Life Processes: Class 10 Science · Chapter 5

A cat curled up asleep on a warm windowsill is not hunting, eating or making a sound, yet nobody mistakes it for a stuffed toy. A packet of dry moong seeds can sit in the kitchen for months without any change. Soak a handful overnight, though, and by morning tiny white shoots appear. How do we decide what is alive?

Visible movement turns out to be a weak test. A banyan tree hardly moves, and a sleeping animal barely stirs. Biologists look much deeper, at the level of molecules. Inside every living cell, molecules are carried from place to place, taken apart and put together again, every second of the day. A virus sitting on a door handle shows none of this activity until it gets inside a cell, which is one reason scientists still debate whether viruses are truly alive.

Why must molecules keep moving? A living body is highly organised: organs are built from tissues, tissues from cells, and cells from even smaller parts. Ordered arrangements tend to fall apart over time, much as a tidy room turns messy unless someone keeps putting things back. An organism stays alive only by repairing and rebuilding itself all the time, and since every structure is made of molecules, that repair means moving molecules around without a break.

5.1 What are Life Processes?

The upkeep of a body cannot wait until we are busy. It carries on while you sit in class, while you watch a match and while you are fast asleep. The set of jobs that together keep an organism alive and in working order are its life processes (जैव प्रक्रम).

Keeping a body in good repair raises four practical questions, and a life process answers each one:

1. Where does the energy come from? From outside the organism, as food. Taking food in is nutrition (पोषण). Food also supplies raw material for growth. Living things are built from carbon compounds, so foods are mostly carbon compounds too: simple ones for some organisms, complex ones for others, which is why organisms feed in such different ways.
2. How does food become usable energy? Foods come in many forms, but a cell needs energy in one standard form that it can spend anywhere. Chains of chemical reactions, most of them oxidation–reduction reactions, take the food apart, and many organisms use oxygen from the air to do it. Taking in oxygen and using it to break down food inside cells is respiration (श्वसन).
3. How does everything reach every cell? A single cell floating in a pond touches water on every side, so food and oxygen diffuse straight in and wastes diffuse out (Fig. 5.1). In a large animal most cells lie deep inside, and diffusion over such distances is hopelessly slow. So special organs take over: the gut takes in food, the lungs take in oxygen. Every cell needs both, so a delivery network, transportation, has to carry them from those organs to the rest of the body.
4. What happens to the waste? Energy-releasing reactions leave by-products that cells cannot use, and some are poisonous. Collecting them and throwing them out of the body is excretion (उत्सर्जन). In a large body a special tissue does this too, and the transport system brings the wastes to it.

5.2 Nutrition

Food does two jobs for every organism. It is fuel, supplying energy for everything from a cricket match to the quiet upkeep of a body at rest. It is also building material, supplying the atoms for new cells, new proteins and the healing of a scraped knee.

Trace the paneer on your plate backwards: it came from milk, the milk from a cow, and the cow's body from the grass it ate. The grass ate nothing. It made its own food from carbon dioxide and water, using sunlight. Organisms like the grass, which build food from simple inorganic substances, are autotrophs (स्वपोषी): green plants and some bacteria. Organisms like the cow and you, which take in complex food made by others, are heterotrophs (विषमपोषी): all animals and fungi. A heterotroph has to break complex food into simpler pieces with enzymes, biological catalysts, before it can use it. And every heterotroph depends on autotrophs in the end.

5.2.1 Autotrophic Nutrition

A leaf in sunlight works as a small food factory. Its raw materials are carbon dioxide from the air and water from the soil. Its power supply is sunlight, captured by the green pigment chlorophyll. Its product is carbohydrate. This is photosynthesis, and it is how autotrophs meet their needs for both carbon and energy. The plant spends some of the carbohydrate at once and stores the rest as starch, a reserve it can draw on later; that is why potatoes, rice grains and carrots are so filling. We keep a reserve too: our liver and muscles store carbohydrate as glycogen.

What actually happens inside a leaf cell? Three events take place (Fig. 5.2):

1. Light is absorbed by chlorophyll.
2. Light energy becomes chemical energy, and water molecules are split into hydrogen and oxygen. The oxygen escapes as a gas.
3. Carbon dioxide is reduced to carbohydrate, using the hydrogen and the stored chemical energy.

Where is the chlorophyll? Look at a very thin slice cut across a leaf under a microscope (Fig. 5.3). Several layers show up. Many cells carry small green bodies called chloroplasts, which hold the chlorophyll. They are packed most tightly in the tall, closely set cells just under the upper surface, where light is strongest. The next activity tests whether chlorophyll is really needed.

Every leaf faces a trade-off. To make food it must let carbon dioxide in, but any opening that lets a gas in also lets water vapour out. The leaf's answer is a set of tiny adjustable pores, the stomata (रंध्र; singular stoma), mostly on its underside. Most of the gas exchange for photosynthesis happens through them, although stems and roots also exchange some gases across their surfaces. When carbon dioxide is not needed, for example at night, the plant closes the pores and saves water.

Two bean-shaped guard cells surround each pore and act as its doors (Fig. 5.4). Water flowing into them makes them swell and bow apart, which opens the pore. When they lose water they go limp, sag together and shut it.

Carbohydrates contain only carbon, hydrogen and oxygen, but a plant body needs more elements than these. Proteins need nitrogen, chlorophyll needs magnesium, and phosphorus and iron are needed too. All of them come from the soil, absorbed by the roots along with water (Table 5.2). Plants cannot use the nitrogen gas of the air directly. They take nitrogen up as nitrates or nitrites, or as organic compounds made from atmospheric nitrogen by soil bacteria, such as those in the root nodules of chana and moong.

5.2.2 Heterotrophic Nutrition

A goat grazing on a hillside and an eagle circling above it are both heterotrophs, but they feed in completely different ways. The goat's food stays put, so it needs only teeth that grind and a gut long enough to deal with tough leaves. The eagle's food runs away, so it needs sharp eyes, speed, curved talons and a hooked beak. How an organism feeds depends on what its food is, whether the food moves, and how its own body is built.

Heterotrophs follow three broad strategies. Most animals, including us, take food into the body and break it down there. Fungi do the reverse: a mushroom or a patch of bread mould releases enzymes onto its food, digests it outside its body and absorbs whatever dissolves. This is often called saprophytic nutrition. The third group are parasites, which feed on a living organism, the host, usually without killing it. Amarbel (Cuscuta), the yellow leafless vine you may see smothering a hedge, is a parasitic plant. Ticks on a stray dog, head lice, leeches and tapeworms are parasitic animals.

5.2.3 How do Organisms obtain their Nutrition?

Single-celled organisms have the simplest feeding equipment of all, since the whole cell touches the water around it. An Amoeba has no mouth and no fixed shape (Fig. 5.6). When it meets a food particle, its cytoplasm flows out into two temporary arms, called pseudopodia ('false feet'), one on each side of the particle. Their tips meet and join, trapping the food inside a small bubble, the food vacuole. Enzymes released into the vacuole break the complex food into simple substances, which diffuse out into the cytoplasm. The leftover undigested bits are carried to the cell surface and pushed out.

Paramoecium shows the first hint of specialisation, even within one cell. It keeps a fixed slipper shape and can take food in at only one place, a kind of mouth at the end of a groove. Thousands of tiny hair-like cilia cover the cell, and their beating drives water, with the bacteria in it, down the groove to that spot.

5.2.4 Nutrition in Human Beings

Our digestive system is built around the alimentary canal, a single long tube that runs from the mouth to the anus (Fig. 5.7). Different stretches of the tube, helped by glands that pour juices into it, do different jobs. Let us follow a meal of dal, rice and a little ghee along the tube, starting with an experiment on saliva.

The mouth does four jobs at once:

1. Teeth cut and grind the food into small pieces of similar size, so that it can pass smoothly along the tube.
2. Saliva from the salivary glands wets and softens the food, because the inner surface of the gut is delicate. That is the watery feeling when you smell something you like.
3. Saliva also starts chemical digestion. Large food molecules must be broken into small ones before they can be absorbed, and enzymes do this. Saliva contains the enzyme salivary amylase, which breaks starch down into a simple sugar.
4. The muscular tongue turns the food over, mixes it with saliva and pushes it back for swallowing.

Food does not simply fall down the canal: astronauts can swallow in space, and you can drink lying down. The wall of the canal has muscles that contract in waves, one ring after another, squeezing the food forward the way you push toothpaste along a tube. These waves, called peristalsis (क्रमाकुंचन), keep the food moving at a controlled pace all along the gut, so each part has time to do its job. From the mouth, food travels down the food pipe, or oesophagus, to the stomach.

Next comes the stomach, a J-shaped bag of muscle that can stretch to hold a whole meal. For a few hours it squeezes and churns the food, mixing it with gastric juice from gastric glands in its lining. Gastric juice holds three important things:

• Hydrochloric acid, which makes the stomach contents strongly acidic. The protein-digesting enzyme works only in acid, and the acid also kills most germs swallowed with food.
• Pepsin, an enzyme that begins the digestion of proteins.
• Mucus, which coats the stomach lining and normally protects it from its own acid.

Food leaves the stomach a little at a time: a ring of muscle at its exit, a sphincter, opens briefly and closes again. Each portion enters the small intestine, the longest stretch of the canal, folded into the abdomen in tight coils. Its length suits an animal's diet. Cows and goats eat grass, whose cellulose is slow to digest, so their small intestine is long. Tigers and cats eat meat, which is digested more easily, and theirs is shorter.

The small intestine completes the digestion of carbohydrates, proteins and fats. Two large glands, the liver and the pancreas, send their juices into it, and its own wall adds a third:

• Bile, made by the liver and stored in the gall bladder, contains no enzyme. It makes the acidic food from the stomach alkaline, which the pancreatic enzymes need. Its bile salts also break large fat globules into tiny droplets, an emulsion, much as soap breaks up grease (Chapter 4). Tiny droplets give enzymes far more surface to work on (Fig. 5.8).
• Pancreatic juice, from the pancreas, contains trypsin, which digests proteins, and lipase, which digests the emulsified fats. It also has an amylase that continues the digestion of starch.
• Intestinal juice, from glands in the wall of the small intestine, finishes the job: proteins end up as amino acids, complex carbohydrates as glucose, and fats as fatty acids and glycerol.

Digestion is wasted unless its products reach the blood. The inner lining of the small intestine is built for this. It is folded, and the folds carry millions of tiny finger-like villi (दीर्घरोम; singular villus), which make the absorbing surface enormous. Inside each villus runs a mesh of blood capillaries, plus a small lymph vessel that takes up fats. Glucose and amino acids pass into the blood and are carried to cells all over the body, to be used for energy, growth and repair.

Food that cannot be absorbed, such as fibre, moves on into the large intestine. Its main task is to reclaim water, so the leftovers become firmer as they go. This waste is stored for a while and then passed out through the anus when the anal sphincter, another ring of muscle, relaxes.

5.3 Respiration

Food taken in by nutrition is only stored energy. To use it, cells must break the food down, and this releases carbon dioxide. Two quick experiments show where that carbon dioxide goes.

Three ways to break down glucose

Whatever the organism, glucose is not taken apart in one go. The first stage is shared by yeast, bacteria, plants and you (Fig. 5.9). In the cytoplasm, a glucose molecule with its six carbon atoms is cut into two pyruvate molecules of three carbons each, and a small amount of energy is released. After that the routes divide, and oxygen decides which one is taken.

The three routes for pyruvate are:

• No oxygen, in yeast: pyruvate becomes ethanol, a two-carbon molecule, and carbon dioxide. This is fermentation. Because it happens without air (oxygen), it is called anaerobic respiration (अवायवीय श्वसन).
• Too little oxygen, in our muscles: during a sprint, muscle cells use oxygen faster than blood can bring it. Pyruvate is then turned into lactic acid, another three-carbon molecule. Lactic acid building up in the muscles causes cramps.
• Plenty of oxygen, in mitochondria: pyruvate is broken down completely, each molecule giving three molecules of carbon dioxide, plus water. This is aerobic respiration (वायवीय श्वसन). It releases far more energy than either route without oxygen.

Adding up the stages, aerobic respiration of one glucose molecule can be summed up as:

Cells do not spend this energy directly. They use it at once to make a molecule called ATP (adenosine triphosphate). ATP then supplies a fixed amount of energy wherever the cell needs it, driving reactions that take in energy.

Taking in oxygen

Many people think plants 'breathe in' carbon dioxide and 'breathe out' oxygen. In fact every living plant cell respires all the time, using oxygen and giving out carbon dioxide, just as ours do. What changes between day and night is the balance. In the dark, respiration is the only process running, so a leaf gives out carbon dioxide. In bright light, photosynthesis uses up all the carbon dioxide that respiration makes, and more besides, so the leaf's main output is oxygen. Either way the gases simply diffuse, through the stomata and the air spaces that keep every leaf cell in touch with air.

Animals rely on a whole range of breathing organs: gills, lungs, even the moist skin of a frog. A crow or a cow takes oxygen from the air. A fish or a prawn has to take it from the water, where it is dissolved, and water holds only about one-thirtieth as much oxygen as the same volume of air.

A fish takes water in through its mouth and forces it over the gills, where the blood picks up the dissolved oxygen. The water then leaves through the gill slits.

Breathing organs on land, whatever the animal, are built to a similar plan:

• Big: a very large surface touches the air, since that is where oxygen and carbon dioxide cross.
• Thin: the surface is extremely thin and moist so gases pass quickly, which also makes it fragile.
• Protected: because it is fragile, it is tucked away inside the body, and tubes bring the air to it.
• Ventilated: muscles keep pumping fresh air in and stale air out.

The human respiratory system

Let us follow a breath of air into the body (Fig. 5.10):

1. Air enters through the nostrils. Fine hairs and sticky mucus lining the nasal passage trap dust and germs.
2. It passes through the throat (pharynx) and the voice box into the trachea, or windpipe. Rings of cartilage in its wall stop the airway from collapsing.
3. The trachea divides into two bronchi, one for each lung. Inside the lungs they branch again and again into finer tubes, the bronchioles.
4. Each bronchiole ends in a cluster of tiny balloon-like air sacs, the alveoli (कूपिका; singular alveolus). Their thin walls are wrapped in a dense network of blood capillaries. This is where gases are exchanged.

Breathing works like a pair of bellows. To breathe in, the muscles between the ribs pull the rib cage upwards and outwards, while the diaphragm, a dome-shaped sheet of muscle below the lungs, tightens and flattens. Both movements enlarge the chest. The lungs stretch to fill the extra room, and air flows down the airways into the alveoli. To breathe out, the muscles relax: the ribs fall back, the diaphragm rises into its dome, the chest shrinks and air is pushed out.

Across the thin alveolar wall, traffic runs both ways at once: oxygen moves from the air into the blood, and carbon dioxide carried from the tissues moves out into the air. Even a deep breath out leaves over a litre of air behind, the residual volume. Because the lungs are never empty, the exchange carries on in the pause between one breath and the next.

Oxygen dissolves poorly in water, so blood plasma on its own could carry very little of it. Our blood solves this with haemoglobin (हीमोग्लोबिन), a red pigment packed inside the red blood cells. Haemoglobin grabs oxygen where it is plentiful, in the lungs, and lets it go where it is scarce, in working tissues. Pigments that carry oxygen like this are called respiratory pigments, and every large animal depends on one, because diffusion is far too slow over long distances. Carbon dioxide is the opposite case: it is much more soluble, so most of it is simply carried back to the lungs dissolved in the blood.

5.4 Transportation

5.4.1 Transportation in Human Beings

Blood is the body's delivery fluid. Its liquid part, plasma, is mostly water with things dissolved in it: digested food, carbon dioxide, urea and other nitrogen-containing wastes, salts and more. Floating in the plasma are red blood cells, which carry oxygen, along with white blood cells and platelets. (You met blood as a fluid connective tissue in Class 9.) A delivery service needs more than parcels, though. The body also needs an engine to keep the blood moving, a road network that reaches every tissue, and a repair crew for damaged roads: the heart, the blood vessels and the platelets.

The heart: our pump

Your heart, a muscular pump about the size of your clenched fist, sits in the chest between the lungs, tilted slightly to the left. Blood has to carry oxygen to the cells and carbon dioxide away from them. If oxygen-rich and carbon-dioxide-rich blood mixed, the body would get a diluted oxygen supply, so the heart is divided into separate chambers (Fig. 5.11). A thick wall, the septum, keeps the right side, which handles deoxygenated blood, apart from the left side, which handles oxygenated blood. On each side a thin-walled upper chamber, the atrium, receives blood, and a thick-walled lower chamber, the ventricle, pumps it out.

Follow one round, starting with blood that has just come back from the body:

1. Deoxygenated blood from the body flows through the vena cava into the relaxed right atrium.
2. The right atrium contracts. At the same moment the right ventricle relaxes, so it fills.
3. The right ventricle contracts and sends the blood through the pulmonary artery to the lungs, where it gives up carbon dioxide and picks up oxygen.
4. Oxygen-rich blood returns through the pulmonary veins into the relaxed left atrium.
5. The left atrium contracts while the left ventricle relaxes and fills.
6. The left ventricle contracts strongly and pumps the blood into the aorta, which carries it to the whole body.

In a real heartbeat both sides work together: the two atria contract at the same time, then the two ventricles. Because the ventricles push blood out to the lungs or the whole body, their walls are much thicker than those of the atria, and the left ventricle's wall is the thickest of all. Valves at the openings stop blood flowing backwards when a chamber contracts. The 'lub-dub' a doctor hears through a stethoscope is the sound of valves snapping shut.

Double circulation

Compare the hearts of different vertebrates and a pattern appears (Table 5.8).

In a fish, the heart pumps blood to the gills. There it picks up oxygen and flows straight on round the body before returning to the heart (Fig. 5.12). One round, one pass through the heart: this is single circulation. In our bodies each round has two halves: heart to lungs and back, then heart to body and back. Blood passing through the heart twice in each round is double circulation (दोहरा परिसंचरण).

Why do birds and mammals need four chambers and complete separation? They keep their bodies warm at a steady temperature, whatever the weather. That burns fuel all the time and demands a rich, undiluted supply of oxygen. A frog or a lizard lets its body temperature follow its surroundings, spends far less energy, and can tolerate some mixing of the two streams.

The tubes: blood vessels

Blood travels in a closed loop of tubes of three kinds, each built for the pressure it carries (Fig. 5.14). Blood leaves the heart in powerful spurts into the arteries, which carry it away from the heart. Their thick, elastic, muscular walls stretch and recoil with every beat; that recoil is the pulse you feel at your wrist. Within an organ the arteries branch into finer and finer vessels, ending in capillaries whose walls are just one cell thick. Food, oxygen, carbon dioxide and wastes cross these thin walls to and from the cells.

Capillaries then merge into veins, which bring the blood back to the heart. The pressure has dropped by now, so vein walls are thinner, and flap-like valves inside them stop blood slipping backwards, which matters most in the legs. Remember that arteries and veins are named for their direction, not for their oxygen: the pulmonary artery carries deoxygenated blood, and the pulmonary veins carry oxygenated blood.

Maintenance by platelets

Platelets are tiny cell fragments that drift through the blood until a vessel is damaged. Then they gather at the injury and help the blood to clot, sealing the gap. This matters for two reasons: it saves blood, and it keeps up the pressure the heart works so hard to make, because a leaking system pumps poorly. That is why a small cut stops bleeding within a few minutes.

Lymph

Every day a little more fluid leaks out of the capillaries into the tissues than seeps back in. Without a drain, the tissues would swell. The leaked fluid, called tissue fluid or lymph (लसीका), is plasma that has squeezed through tiny pores in the capillary walls, carrying a little protein and some white blood cells. It is colourless and has less protein than plasma. Lymph collects in fine lymphatic capillaries, which join into larger lymph vessels that empty into big veins near the heart, so the fluid returns to the blood. Lymph vessels have a second job too: the small lymph vessel inside each villus takes up fat absorbed in the intestine and carries it to the blood.

5.4.2 Transportation in Plants

A coconut palm may stand twenty-five metres tall. Its leaves make food at the top, while its roots take in water and minerals such as nitrogen and phosphorus from the soil at the bottom. Diffusion could bridge the gap in a seedling a few centimetres high, but never across twenty-five metres. So plants need a transport system, with two separate sets of pipes, the tissues you met in Class 9 (Fig. 5.15). Xylem carries water and dissolved minerals up from the roots. Phloem carries the food made in the leaves to every other part.

A plant's transport can afford to be slow. Plants do not walk or run, and much of their bulk, such as wood, is dead tissue, so they need little energy compared with an animal of the same size. What makes their task hard is distance: the tallest trees are more than a hundred metres high.

Transport of water

Think of the xylem as one long, unbroken set of pipes running from root tip to leaf tip: its tube-like vessels and tracheids are joined end to end through roots, stem and leaves. Water enters at the bottom by a clever use of osmosis. Root cells touching the soil use energy to take in mineral ions, so the root ends up with a higher concentration of ions than the soil around it. Water then moves from the soil into the root to even out the difference. A steady inflow of water into the root xylem results, and it pushes the column of water upwards. This push is called root pressure.

Root pressure alone can lift water only a few metres, nowhere near the top of a tall tree. Plants have a second, much stronger way of moving water up. The next activity hints at it.

Leaves are always losing water vapour through their open stomata. As water evaporates from the leaf cells, they draw in more water from the xylem in the leaf. Because the water in the xylem forms an unbroken column, this pull is passed all the way down the stem to the roots, much like sucking a drink up through a long straw. The loss of water as vapour from the parts of a plant above the ground is called transpiration (वाष्पोत्सर्जन).

Losing water sounds wasteful, but transpiration earns its keep. It is the engine that lifts water, and the minerals dissolved in it, from the roots to the leaves, and the evaporation cools the leaves on a hot day, much as drying sweat cools your skin. Root pressure and transpiration share the lifting according to the time of day. At night, with most stomata closed, root pressure does more of the work. In daylight, with the stomata open, the transpiration pull takes over as the main force.

Transport of food and other substances

Every sweet mango, every potato and every grain of wheat was filled with food that travelled from the leaves through the phloem. Moving the soluble products of photosynthesis around the plant is called translocation (स्थानांतरण). Phloem carries amino acids and other substances as well as sugar. Its conducting cells are sieve tubes, each with companion cells alongside that help it work, and the flow can run upwards or downwards, especially to parts that store food (roots, fruits, seeds) and to parts that are growing.

Unlike the xylem, the phloem needs the plant to spend energy. Loading works in three moves:

1. Sugar, mostly sucrose, is loaded into the sieve tubes using energy from ATP.
2. With so much dissolved sugar inside, the osmotic pressure of the sieve tube rises and water moves in from the neighbouring xylem.
3. The pressure pushes the sugary sap along the phloem towards tissues where the pressure is lower, because sugar is being used or stored there.

Because the plant controls where sugar is loaded and unloaded, it can send food wherever it is needed at the time. When an old potato sprouts in the kitchen, for example, food stored in the tuber is sent into the growing shoots.

5.5 Excretion

Every working cell makes waste. Carbon dioxide from respiration is easy to get rid of, as we have seen: it leaves through the lungs or the stomata. Nitrogen is the real problem. When the body breaks down proteins and amino acids, the nitrogen ends up in ammonia, urea or uric acid, all poisonous if they pile up. Getting rid of harmful wastes made by the body's own chemistry is excretion (उत्सर्जन).

How it is done depends on size. An Amoeba in a pond just lets its wastes diffuse out across its surface into the water. Larger, multicellular animals need organs that do nothing else.

5.5.1 Excretion in Human Beings

Our excretory system has four parts (Fig. 5.16): a pair of bean-shaped kidneys, a pair of thin tubes called ureters, a muscular bag, the urinary bladder, and a single tube to the outside, the urethra. The kidneys sit high up at the back of the abdomen, one on each side of the backbone. Blood carrying wastes enters each kidney through a renal artery and leaves, cleaned, through a renal vein. The kidneys make urine, which runs down the ureters into the bladder, is stored there and finally leaves the body through the urethra.

Making urine is really a way of cleaning blood. Each kidney is packed with about a million microscopic filtering units called nephrons (वृक्काणु). Follow the parts of one nephron (Fig. 5.17):

1. Glomerulus. A branch of the renal artery breaks up into a tight knot of thin-walled capillaries. Blood pressure here squeezes water and small dissolved substances out of the blood.
2. Bowman's capsule. The cup-shaped start of the nephron's tube wraps around the glomerulus and collects this liquid, the filtrate. It contains urea, but also useful glucose, amino acids and salts, and lots of water.
3. Tubule. As the filtrate flows along the long, coiled tube, useful substances are taken back into the capillaries around it: all the glucose and amino acids, many of the salts and most of the water. This is selective reabsorption.
4. Collecting duct. What is left, water with urea and extra salts, is now urine. It drains from many nephrons into collecting ducts and then into the ureter.

The tubules adjust how much water they reclaim. They keep more when the body is short of water and let more go when there is water to spare, also allowing for how much dissolved waste must be flushed out. After a long cricket match on a hot day, little water is spare, so the tubules reclaim almost all of it and you pass a small amount of dark, concentrated urine. After several glasses of nimbu-paani, less is reclaimed and you pass plenty of pale urine. This is how the body keeps its water content steady.

Why can you wait until you reach a toilet? The bladder is a stretchy, muscular bag. As it fills, nerves that sense the stretch signal the brain, and you feel the urge to pass urine. The muscles that let urine out are under the control of the nervous system, so you can usually hold on until it is convenient. Chapter 6 explains how such control works.

5.5.2 Excretion in Plants

A plant has no kidneys, yet it copes with its waste well. Gases are the easy part. Oxygen, a by-product of photosynthesis, and carbon dioxide from respiration diffuse out through the stomata, and spare water leaves by transpiration. For everything else a plant relies on storage rather than removal. It can afford to, because much of its body is dead tissue and it can shed whole parts. Here is where the waste goes:

• Into vacuoles: many wastes are locked away in the large vacuoles of living cells, where they can do no harm.
• Into gums and resins: these collect especially in the old, dead xylem at the centre of a trunk. The edible gum (gond) of the babool tree and the resin of pine trees are examples.
• Out with old parts: wastes pile up in leaves that later fall, and in bark that peels away.
• Into the soil: roots release some wastes into the soil around them.