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

Put one drop of pond water under a microscope and a crowd appears: green specks that spin, slipper-shaped swimmers and grey blobs that creep by changing shape. Each of them is a complete living thing made of a single cell (कोशिका). Now look at the back of your hand. It is built of cells too, and so is every other living thing you have ever seen.

Some organisms, such as Amoeba, the yeast that makes bread dough rise and the bacteria that set milk into curd, live their whole lives as one cell. We call them unicellular. A mango tree, a sparrow, a rohu fish and you are multicellular: an adult human body holds roughly 30 trillion cells. In both cases the cell is the smallest unit that can stay alive on its own.

In a multicellular body, cells work in teams. Cells of one kind doing one job form a tissue. A few kinds of tissue together make an organ, and organs that share a larger task form an organ system (Fig. 2.1). The inner lining of your stomach, for example, is a tissue of cells that pour out digestive juice. With muscle and other tissues it makes the stomach, which joins the mouth, intestines and liver in the digestive system. Strip away the cells and nothing remains at any level.

2.1 How to Study Cells?

Hold a piece of fine muslin cloth at arm's length and it looks like a smooth sheet. Bring it close and the separate threads appear. Your eye can tell two points apart only when the gap between them is large enough. The smallest gap at which two points still look separate is the eye's limit of resolution. For a normal eye looking at something 25 cm away, the nearest distance at which most of us see sharply, that gap is about 0.1 mm.

Most plant and animal cells are 10 to 100 micrometres across (1 µm is one-thousandth of a millimetre). Most bacteria are only 1 to a few micrometres long. So a typical cell is at or below the eye's 0.1 mm limit, and a bacterium is about a hundred times smaller still. A few cells are exceptions: the yolk of a hen's egg is one giant cell. Everything else needs help, and Fig. 2.2 shows which tool can see what.

You have already used a convex lens as a magnifying glass. A compound microscope uses two sets of lenses one after the other. The objective lens, close to the specimen, makes an enlarged image, and the eyepiece enlarges that image again. In 1665 Robert Hooke built such an instrument himself. Through it, a sliver of bottle cork turned out to be a honeycomb of tiny empty chambers. He called them cells, after the small rooms that monks lived in.

A school microscope is a light microscope: it sends ordinary light up through a thin, almost transparent specimen. It usually carries two or three objectives, such as 10X and 40X, on a turning nosepiece, so you can switch from a wide view to a closer one. Learn its parts in Fig. 2.3 before you use it, and always focus first with the lowest-power objective.

A microscope shows an enlarged picture, so how can you tell the real size of what you see? The trick is to measure the circle of light you see through the eyepiece, the field of view, and then count how many cells fit across it.

How much bigger does the cell look? Each lens magnifies by a fixed amount, printed on it as 10X, 40X and so on. The image made by the objective is magnified again by the eyepiece, so the two numbers multiply, not add.

2.2 Structure of a Cell

A cell is never sealed off. A root hair cell draws in water and minerals, a nerve cell picks up signals, and an Amoeba swallows food and pushes out waste. Cells in a tissue also pass materials and messages to their neighbours. All this traffic crosses one place, the edge of the cell, so that is where we begin.

2.2.1 Cell membrane — The universal feature of a cell

Picture the security gate of a busy railway station: some people walk straight through, some are checked and allowed in, and some are turned away. Every cell, from a bacterium to a nerve cell, has a boundary that works like this gate. It is the cell membrane, also called the plasma membrane (प्लैज़्मा झिल्ली). Because it chooses what crosses, we call it selectively permeable. It also holds the cell's contents together and marks where one cell ends and the next begins.

Which substances get through, and which way do they go? Oxygen and carbon dioxide slip across easily, as they do in the air sacs of your lungs. Water crosses too. A raw potato can show you which way it moves, and what decides the direction.

Particles always tend to spread from where they are crowded to where they are scarce. Drop some rooh afza syrup into a glass of water and the red colour slowly spreads through it; the smell of frying pakoras drifts into the next room. This net movement from a higher to a lower concentration is diffusion (विसरण), and it needs no membrane at all.

Now put a membrane in the way that lets water molecules through but holds back most dissolved particles (the solute). Only the water can spread, so water moves from the side where it is plentiful (a dilute solution) to the side where it is scarcer (a concentrated solution). This is osmosis (परासरण): water diffusing through a selectively permeable membrane (Fig. 2.5). Root hair cells take in water from damp soil in exactly this way.

This explains the potato strips. The sap inside each potato cell is water with sugars and salts dissolved in it. Plain water outside is more dilute than the sap, so more water moved into the cells than out, and the strip swelled and stiffened. Strong sugar solution is more concentrated than the sap, so the cells lost water and the strip went limp.

What happens to a cell, then, depends on how the solution outside it compares with the solution inside. Biologists use three words, each describing the outside solution, as Fig. 2.6 and Table 2.3 show. A memory hook: iso means equal, hypo means under (less solute outside) and hyper means over (more solute outside).

Hospitals rely on this every day. A drip put into a patient's vein is usually 0.9 % salt solution, which is isotonic with blood, so red blood cells neither swell nor shrink. Pure water in a vein would make red cells swell and burst.

What is this choosy boundary made of? It is astonishingly thin, only about 7 to 10 nanometres: a pile of ten thousand membranes would be about as thick as a sheet of paper. It is built from lipids (fat-like molecules) and proteins, arranged as the fluid mosaic model describes (Fig. 2.8).

A soap bubble is a good place to start. Soap molecules have a head that is attracted to water and a tail that avoids it, so they line up into a thin film that bends, wobbles and even seals small holes. Membrane lipids behave in the same way. They form a lipid bilayer: two sheets of molecules, with their water-loving heads facing the watery fluid outside and inside the cell, and their water-avoiding tails hidden in the middle.

Proteins float in this bilayer like icebergs in a sea. Some sit on one surface. Others cross the whole membrane, forming channels and carriers that let particular substances through, which is how the membrane does its choosing. Together, lipids and proteins make a mosaic, a pattern of many different pieces. The pieces are not fixed in place: they slide sideways past each other, spin, and now and then flip between the two layers, so the whole sheet is fluid. Hence the name fluid mosaic model.

Every cell has a membrane. The cells of plants, fungi and most bacteria also have a second, tougher layer outside it, the cell wall. To see what difference the wall makes, compare a plant cell with one of your own.

2.2.2 Cell wall — The outer covering of cells

Look again at what the strong sugar solution did in Activity 2.3. Both kinds of cell lost water by osmosis, yet only the cheek cell collapsed. The onion cell kept its outline because of a tough layer outside its membrane, the cell wall (कोशिका भित्ति). The wall could not shrink, so the membrane and everything inside it pulled away from the wall. This is called plasmolysis. Put the cell back in plain water and it usually recovers.

Plant cell walls are made mostly of cellulose, a carbohydrate built from long chains of glucose units. We cannot digest cellulose, but it is still useful in our food as fibre (roughage), which helps food move smoothly through the gut. Fungi and most bacteria have walls too, made of other materials. Fungal walls, for example, contain chitin, the same tough material found in an insect's outer skeleton.

Why do plants need walls when we manage without them? A tree cannot walk out of a storm or into the shade, and it must hold up its own weight for as long as it lives. Billions of walled cells pressed together keep stems upright and leaves and petals in shape. An animal is held up by a skeleton instead, and its cells must be able to change shape: muscle cells shorten when they contract, and white blood cells squeeze through gaps between other cells. Rigid walls would make both impossible.

A wall is strong but not choosy. Water and dissolved minerals pass through it freely, as through the gaps of a cane basket. The choosing is done by the membrane just inside it. A root needs both: the open wall lets soil water reach the cell, and the selective membrane decides what actually gets in.

2.3 The Cell Interior — A Coordinated Working System

Under a school microscope, a stained cell shows only a few features: the membrane at its edge, a jelly-like fluid called the cytoplasm (कोशिकाद्रव्य) filling it, and a dark, rounded nucleus. An electron microscope reveals far more. Suspended in the cytoplasm are many smaller structures, each with its own job. These are organelles (कोशिकांग), the 'little organs' of the cell.

Did you notice the biggest difference? Biologists use it to split all cells into two great groups, named after the Greek karyon, a kernel, meaning the nucleus. Eukaryotic cells (eu, true) keep their DNA inside a true nucleus and divide their interior into membrane-bound organelles. Plant, animal and fungal cells are eukaryotic, and so are single-celled organisms such as Amoeba and yeast.

Prokaryotic cells (pro, before) are simpler, and they appeared first in the history of life. A bacterium has no nucleus: its DNA lies in a tangle in a region of the cytoplasm called the nucleoid, none of its parts has a membrane of its own, and almost all its chemistry happens directly in the cytoplasm. Table 2.6 sets the two groups side by side.

Why does a cell not sag like a water balloon, and how does a package get from one end of it to the other? Eukaryotic cells have an inner framework, the cytoskeleton: a mesh of very fine protein fibres running through the cytoplasm. It works like the poles of a tent and the tracks of a railway at once. It gives the cell its shape and strength, lets some cells crawl or change shape, and carries organelles and packages along its fibres. Its fibres are too fine to see without an electron microscope.

The cytoplasm can also hold non-living stored material, called cell inclusions. Many plant cells store starch grains. Some hold needle-like crystals of calcium oxalate, which is why raw arbi (colocasia) leaves can make your throat itch. Others build silica into their tissues, which is why rice husk and bamboo feel so hard and scratchy.

2.3.1 Why do eukaryotic cells need these organelles?

A cell must do many jobs at once: build proteins, release energy, clear away waste, store food and look after its instructions. Some jobs need conditions that would ruin others. Digestive enzymes, for example, would break down the cell's own proteins if they roamed freely. Membrane-bound organelles solve this by giving each job its own room, with the right conditions inside.

Picture the langar kitchen of a large gurdwara, which can feed tens of thousands of people in a day. Recipes are kept in one place, cooks work at their own counters, food is packed at another station, stoves are kept burning, and a cleaning team keeps everything usable. Each organelle of a eukaryotic cell plays a similar part (Table 2.7).

Nucleus: the control centre

The nucleus is usually the largest organelle. It is wrapped in a nuclear envelope of two membranes, pierced by many nuclear pores, openings that let materials and messages pass between the nucleus and the cytoplasm (Fig. 2.12). Inside lies a dense, rounded body, the nucleolus. It makes the two parts, one large and one small, from which ribosomes are built. These parts leave through the pores and join up in the cytoplasm as working ribosomes.

Most of the nucleus is filled with a tangle of long, thin threads called chromatin. These threads are the cell's instructions, written in long molecules of DNA (deoxyribonucleic acid) wound around proteins. When the cell gets ready to divide, the threads coil up tightly into short, thick chromosomes (गुणसूत्र) that show clearly under a microscope. Think of a long charger cable: loose, it tangles all over the drawer; coiled and tied, it is easy to hand over. DNA is coiled for the same reason before it is shared between two new cells.

A section of DNA that does one particular job, such as carrying the code for one protein, is a gene. Each chromosome carries many genes. Genes are passed from parents to children, which is how you may have inherited your mother's dimples or your grandfather's curly hair.

Prokaryotes manage without a nucleus. A bacterium's DNA is usually one circular molecule, with some proteins attached, lying in the nucleoid.

Ribosomes: protein builders

Ribosomes are the smallest organelles and have no membrane around them. Each one is a tiny machine that reads instructions copied from the DNA and links building blocks called amino acids into a protein chain. Some ribosomes float free in the cytoplasm and make proteins for use inside the cell. Others sit on the endoplasmic reticulum and make proteins that will go into membranes or leave the cell. Bacteria have ribosomes too, because every cell needs proteins.

Endoplasmic reticulum: a network of channels

The endoplasmic reticulum (ER) is a large system of membrane-lined tubes and flattened sacs spread through the cytoplasm. Its membrane is joined to the outer membrane of the nuclear envelope. The ER makes proteins and lipids, carries them through the cell, and in some cells makes hormones. It comes in two forms:

• Rough ER (RER) is dotted with ribosomes, which give it a grainy look under the electron microscope. Many of its proteins are made for export. Cells of the salivary glands, which pour enzymes into your mouth, are packed with rough ER.
• Smooth ER (SER) has no ribosomes. It makes lipids, including those of new membranes, and some hormones. In liver cells it also helps break down drugs and poisons.

Golgi apparatus: sorting and dispatch

Look closely at the cell diagrams and you will find stacks of flat, curved sacs, like a pile of chapatis pressed into a bowl shape. This is the Golgi apparatus (गॉल्जी उपकरण). Proteins and lipids made in the ER arrive at one face of the stack inside small membrane bubbles called vesicles. As they pass through the sacs they are finished off, for example by having sugar chains attached, and then sorted and packed into new vesicles at the other face.

Like a courier hub that stamps an address on every parcel, the Golgi sends each package where it is needed: to the cell membrane, out of the cell, or into a lysosome. Fig. 2.13 follows one protein along this route.

Lysosomes: the clean-up crew

A lysosome (लयनकाय) is a small bag of powerful digestive enzymes wrapped in a single membrane. The enzymes can break down proteins, carbohydrates, fats and even whole organelles. The membrane keeps them away from the rest of the cell. If a cell is badly damaged, its lysosomes may burst and digest the cell itself, which is why they are nicknamed the cell's 'suicide bags'.

Why keep such a dangerous bag? Every busy cell makes rubbish: worn-out organelles, faulty proteins and, in some cells, bacteria swallowed from outside. Lysosomes digest it, and the simple pieces that result are released into the cytoplasm and reused. A lysosome is a recycling unit as much as a dustbin.

Mitochondria: where food becomes usable energy

Everything a cell does, from building a protein to moving a tiny hair, costs energy, and the cell pays in one currency: a small molecule called ATP (adenosine triphosphate). ATP works like a prepaid card that every machine in the cell accepts. Most of it is made in mitochondria (सूत्रकणिका; singular mitochondrion). There, glucose and other food molecules are broken down with the help of oxygen, a process called cellular respiration, which you have met before, and the energy released is stored in ATP.

The structure suits the job (Fig. 2.14). A mitochondrion has two membranes. The outer one is smooth and lets small molecules through. The inner one is folded into many shelves called cristae, which give it a large surface on which the energy-releasing reactions take place. That is why mitochondria are called the powerhouses of the cell, and why heart muscle cells, which never rest, contain thousands of them.

Plastids: food factories and colour stores

Why is a carrot orange, a potato white and a leaf green? All three colours come from plastids (लवक), organelles found in plants and algae, which make, colour or store food. Chromoplasts (Greek chroma, colour) hold yellow, orange or red pigments. They colour marigold petals, ripe mangoes and carrots, and the bright colours attract the insects that pollinate flowers and the birds and animals that eat fruit and scatter its seeds. A ripening tomato shows a switch: as it turns from green to red, its chloroplasts change into chromoplasts. Leucoplasts (leukos, white) have no pigment. They store food: starch in a potato tuber or a wheat grain, and oils or proteins in some seeds.

The green plastids are chloroplasts, the food factories of the plant. They hold the pigment chlorophyll, which traps sunlight for photosynthesis. Like a mitochondrion, a chloroplast has two membranes. Inside is a fluid called the stroma, in which sit stacks of disc-shaped membrane sacs. The chlorophyll is in these discs, where light energy is captured. The sugar made is used at once or stored in the stroma as starch grains.

Mitochondria and chloroplasts share a surprising feature: each has its own DNA and ribosomes and can make some of its own proteins. Their DNA is circular, like a bacterium's, and they multiply by splitting in two. Most biologists read this as strong evidence that, long ago, both were free-living bacteria that came to live inside larger cells and never left.

Vacuoles: storage tanks

Forget to water a tulsi plant on a hot day and by evening its leaves droop. Water it, and within a few hours it stands up again. The change happens in its vacuoles (रसधानी). In a mature plant cell, one large central vacuole can fill most of the cell, pushing the cytoplasm into a thin layer against the wall. It is a sac bounded by a single, selectively permeable membrane and filled with a watery cell sap that holds dissolved sugars, salts, pigments and wastes.

When water is plentiful, the vacuole swells and presses the cytoplasm against the wall, the way air inflates a football. This pressure keeps the cell firm, and firm cells hold soft stems and leaves up. Short of water, the vacuoles shrink, the pressure falls and the plant wilts. Animal cells have only small, temporary vacuoles, if any, for storing or carrying materials, such as the food vacuoles of Amoeba.

2.4 How do Normal Cells Grow and Divide?

A scraped knee closes over in about a week. A short haircut grows out in a month or two. A newborn of 3 kg becomes an adult of 50 kg or more. Each time, new cells are being made. Growth is mostly not cells getting bigger: a cell can only grow so large, and then it divides in two.

Why is there a limit? Everything a cell takes in or gets rid of must cross its surface. As a cell grows, its volume, the living material that needs supplies, increases faster than its surface area, through which the supplies arrive. A very large cell would starve in the middle. Dividing in two restores a better balance, as Example 2.3 shows.

Your body replaces cells all the time: by one careful estimate, a few hundred billion every day, about 1 % of all your cells. Most are blood cells and cells lining the gut. Bacteria multiply too, simply by copying their DNA and splitting in two. A eukaryotic cell, though, divides only at the end of a tightly controlled series of stages called the cell cycle. The cell grows, copies its DNA, checks the copy, and only then divides. You will study the stages in higher classes.

2.4.1 Cell division

Some cells in your body never stop dividing: those in the deepest layer of your skin make new cells all your life. Others, such as most nerve cells, never divide again once they are formed. Whenever a cell does split in two, the process is called cell division. It builds a growing body, patches it when it is hurt, and makes the cells that start the next generation. It comes in two kinds, with very different results.

Mitosis (समसूत्री विभाजन) makes two daughter cells that are copies of the parent cell. Before dividing, the cell copies every chromosome. The two copies are then pulled apart, so each daughter cell receives a complete set: the same number of chromosomes and the same DNA as the parent (Fig. 2.15).

You began life as one fertilised egg. Mitosis, repeated again and again, turned it into trillions of cells, all carrying the same instructions. Mitosis also replaces worn-out cells and heals wounds. It even lets some organisms produce new individuals without seeds or gametes (asexual reproduction), as when a potato tuber sprouts or new plantlets grow along the edge of a Bryophyllum leaf.

Meiosis (अर्धसूत्री विभाजन) is the division that makes gametes, the sex cells that fuse at fertilisation. Its result is very different from that of mitosis: one cell divides twice in a row and ends up as four cells, each with only half the number of chromosomes (Fig. 2.16). Since gametes are made only in reproductive organs, that is the only place meiosis happens: in the testes and ovaries of animals, which make sperm and eggs, and in the anthers and ovary of a flower, which give rise to pollen grains and the egg cell. The two divisions do different jobs:

• First division: the chromosome number is halved. Each of the two new cells gets one chromosome from every matching pair.
• Second division: each of those two cells divides again, much as in mitosis. The result is four cells, each with half the parent's number of chromosomes.

Why halve the number? At fertilisation a sperm and an egg fuse. If each carried the full set, the child would have twice as many chromosomes as its parents, and the number would double in every generation. Halving in meiosis and joining at fertilisation keep it steady: 23 + 23 = 46 in humans. Meiosis also shuffles the chromosomes, so every gamete gets a different mixture. That is why brothers and sisters resemble their parents, and each other, without being exact copies, unless they are identical twins. Chapter 11 takes this further.

2.5 Cell Theory — The Unifying Principle of Biology

By the 1830s, better microscopes let biologists examine many kinds of tissue, and a pattern emerged. In 1838 the German botanist Matthias Schleiden concluded that every part of a plant is made of cells. A year later the German zoologist Theodor Schwann reached the same conclusion for animals. In 1855 the German doctor Rudolf Virchow added the missing piece: cells do not appear out of nowhere, but always come from cells that already exist. His Latin motto was omnis cellula e cellula, 'every cell from a cell'.

These three statements tie all of biology together. A bacterium, a banyan tree and a human are built on the same plan, and their cells grow and divide in similar ways. The third statement also explains the continuity of life: every cell in your body traces back, division by division, to the fertilised egg you began as, and that egg came from cells of your parents.

2.5.1 Do cells grow and reproduce forever?

Watch animal cells growing in a culture dish and you see a built-in brake. They divide until they cover the bottom in a single layer and touch neighbours on every side, and then they stop. This is called contact inhibition (Fig. 2.17). Cancer cells have lost the brake. They keep dividing even when crowded, pile up on top of one another and form a tumour.

Division is only half of the balance; the other half is death. A healthy cell does its job and dies when its time is up, to be replaced by a new one. Life spans vary enormously: cells lining your gut last only a few days, red blood cells about four months, and most nerve cells as long as you live. If cells divide too little or die too early, a tissue wastes away; if they divide too much or refuse to die, they build up where they are not wanted.

Plant cells are held in place by their walls and never crawl over each other, so contact inhibition plays no part in them. Plants control growth in another way: only special growing regions, such as the root tip you studied in Activity 2.5 and the tip of the shoot, keep dividing, while most other cells stop dividing once they mature.

Next time a scrape on your knee heals, picture what is happening under the scab. Cells at the edge of the gap divide by mitosis, build new proteins on their ribosomes, power the work with ATP from their mitochondria, and stop dividing once the gap is closed. Nearly everything in this chapter is at work in that small patch of skin.