Class 9 Science Chapter 2: Cell: The Building Block of Life

The basic level at which life exists. Every living organism is made of cells, and no matter how cells are organised into tissues and organs above them, the cell stays the fundamental unit of structure and of function.

The smallest separation at which two close points are still seen as two rather than one. For the human eye, viewing from its near point of about 25 cm, that separation is 0.1 mm, which is why nearly all cells are invisible without help.

How much larger an object is made to appear. In a light microscope it is the magnifying power of the eyepiece multiplied by that of the objective lens, so 10X with 10X gives 100X.

Resolution is a measure of clarity, meaning how well two close details are kept apart. Contrast is the difference in brightness between different parts of an object. Both were improved alongside magnification to make the microscope genuinely useful.

An instrument using a beam of electrons in place of light. It produces highly magnified images that reveal cell structure at the nanometre scale, a nanometre being one-billionth of a metre.

The cell represents the basic level at which life exists, and all living organisms are made of cells. Unicellular organisms such as bacteria and yeast consist of only one cell, while multicellular organisms such as plants, fish, birds and humans are made of millions of cells working together.

A group of similar cells performing similar functions forms a tissue. Several kinds of tissue together make up one organ, and a set of organs sharing a job makes an organ system. The nasal pores, nasal cavity, trachea and lungs together form the respiratory system.

Viewed from about 25 cm, the near point of the eye, two points can only be distinguished if they are at least 0.1 mm apart; closer than that they appear as a single point. Since cells are usually far smaller than this, they cannot be seen with the unaided eye and a microscope is required.

Using a microscope of his own design capable of about 200 to 300X, Hooke examined a thin slice of cork and saw small box-like compartments, which he named cells. Cork is dead plant material, so these were the structures left behind rather than living cells at work, though the name has lasted.

It is the magnifying power of the eyepiece multiplied by that of the objective lens. If both the eyepiece and the objective are 10X, the total magnification is 100X, so an object appears one hundred times larger. The two powers multiply and are never added.

Place a millimetre ruler on the stage and measure the diameter of the circular field of view, then convert it to micrometres using 1 mm equals 1000 micrometres. Replace the ruler with the onion peel slide and count the cells lying along that same diameter. Dividing the field diameter in micrometres by that count gives the size of one cell, so a 5 mm field holding 25 cells gives about 200 micrometres.

Resolution, which is a measure of clarity, contrast, which is the difference in brightness between various parts of an object, and magnification. Magnification alone is not enough because enlarging an image that is already unclear simply produces a larger unclear image, so resolution and contrast had to improve with it.

An electron microscope uses a beam of electrons instead of light. This produces highly magnified images that reveal cell structure at the nanometre scale, a nanometre being one-billionth of a metre, with a clarity no light microscope can match.

The human eye has a fixed limit. Viewed from about 25 cm, its near point, two points can be distinguished only if they are at least 0.1 mm apart. Closer together than that, they merge and appear as a single point.

This is the limit of resolution of the human eye, and a cell is usually very much smaller than it. No amount of looking harder can overcome the limit, so the study of cells could not begin until an instrument existed that went beyond it.

A convex lens, or a combination of an objective lens and an eyepiece, magnifies an object so that it appears larger. Robert Hooke built such an instrument for himself, reaching about 200 to 300 times magnification.

In 1665 he used it to examine a thin slice of cork, and saw small box-like compartments running through it. He named these compartments cells, and the name has been used ever since.

What he was actually seeing deserves notice. Cork is dead plant material, so the compartments were structures left behind rather than living cells carrying out their work. The observation nevertheless opened an entire field, because it showed that living material has a repeating underlying structure at a scale nobody had been able to reach.

Nothing alive is built of anything else, and it is at this level, and no lower, that anything can be said to be living at all. Some organisms, such as bacteria and yeast, consist of a single cell, while plants, fish, birds and humans are made of millions.

In a multicellular organism these cells are organised. A group of similar cells performing similar functions forms a tissue.

Different tissues are then organised to form an organ, and several organs working together form an organ system. The respiratory system illustrates this, since the nasal pores, nasal cavity, trachea and lungs are separate organs that together carry out one function.

Despite this organisation, the cell remains the fundamental unit of both structure and function in every living organism. The higher levels are arrangements of cells rather than replacements for them, and every activity carried out by a tissue or an organ is ultimately carried out by the cells within it.

This is why the cell is described as the structural and functional unit of life, and why understanding cells is the starting point for understanding organisms of any size.

Begin by placing a transparent ruler with millimetre markings on the stage of the microscope. Focus on it with the adjustment knob and measure the diameter of the circular field of view in millimetres.

Convert that diameter into micrometres, since 1 mm equals 1000 micrometres. Cells are described on the micrometre scale, so working in millimetres would leave the final answer in an awkward unit. A field measuring 5 mm across is therefore 5000 micrometres.

Next remove the ruler and place the onion peel slide on the stage without changing the objective lens, because the field of view just measured applies only to that lens. Focus, and count how many cells lie along the diameter in one straight line.

Now divide. The estimated size of one cell is the diameter of the visible field in micrometres divided by the number of cells along it. If 25 cells span the field, each cell is about 5000 divided by 25, which is 200 micrometres.

Finally, the magnified appearance is a separate question from the real size. With a 10X eyepiece and a 10X objective the total magnification is 100X, so that 200 micrometre cell appears one hundred times larger than it actually is.

Over the years scientists improved microscopes along three features: resolution, which is a measure of clarity, contrast, which is the difference in brightness between various parts of an object, and magnification, which is how much larger the object is made to appear.

Magnification on its own is of limited use. If two details in a specimen are not kept separate, enlarging the image simply produces a larger version of the same indistinct picture. Resolution is what decides whether those close details remain distinguishable at all.

Contrast matters for a related reason. If different parts of an object differ very little in brightness, they cannot be told apart however sharply they are resolved or however much they are enlarged.

These improvements together turned the microscope into a genuinely powerful tool for studying cells, rather than simply a device that made things look bigger.

Electron microscopes went further still. By using a beam of electrons in place of light, they produce highly magnified images revealing cell structure at the nanometre scale, a nanometre being one-billionth of a metre. A scanning electron micrograph of the lower surface of a Colocasia leaf shows detail that no light microscope could separate.

The property of letting some substances cross a boundary while refusing others. It is what separates a membrane from a simple filter, and it is how a cell keeps control of its own contents while still trading with its surroundings.

The net drift of particles from a crowded region to a less crowded one, caused by the difference in concentration between them. No membrane is required; a dye spreading through still water is diffusion.

Diffusion of water specifically, across a selectively permeable membrane. Water leaves the side with more water and less dissolved matter and enters the side with less water and more dissolved matter, until both sides match.

The accepted picture of membrane structure. Two layers of fat molecules sit tail to tail, water-attracting heads outward, with proteins embedded among them. The parts drift and turn, which makes it fluid, and they sit together like tiles, which makes it a mosaic.

A rigid outer covering found in plants, fungi and bacteria, lying outside the membrane. In plants it is mostly cellulose. Unlike the membrane it is simply permeable, so water and dissolved minerals cross it freely, and its job is support rather than selection.

It means some substances are allowed to cross the membrane while others are turned back. This matters because it is how a cell controls what enters and leaves, keeping its contents in order while still exchanging materials with its surroundings, as oxygen and carbon dioxide do across the membranes of the alveoli.

Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration, and it needs no membrane at all. Osmosis is the narrower case in which water alone moves across a selectively permeable membrane. Every case of osmosis is diffusion, but most diffusion is not osmosis.

The piece in plain water swells and gains weight, because the surroundings are more dilute than its contents and water moves inward. The piece in salt solution shrinks and loses weight, because the surroundings are more concentrated and water moves outward. In both cases the membrane allowed water across but blocked the salt and sugar.

A solution is isotonic when its solute concentration equals that inside the cell, so no net water movement occurs. It is hypotonic when the outside concentration is lower than inside, so the cell takes in water. It is hypertonic when the outside concentration is higher, so the cell loses water.

The membrane consists of a bilayer of lipid molecules with water-attracting heads facing outwards and water-repelling tails facing inwards, with proteins embedded among them. It is fluid because the molecules move sideways, flip and rotate, and a mosaic because they sit together like tiles. The embedded proteins act as gatekeepers for substances crossing over.

A plant generally cannot move from place to place, so it has to withstand wind and rain where it stands. The rigid wall provides that strength while keeping leaves and flowers firm and holding the plant upright. An animal cell has no wall, which lets it change shape easily, and that flexibility supports the movement and functioning of animal tissues.

Both lose water by osmosis. In the plant cell the rigid wall holds the outer boundary in place, so the outline is unchanged while the inner content shrinks away and the gap between content and wall widens. The cheek cell has no wall to hold its shape, so the whole cell shrinks considerably.

The cell wall is permeable rather than selective, so water and some dissolved minerals pass straight through it. Behind it the selectively permeable membrane decides what is actually admitted into the cell. The wall supplies support and free passage while the membrane supplies the selection, and water itself enters by osmosis from the more dilute soil.

Diffusion is the net movement of particles from a region where they are more concentrated to one where they are less concentrated. The difference driving it is called a concentration gradient, and no membrane is needed: a dye spreading through still water is diffusion.

Osmosis is narrower. It is the diffusion of water specifically, and it takes place across a selectively permeable membrane. Water travels from the side with more water and less dissolved matter toward the side with less water and more dissolved matter, until the two concentrations become equal.

An experiment shows this plainly. Cut a potato into two pieces of roughly equal size and record the weight of each, since without a starting value there is nothing to compare against. Place one piece in plain water and the other in 20 per cent salt or sugar solution, and leave them about an hour.

The piece in plain water swells and gains weight, because its surroundings are more dilute than its contents, so water moves inward. The piece in the concentrated solution shrinks and loses weight, because water moves outward toward the stronger solution.

The result also shows the selectivity of the membrane. Water crossed freely in both directions, but the salt and sugar molecules did not, which is why the changes were changes in water content alone.

The cell membrane is extremely thin, about 7 to 10 nanometres, and is built from lipids and proteins. The fluid-mosaic model describes how these are arranged.

At its base is a lipid bilayer: two layers of special fat molecules, with their water-attracting heads facing outwards and their water-repelling tails facing inwards. Proteins are embedded among these lipid molecules.

The membrane is described as fluid because its molecules are not fixed in place. They move sideways, flip and rotate within the layer. It is described as a mosaic because those molecules sit together like tiles.

This structure suits the membrane's job well. Because it is selectively permeable, some substances are allowed across while others are turned back, and the embedded proteins act as gatekeepers helping particular substances pass through.

The arrangement also explains why the membrane can serve as the cell's only channel of communication. Every living cell exchanges materials with its surroundings and with neighbouring cells across this layer, so a boundary that is both stable and able to permit controlled passage is exactly what is required.

Prepare a peel of Rhoeo or onion leaf and, separately, a slide of cheek cells. Under the microscope the plant cells appear box-shaped and regularly arranged, while the cheek cells are irregular. Now place both in 20 per cent sugar solution and observe after half an hour.

In the plant cell, the outer boundary stays exactly where it was. The inner content shrinks, pulling away from the wall, and the space between the inner content and the outer boundary widens.

In the cheek cell, the whole cell shrinks considerably, with nothing to hold its outline in place.

Both cells lost water by osmosis, since the surroundings were far more concentrated than their contents. The difference in appearance is due entirely to the cell wall.

This reveals the wall's function. Being rigid, it maintains the plant cell's shape even after water has left, which is what keeps leaves and flowers firm and the plant upright. An animal cell has no wall, so it changes shape easily, and that flexibility is itself useful because it supports the movement and functioning of animal tissues.

The cell membrane is the thin boundary around every cell, about 7 to 10 nanometres thick and made of lipids and proteins. It is selectively permeable, letting some substances across while turning others back, and it is present in every cell without exception.

The cell wall lies outside the membrane and is found only in plants, fungi and bacteria. In plants it is made chiefly of cellulose, a carbohydrate formed from many glucose units linked together, and it is rigid rather than flexible.

Their functions differ accordingly. The membrane controls what enters and leaves, which is why it is called selectively permeable. The wall provides strength and shape, allowing a plant that cannot move away from wind and rain to withstand them where it stands.

Crucially, the wall is permeable but not selective. Water and some dissolved minerals pass straight through it without being sorted.

In a root this division of labour is what makes absorption possible. Soil water is more dilute than the root cell contents, so water moves inward by osmosis. The permeable wall offers no obstacle to water and dissolved minerals reaching the membrane, and the selectively permeable membrane behind it then decides what is actually admitted into the cell.

The semi-fluid, jelly-like substance filling the cell inside the membrane. It holds the organelles and, in a prokaryotic cell that has no internal compartments, it is where most cellular activity actually happens.

A sub-cellular component wrapped in a membrane of its own. This wrapping is the dividing line between the two classes of cell: eukaryotic cells have such organelles, prokaryotic cells have none.

A cell with no well-defined nucleus and no membrane-bound organelles, its genetic material lying uncovered in a region called the nucleoid. The name comes from pro, meaning primitive, and karyon, meaning nucleus. Bacteria are the standard example.

A cell with a well-defined nucleus and several membrane-bound organelles, so named from eu, meaning true, and karyon, meaning nucleus. Plant and animal cells are both eukaryotic.

A network of fine fibres inside eukaryotic cells. It gives structural support, maintains the cell's shape, and allows both movement of the cell and transport of materials within it. An electron microscope is needed to see it as a distinct structure.

The plasma membrane is a selectively permeable boundary around the cell. The cytoplasm is a semi-fluid, jelly-like substance filling the interior and holding the organelles. The nucleus is a prominent structure containing the cell's genetic material.

Both words end in karyon, which means nucleus. Pro means primitive, so a prokaryotic cell has only a primitive nucleus and no membrane around its genetic material. Eu means true, so a eukaryotic cell has a genuine, well-defined nucleus.

It lacks a well-defined nucleus and membrane-bound organelles, meaning organelles surrounded by their own membranes. Instead of a nucleus it has a nucleoid, a region holding its genetic material without any membrane around it.

A typical prokaryotic cell measures 1 to 10 micrometres across, while a typical eukaryotic cell measures 10 to 100 micrometres. Prokaryotic organisms are usually unicellular, whereas eukaryotic organisms may be either unicellular or multicellular.

All three have a cell membrane and cytoplasm. A cell wall is present in bacterial and plant cells but not in animal cells. A well-defined nucleus and membrane-bound organelles are present in plant and animal cells but absent from bacterial cells, which have a nucleoid instead.

The cytoskeleton is a network of fine fibres found in eukaryotic cells. It provides structural support, maintains the shape of the cell, and enables both cell movement and internal transport. It can be seen as a separate structure only under an electron microscope.

Cell inclusions are materials stored in the cytoplasm, grains of starch, for instance, or the calcium oxalate and silica crystals that some plants lay down. They differ from organelles in being stored substances rather than working components of the cell.

A virus consists of genetic material inside a protein coat. A viroid has genetic material but lacks the protein coat. A prion is a misfolded protein with no genetic material at all. All three are called acellular because they are not made of cells, and all are too small to be seen under a light microscope.

Both names end in the Greek root karyon, meaning nucleus, so the classification turns entirely on the prefix. Pro means primitive and eu means true.

A prokaryotic cell, such as a bacterial cell, has no well-defined nucleus and no membrane-bound organelles, meaning no organelles wrapped in membranes of their own. Its genetic material is not absent but uncovered, lying in a region called the nucleoid.

A eukaryotic cell, such as a plant or animal cell, has a well-defined nucleus in which the genetic material is enclosed by a membrane, together with several membrane-bound organelles.

This difference in architecture has consequences. With no internal compartments, most cellular activity in a prokaryotic cell takes place directly in the cytoplasm, whereas a eukaryotic cell can run different processes independently and at the same time in separate organelles.

The two also differ in size and in the organisms they build. A typical prokaryotic cell is 1 to 10 micrometres across against 10 to 100 micrometres for a eukaryotic one, and prokaryotic organisms are usually unicellular while eukaryotic organisms may be unicellular or multicellular.

All three cells share a cell membrane and cytoplasm, which are among the basic parts of any cell.

Beyond that they diverge. A cell wall is present in the bacterial cell and the plant cell but absent from the animal cell, which is why animal cells change shape so easily.

A well-defined nucleus, with genetic material enclosed by a membrane, is present in the plant and animal cells. The bacterial cell has no such nucleus; it carries a nucleoid instead, where the genetic material lies without any membrane around it.

Membrane-bound organelles follow the same pattern: present in plant and animal cells, absent from the bacterial cell. Looking at labelled diagrams adds further detail, since a plant cell shows chloroplasts, a large vacuole and a nucleolus, an animal cell shows lysosomes, and a bacterial cell often shows an appendage for locomotion. Ribosomes appear in all three.

On this evidence the bacterial cell is prokaryotic, while the plant and animal cells are both eukaryotic.

The cytoplasm is the semi-fluid, jelly-like substance filling the cell, and it holds more than the organelles alone.

Running through it is the cytoskeleton, a network of fine fibres. This provides structural support to the cell, maintains its shape, and enables both movement of the cell and transport of materials within it.

The cytoskeleton is easy to overlook because, despite performing three distinct functions, it can be seen as a separate entity only under an electron microscope.

The cytoplasm may also hold stored materials. Plant cells commonly store starch, and some store crystals of calcium oxalate or silica. Stored substances of this kind are known as cell inclusions.

The distinction between inclusions and organelles is worth keeping clear. Organelles are working components that carry out processes, while inclusions are materials the cell has set aside, so the two play quite different roles in the life of the cell.

A cell is built from certain basic parts: a plasma membrane, cytoplasm, and genetic material either in a nucleus or in a nucleoid. Viruses, viroids and prions have none of this organisation, which is why they are called acellular, meaning not made of cells.

They are also all too small to be seen under a light microscope, so their structure was worked out by other means.

A virus consists of genetic material with a protein coat around it, so it has two components.

A viroid drops one of them. It carries genetic material but lacks the protein coat entirely.

A prion drops the other. It is a misfolded protein and carries no genetic material at all, which makes it the most unusual of the three, since an infectious agent with no genes overturns the expectation that infection requires something to copy.

Calling a virus a very small cell is therefore mistaken. The difference is one of kind rather than of size.

Two states of the same material. In a cell that is not dividing, DNA sits in chromatin, an entangled mass of thread-like structures. When division approaches, that material organises itself into rod-shaped chromosomes, which is the only time they can be seen as such.

A functional segment of DNA. Chromosomes are made of DNA together with specific proteins, the DNA carries the genetic information, and the working portions of that DNA are the genes.

A large organelle spreading like a network through the cytoplasm, continuous with the outer membrane of the nuclear envelope. It synthesises and transports proteins, fats and, in some specialised cells, hormones. Its rough and smooth forms differ by whether ribosomes sit on the surface.

The finger-like folds of a mitochondrion's inner membrane. Folding increases the surface area available for chemical reactions, which is what makes the organelle's energy production efficient.

Adenosine triphosphate, the molecule in which the energy released by cellular respiration is stored. It is described as the energy currency of the cell because most cellular activities are paid for with it.

The nucleus is enclosed by a double-layered nuclear membrane carrying pores, which allow material to be transferred between the nucleus and the cytoplasm. Inside it lies the nucleolus, a dense round body where ribosomal subunits are made, and the genetic material in the form of chromatin or chromosomes.

In a non-dividing cell the DNA is present as chromatin, an entangled mass of thread-like structures. When the cell is about to divide, the chromatin organises itself into rod-shaped chromosomes. Chromosomes are composed of DNA together with specific proteins, the DNA carries the genetic information, and the functional segments of that DNA are called genes.

Rough endoplasmic reticulum has ribosomes attached to its surface, which is why it appears rough, and it is mainly involved in protein synthesis and secretion, as in gland cells such as pancreatic cells. Smooth endoplasmic reticulum has no ribosomes on its surface and handles the synthesis and storage of fats and hormones.

It consists of stacks of flattened, sac-like structures and is functionally linked to the endoplasmic reticulum, the cell membrane and other organelles. It modifies, sorts and packages proteins and lipids into vesicles for transport, for secretion, or for the formation of lysosomes.

A mitochondrion has two membranes. The outer membrane is smooth and porous, while the inner membrane throws up finger-like folds called cristae. The folding increases the surface area available for chemical reactions, which helps energy production during cellular respiration.

Chloroplasts contain the green pigment chlorophyll and carry out photosynthesis. Chromoplasts contain yellow, orange or red pigments and give flowers and fruits their bright colours. Leucoplasts are colourless and store food materials such as starch, oils or proteins.

In a grown plant cell a single big vacuole sits at the centre, brimming with cell sap, and into it go water, minerals, sugars and the cell's waste. The sheer volume of water held there presses outward and that pressure is what keeps the cell taut. Starve the plant of water and the vacuole gives its water up, the cells slacken, and the whole plant droops.

Both carry their own DNA and their own ribosomes, and can therefore make some of their own proteins. These are features found in certain bacteria, which suggests that mitochondria and plastids share an evolutionary history with single-celled organisms.

The nucleus is enclosed by a double-layered nuclear membrane. This covering is not sealed: it carries pores that allow material to be transferred between the nucleus and the cytoplasm.

Within the nucleus lies the nucleolus, a dense round body. Ribosomal subunits are synthesised here, and then leave for the cytoplasm, where one large and one small subunit assemble to form a ribosome.

The nucleus also holds the genetic material, and the form this takes depends on what the cell is doing. Between divisions the DNA lies about as chromatin, which under the lens looks like a tangle of fine threads.

Whenever the cell is about to divide, that chromatin organises itself into chromosomes, which is why chromosomes can be seen as rod-shaped structures only at that stage.

Chromosomes are composed of DNA together with specific proteins. The DNA molecules contain the genetic information, and the functional segments of DNA are called genes. It is through the chromosomes that characters are passed from parents to the next generation.

Protein synthesis begins at the ribosomes, which are the sites where proteins are made. Those destined for secretion are built on ribosomes attached to the endoplasmic reticulum.

Because those ribosomes stud its surface, this part of the network appears rough under an electron microscope and is called rough endoplasmic reticulum. It is mainly involved in protein synthesis and protein secretion, and is abundant in gland cells such as pancreatic cells.

The endoplasmic reticulum itself spreads like a network through the cytoplasm and is continuous with the outer membrane of the nuclear envelope, so it also transports what it helps to make.

The protein then reaches the Golgi apparatus, a set of stacks of flattened, sac-like structures functionally linked to the ER, the cell membrane and other organelles. Acting as the cell's post office, it modifies, sorts and packages the protein into vesicles.

From there the vesicle carries its contents onward, for transport within the cell, for secretion outside it, or for the formation of a lysosome.

Both are double-membrane-bound organelles, and both are concerned with energy, though from opposite directions.

A mitochondrion has a smooth, porous outer membrane and an inner membrane folded into finger-like projections called cristae. Those folds increase the surface area available for chemical reactions. Inside, glucose and other molecules are broken down during cellular respiration, and the energy released is stored as ATP, the cell's energy currency.

Inside a chloroplast — one kind of plastid — there is a thickish fluid, the stroma. Within the stroma are disc-shaped membrane structures containing chlorophyll, the green pigment that absorbs sunlight. Light energy is absorbed there during photosynthesis, and the sugars produced are stored in the stroma along with starch granules.

So a mitochondrion releases energy from food while a chloroplast captures light energy to make food, which is why chloroplasts are found in plant cells and mitochondria in both plant and animal cells.

What they share is striking. Both carry their own DNA and their own ribosomes, and can make some of their own proteins. These are features of certain bacteria, which suggests that mitochondria and plastids share an evolutionary history with single-celled organisms.

Cells produce waste materials and wear out their own organelles as they work, and something must prevent this accumulating inside the cell.

That task falls to the lysosomes. These are single membrane-bound sacs filled with enzymes, and the enzymes are capable of breaking down unwanted proteins, carbohydrates and fats, and even damaged parts of the cell itself.

The result is that the cell is kept clean and healthy rather than clogged with material it can no longer use.

Importantly, the breakdown products are not discarded. What the breakdown yields is let out into the cytoplasm and much of it is put back to work elsewhere, so the lysosome is as much a salvage yard as a rubbish chute.

The same enzymes are put to a quite different use elsewhere. A human sperm carries lysosomal enzymes of its own, and on reaching an egg they cut through its outer coat so that fertilisation can follow. Machinery evolved for housekeeping turns out to serve reproduction as well.

The making of new cells out of cells that already exist. It is what allows an organism to grow, to repair damaged tissue and to reproduce, and in eukaryotes it proceeds in an orderly way through the cell cycle.

Division producing two genetically identical daughter cells from one parent cell, each with the same DNA and the same chromosome number. It serves growth, repair, maintenance and asexual reproduction, and is the most common type of cell division.

A two-step division occurring only in reproductive organs. The parent cell divides twice in succession to give four daughter cells, each with half the parent's chromosome number, and these become the gametes used in sexual reproduction.

The control by which many animal cells stop dividing once they come into contact with neighbouring cells. Plant cells, having rigid walls, do not show it and follow a different pattern of growth.

The ability of a living plant cell, even a fully mature one from permanent tissue, to develop into a complete plant given suitable nutrients and favourable conditions. Haberlandt proposed it in 1902, and it underlies plant tissue culture technology.

A cell can grow only up to a certain size, so an organism cannot become larger simply by its existing cells swelling. Growth happens because cells divide to form new cells. The same process replaces old, dead or damaged cells, which is why a cut heals and lost hair grows back.

Mitosis produces two daughter cells while meiosis produces four. The cells produced by mitosis have the same chromosome number as the parent, whereas those produced by meiosis have half that number. Mitosis serves growth, repair, maintenance and asexual reproduction, while meiosis produces gametes for sexual reproduction and creates genetic diversity.

In the first division the cell divides into two daughter cells and the chromosome number in each is reduced to half. The second division resembles mitosis, with each daughter cell dividing into two, giving four daughter cells in all, each carrying half the chromosome number of the parent.

Firstly, every living organism consists of a cell or of several. Secondly, the cell is the unit on which both structure and function in living things are founded. Thirdly, no cell comes into being except from a cell that was already there.

The German botanist Matthias Schleiden announced in 1838 that plants are built throughout from cells. A year later, in 1839, the German zoologist Theodor Schwann showed the same held for animals. Rudolf Virchow completed the picture in 1855 by insisting that no cell arises except from one already present.

Contact inhibition is the process by which cell division in many animal cells stops once the cells come into contact with neighbouring cells. Plant cells do not show it because their rigid cell walls mean they follow a different pattern of growth altogether.

An ordinary cell grows, ages and dies to a schedule. A cancer cell has slipped that schedule and goes on dividing without restraint, which is how a tumour builds up. Tumours may be benign or malignant. The cancerous sort pushes into the tissue around it, and may travel elsewhere in the body and start again there.

Totipotency is the ability of any living plant cell, even a fully mature cell from permanent tissue, to develop into a complete plant if provided with suitable nutrients and favourable conditions. Gottlieb Haberlandt proposed it in 1902, and the idea laid the foundation for plant tissue culture technology.

Both are kinds of division — new cells made out of cells that already exist — but they are done for quite different reasons.

Mitosis is the most common type. A single parent divides once and yields two daughters of identical inheritance, each with the DNA the parent had and as many chromosomes as the parent had. This keeps genetic information largely uniform across the body's cells, which is what makes it suitable for normal growth, repair, maintenance and asexual reproduction. Every human begins as a single fertilised egg that divides repeatedly by mitosis to form trillions of cells.

Meiosis produces gametes and happens only in the cells of reproductive organs, meaning the testes and ovaries in animals and the anthers and ovaries in plants. The parent cell divides twice, one division after the other.

In the first division the cell gives two daughter cells and the chromosome number in each is reduced to half. The second division resembles mitosis, each daughter cell dividing again, so that four daughter cells result, each with half the parent's chromosome number.

That halving is essential. Gametes carrying half the DNA allow the original chromosome number to be restored at fertilisation, when gametes from two individuals combine, and the process creates the variation that makes children resemble their parents without being identical to them.

An important observation about living things is that all organisms are made of cells, and three scientists turned that observation into a theory.

In 1838 Matthias Schleiden, a German botanist, reported that every plant is made up of cells. In 1839 Theodor Schwann, a German zoologist, found that all animals are also made up of cells. Rudolf Virchow completed it in 1855 with the point that a cell can arise from another cell and from nothing else.

Their combined work produced the classical Cell Theory, which says three things at once: that everything alive consists of a cell or of many; that nothing smaller than a cell does the work or forms the building block; and that every cell has come from a cell before it.

It counts as unifying because it applies without exception across the whole of biology, from a single bacterium to a human being. However different two organisms appear, the theory says the same three things about both.

Virchow's contribution matters particularly, because by ruling out any other source of cells it explains life's continuity through cell division: every cell alive today traces back through an unbroken line of divisions to cells that came before it.

A normal cell enlarges and divides under restraint, keeps to the position assigned it, does the work it is there for, and is disposed of once it is surplus. Another takes over the same duty, so every cell has a term of service and then ends.

This balance can fail in either direction. Problems arise if cells do not die when they should, and equally if they die too early.

One important control operates in many animal cells: division usually stops when a cell comes into contact with neighbouring cells. This is called contact inhibition. Plant cells do not show it, since their rigid cell walls mean they follow a different pattern of growth.

In cancer, this system breaks down. Abnormal cells begin growing and dividing uncontrollably because they have lost contact inhibition, and the result is the formation of tumours.

Tumours may be benign or malignant, so not every tumour is cancerous. The cancerous sort is the one that pushes into the tissue beside it, and may set off through the body to start again somewhere new. Errors in mitosis are one route to such uncontrolled division, and can also leave body cells with abnormal numbers of chromosomes.

Begin by growing the roots. Fill a jar with plain water and stand an onion bulb on it so that the root-bearing base is immersed, then leave the setup for five or six days until roots appear.

Cut 2 to 3 cm of the freshly grown root and transfer it to aceto-alcohol, made from glacial acetic acid and ethanol in a ratio of 1 to 3, for 24 hours. Move it afterwards to 70 per cent ethanol for preservation.

To make the slide, wash a preserved root and place it on a clean slide. Add a drop of dilute hydrochloric acid to soften the tissue, rinse after 10 to 15 minutes, then add two or three drops of aceto-carmine stain and leave it for 5 to 10 minutes.

Warm the slide gently over a spirit lamp, taking care, then cut off the tip portion, add a coverslip and squash it gently with the thumb so that the cells spread out into a layer thin enough to see through.

Under the microscope the cells will not all look alike. That is the expected result, because the cells of a growing root tip divide continuously, so at any given moment different cells are at different stages of cell division and therefore show different structures.

The cell represents the basic level at which life exists, and all living organisms are made of cells. Unicellular organisms such as bacteria and yeast consist of only one cell, while multicellular organisms such as plants, fish, birds and humans are made of millions of cells working together.

A group of similar cells performing similar functions forms a tissue. Several kinds of tissue together make up one organ, and a set of organs sharing a job makes an organ system. The nasal pores, nasal cavity, trachea and lungs together form the respiratory system.

Viewed from about 25 cm, the near point of the eye, two points can only be distinguished if they are at least 0.1 mm apart; closer than that they appear as a single point. Since cells are usually far smaller than this, they cannot be seen with the unaided eye and a microscope is required.

It means some substances are allowed to cross the membrane while others are turned back. This matters because it is how a cell controls what enters and leaves, keeping its contents in order while still exchanging materials with its surroundings, as oxygen and carbon dioxide do across the membranes of the alveoli.

Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration, and it needs no membrane at all. Osmosis is the narrower case in which water alone moves across a selectively permeable membrane. Every case of osmosis is diffusion, but most diffusion is not osmosis.

The piece in plain water swells and gains weight, because the surroundings are more dilute than its contents and water moves inward. The piece in salt solution shrinks and loses weight, because the surroundings are more concentrated and water moves outward. In both cases the membrane allowed water across but blocked the salt and sugar.

The plasma membrane is a selectively permeable boundary around the cell. The cytoplasm is a semi-fluid, jelly-like substance filling the interior and holding the organelles. The nucleus is a prominent structure containing the cell's genetic material.

Both words end in karyon, which means nucleus. Pro means primitive, so a prokaryotic cell has only a primitive nucleus and no membrane around its genetic material. Eu means true, so a eukaryotic cell has a genuine, well-defined nucleus.

It lacks a well-defined nucleus and membrane-bound organelles, meaning organelles surrounded by their own membranes. Instead of a nucleus it has a nucleoid, a region holding its genetic material without any membrane around it.

The nucleus is enclosed by a double-layered nuclear membrane carrying pores, which allow material to be transferred between the nucleus and the cytoplasm. Inside it lies the nucleolus, a dense round body where ribosomal subunits are made, and the genetic material in the form of chromatin or chromosomes.