — Having no fixed shape and moving by pseudopodia, in the manner of Amoeba.
- Anthocyanins — Violet and blue pigments held in the vacuoles of plant cells.
- Asters — Star-shaped groups of fibres that radiate around each centriole during cell division.
- ATP — The chemical in which a cell stores energy in a ready-to-use form.
- Biconcave — Disc-shaped with a dip in the centre of both faces, as in a human red blood cell.
- Carotene — An orange-red pigment found in chromoplasts.
- Cell — The basic structural and functional unit of a living organism.
- Cell inclusions — The non-living parts of a cell, that is the vacuoles and the granules.
- Cell membrane — The living, flexible, semi-permeable lipoprotein boundary of a cell; also called the plasma membrane.
- Cell wall — The non-living, freely permeable outer layer of cellulose lying outside the cell membrane in plant cells.
- Cellulose — The tough carbohydrate of which the plant cell wall is mainly made.
- Centrioles — One or two small rod-like bodies lying within the centrosome of an animal cell.
- Centrosome — The region of cytoplasm near the nucleus of an animal cell that contains the centrioles and organises cell division.
- Chlorophyll — The green pigment of chloroplasts that traps solar energy for photosynthesis.
- Chloroplasts — Green plastids that contain chlorophyll and carry out photosynthesis.
- Chromatin fibres — The network of fine, dark-staining DNA threads in the nucleoplasm of a cell that is not dividing.
- Chromoplasts — Coloured plastids containing xanthophyll and carotene that give colour to flowers and fruits.
- Chromosomes — The short thick bodies into which chromatin fibres condense during cell division; they carry the genes.
- Cisternae — The flattened tubular sacs that make up the stacks of the Golgi apparatus.
- Cristae — The folds of the inner wall of a mitochondrion.
- Cytoplasm — The semi-liquid part of a cell lying inside the cell membrane and outside the nucleus.
- Cytosol — The liquid part of the cytoplasm, apart from the organelles held in it.
- Dictyosomes — The name given to the Golgi bodies of plant cells.
- Diffusion — The spreading of a substance from where it is more concentrated to where it is less concentrated.
- DNA — The substance of the chromatin fibres and chromosomes, which carries the instructions for building and running a cell.
- Endoplasmic reticulum — The network of single-membraned tubules running through the cytoplasm, continuous with the plasma membrane outside and the nuclear membrane inside.
- Enzyme — A protein that speeds up a particular chemical reaction in a cell.
- Epidermis — The outermost layer of cells covering a plant part.
- Freely permeable — Allowing substances in solution to pass through without hindrance, as the cell wall does.
- Genes — The units carried on the chromosomes that pass hereditary information from parents to offspring.
- Golgi apparatus — Stacks of flattened membrane sacs that synthesise, pack and secrete substances such as enzymes and hormones.
- Granules — Small non-living particles, crystals or droplets in the cytoplasm that store starch, glycogen or fat as food for the cell.
- Guard cells — The pair of bean-shaped cells that surround a stoma and open and close it.
- Leucoplasts — Colourless plastids that carry no pigment and store starch.
- Lipoproteins — Molecules that are part lipid and part protein, of which the cell membrane is made.
- Lysosomes — Membranous sacs of digestive enzymes budded off from the Golgi body.
- Metabolism — The whole set of chemical changes by which a cell releases energy, builds materials and breaks them down.
- Micrometre — A unit of length equal to one-thousandth of a millimetre, used for measuring cells.
- Microtubules — The fine tubes that surround the centrioles.
- Mitochondria — Sausage-like, double-walled organelles that are the seat of aerobic cellular respiration and release energy in the form of ATP.
- Multicellular organism — An organism whose body is made up of many cells.
- Nerve cells — The long cells that conduct impulses between distant parts of the body and the brain.
- Nuclear membrane — The double-layered membrane, carrying pores, that surrounds the nucleus.
- Nuclear pores — The fine openings in the nuclear membrane through which substances pass between the nucleus and the cytoplasm.
- Nucleolus — A round-shaped body inside the nucleus that forms and stores RNA and produces ribosomes.
- Nucleoplasm — The semi-solid ground substance that fills the nucleus.
- Nucleus — The large, dense, membrane-bound body of a cell that holds the chromatin material and controls the cell’s activities.
- Organelles — Specialised living structures inside a cell, usually bounded by a membrane, each with a definite function.
- Plasma membrane — Another name for the cell membrane.
- Plastids — Double-membraned organelles found in plant cells only, of three kinds: chloroplasts, leucoplasts and chromoplasts.
- Protoplasm — All the living material of a cell taken together, that is the cytoplasm, the nucleus and the organelles.
- Protoplast — The living material of a plant cell after the cell wall has been removed.
- Pseudopodia — Temporary finger-like extensions of cytoplasm by which amoeboid cells move.
- Red blood cells — The biconcave, disc-shaped cells of the blood that transport oxygen.
- Ribosomes — Small dense granules of RNA and protein, not bounded by a membrane, on which proteins are built.
- Rough endoplasmic reticulum — Endoplasmic reticulum with ribosomes attached to its outer surface.
- Semi-permeable — Allowing only certain substances to pass through while preventing others.
- Smooth endoplasmic reticulum — Endoplasmic reticulum without ribosomes on its surface.
- Spindle fibres — The threads formed at cell division along which the chromosomes are moved.
- Stoma — A tiny pore in the surface of a leaf through which gases enter and leave.
- Surface area to volume ratio — The amount of surface a body has compared with its volume; large in a small cell and small in a large one.
- Thylakoids — The disc-like structures inside a chloroplast in which the chlorophyll is held.
- Tonoplast — The membrane covering a vacuole.
- Turgidity — The firmness of a plant cell produced by the vacuole pressing outwards against the cell wall.
- Unicellular organism — An organism whose body is made up of only a single cell.
- Vacuoles — Clear non-living spaces in the cytoplasm holding water or other substances in solution.
- White blood cells — The amoeboid cells of the blood that can squeeze out through the walls of capillaries.
- Xanthophyll — A yellow pigment found in chromoplasts.
1.12 Points To Ponder
A cell stays small because its volume grows faster than its surface. Suppose a cell could keep its surface large while growing, by folding its membrane deeply inwards. Would size still set a limit on it? What else inside such a cell might begin to run short?
Lysosomes hold enzymes strong enough to digest the cell’s own organelles. What must be true of the lysosome membrane every hour of every day for the cell to stay alive, and what would follow if many lysosomes were damaged at once?
Of all the organelles, only mitochondria and plastids carry DNA and ribosomes of their own. What might that unusual fact suggest about where these two organelles came from?
A plant cell builds a rigid wall around itself and an animal cell does not. Follow that single difference outwards: how does it shape the way a plant grows, holds itself up and responds to its surroundings, compared with an animal?
In the experiment on Amoeba, the nucleus was not only removed but also put back. Why is putting it back the more convincing half of the experiment? Where else in biology would you insist on such a second step before accepting a conclusion?
The cells of an onion peel look almost identical to one another, yet a human body contains about 200 different types of cell — and every one of them carries the same set of genes. How can one identical set of instructions give rise to cells as different as a nerve cell and a red blood cell?
1.13 Objective Questions
Multiple Choice Questions
Choose the correct option.
- The one organelle that is not bounded by a membrane is the (1 mark)
- lysosome
- ribosome
- mitochondrion
- Golgi body
- The folds of the inner wall of a mitochondrion are called (1 mark)
- cisternae
- thylakoids
- cristae
- asters
- Which one of the following is present in a plant cell but absent in an animal cell? (1 mark)
- Centrosome
- Lysosome
- Golgi body
- Leucoplast
- The cell wall of a plant cell is (1 mark)
- living and semi-permeable
- living and freely permeable
- non-living and semi-permeable
- non-living and freely permeable
- Turgidity is given to a plant cell by the (1 mark)
- nucleolus
- vacuole
- centrosome
- ribosome
- The largest single cell in the living world today is (1 mark)
- a nerve cell
- a bacterial cell
- the ostrich egg
- a white blood cell
Assertion And Reason
Each of the following gives an Assertion (A) and a Reason (R). Choose your answer from the codes given below. (1) A is true and R is false. (2) A is false and R is true. (3) Both A and R are true. (4) Both A and R are false.
- Assertion (A): A mitochondrion contains DNA and ribosomes of its own. Reason (R): A mitochondrion carries out the synthesis of respiratory enzymes. (1 mark)
- Assertion (A): Lysosomes are known as ‘suicide bags’. Reason (R): Lysosomes are budded off from the endoplasmic reticulum. (1 mark)
- Assertion (A): In an onion peel cell the nucleus lies towards one side. Reason (R): A large vacuole occupies the middle of the cell and presses the cytoplasm outwards. (1 mark)
- Assertion (A): A chromoplast traps solar energy for photosynthesis. Reason (R): A chromoplast contains the pigments xanthophyll and carotene. (1 mark)
Match The Following
- Match each item in Column A with the correct item in Column B. (5 marks)
| Column A | Column B |
|---|---|
| a. Tonoplast | i. Stacks of the Golgi apparatus |
| b. Cristae | ii. Holds the chlorophyll |
| c. Cisternae | iii. Covering of a vacuole |
| d. Thylakoid | iv. Produces ribosomes |
| e. Nucleolus | v. Folds of the inner wall of a mitochondrion |
Fill In The Blanks
- Complete each of the following. (5 marks)
- The non-living parts of a cell, the vacuoles and granules, are together called ______.
- The colourless plastids that store starch are called ______.
- The semi-solid ground substance filling the nucleus is the ______.
- The Golgi bodies of a plant cell are known as ______.
- The orange-red pigment present in chromoplasts is ______.
True Or False
- State whether each of the following is true or false. (4 marks)
- Ribosomes are the site of protein synthesis.
- The cell wall is a living membrane.
- An amoeba whose nucleus has been removed goes on dividing normally.
- Chromatin fibres condense into chromosomes during cell division.
Name The Following
- Name the following. (6 marks)
- The organelle that forms the acrosome of a sperm.
- The membrane that covers a vacuole.
- The part of the cytoplasm left when the organelles are not counted.
- The pigments that give a violet-blue colour to the vacuoles of plant cells.
- The threads along which chromosomes are moved during cell division.
- The living material of a plant cell after its cell wall has been removed.
Odd One Out
- Pick the odd one out in each group and give a reason for your choice. (3 marks)
- Chloroplast, leucoplast, chromoplast, centrosome.
- Cristae, thylakoid, cisternae, cellulose.
- Amoeba, Paramecium, Volvox, Chlamydomonas.
1.14 Check Your Concepts
Define an organelle. (2 marks)
State two functions of the cell wall. (2 marks)
Give a reason why a human red blood cell is biconcave and disc-shaped. (2 marks)
Name the three kinds of plastid and state the colour of each. (2 marks)
What is meant by a semi-permeable membrane? (2 marks)
Give a reason for each of the following. (3 marks)
- Lysosomes are called ‘suicide bags’.
- The cell wall is described as freely permeable.
- Guard cells are bean-shaped.
Differentiate between chromatin fibres and chromosomes, stating the basis of difference. (3 marks)
Explain why cells remain small, using the idea of surface area and volume. (3 marks)
Describe the structure of the nucleus as it appears in a cell that is not dividing. (3 marks)
Differentiate between a plant cell and an animal cell, giving four points of difference and stating the basis of each. (4 marks)
Explain how the structure of a mitochondrion suits the work it has to do. (4 marks)
What would happen in each of the following cases? (4 marks)
- The cell wall is removed from a plant cell.
- The large vacuole of a plant cell loses most of its water.
- The lysosome membranes of an injured cell break down.
- The nucleus is removed from a living cell.
Describe how you would prepare and mount a slide of onion peel, and state three observations you would record from it. (5 marks)
An experiment on Amoeba is used to show that the nucleus is essential to a cell. Describe the three parts of this experiment, and explain why the third part is needed before the conclusion can be accepted. (5 marks)
1.15 Figure-Based Questions
- Fig. 1.5-1 shows a generalised animal cell (A) beside a generalised plant cell (B). (4 marks)
- Name two parts present in B but absent in A.
- Name one part present in A but absent in B.
- State one function of the large central vacuole of B.
- Give a reason why the cytoplasm in B is only a thin lining against the wall.
- Fig. 1.6-2 shows a single organelle drawn in section. (4 marks)
- Identify the organelle.
- Name the folds of its inner membrane.
- State one advantage the cell gains from these folds.
- Name the substance from which this organelle releases energy, and the form in which that energy is stored.
- Fig. 1.5-2 shows the structure of the nucleus. (3 marks)
- Name the two features of its boundary that are labelled in the figure.
- State one function of the round body lying inside it.
- Name the network seen within it, and state what this network becomes during cell division.
- Fig. 1.8-3 shows cells of an onion peel as seen under the low power of a microscope. (3 marks)
- Give the shape of these cells.
- Give a reason why the nucleus does not lie at the centre of the cell.
- Name one stain that would make the nucleus more distinct.
- Redraw the pair of guard cells shown in Fig. 1.4-1 D and label the guard cells and the stomatal pore. State one reason for the shape of these cells. (3 marks)