Chapter 1 — Nutrition – Food Supplying System

All living organisms perform certain basic activities that keep them alive, such as obtaining food, releasing energy from it, moving materials around the body, removing wastes and responding to the surroundings. These activities are together called life processes. The first and most basic of them is nutrition, the process by which an organism takes in food and uses it for energy, growth, repair and the maintenance of its body.

Food is needed for three main reasons. First, it supplies energy: every movement, every heartbeat, every thought uses energy that ultimately comes from the chemical bonds of food. Second, it supplies raw material for growth and for replacing worn-out cells; proteins in particular are the building blocks of protoplasm. Third, certain substances in food, such as vitamins and minerals, are needed in small amounts to regulate body processes and to protect against disease.

Organisms are placed in two great groups according to how they obtain food. Autotrophs (Greek: auto = self, trophos = feeder) make their own food from simple inorganic substances such as carbon dioxide, water and mineral salts. Green plants, algae and some bacteria are autotrophs; almost all of them use sunlight as the energy source, and their mode of nutrition is called photosynthesis. A few bacteria use the energy of chemical reactions instead of light, a process called chemosynthesis. Heterotrophs (hetero = other) cannot make their own food and depend directly or indirectly on autotrophs. All animals, fungi and most bacteria are heterotrophs.

Because autotrophs produce the food on which every other organism depends, they are called producers. Heterotrophs that eat plants are herbivores, those that eat animals are carnivores, and those that eat both are omnivores; together they are the consumers. Organisms that feed on dead and decaying matter, such as fungi and many bacteria, are decomposers, and they return minerals to the soil so that plants can use them again. The flow of food from producers to consumers and decomposers is the basis of every food chain.

In this chapter we study first how plants make food, and then how food is taken in, broken down, absorbed and used in the human body. Along the way we look at what goes wrong when the diet is short of energy, protein or vitamins.

Photosynthesis (photo = light, synthesis = putting together) is the process in which green plants use light energy to build glucose from carbon dioxide and water, releasing oxygen as a by-product. It is the most important chemical process on Earth: it produces the food and the oxygen on which almost all life depends, and it fixes the carbon that ends up in wood, coal and petroleum.

The raw materials are carbon dioxide and water. Carbon dioxide enters the leaf from the air through tiny pores called stomata. Water is absorbed by the root hairs from the soil and carried up through the xylem to the leaf. The energy is light, usually sunlight. The catalyst that captures light is the green pigment chlorophyll, contained in small organelles called chloroplasts, which are most numerous in the mesophyll cells of the leaf.

The overall reaction may be written as:

6CO₂ + 12H₂O → (light, chlorophyll) → C₆H₁₂O₆ + 6O₂ + 6H₂O

A simpler and commonly used form is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The products are glucose and oxygen. The oxygen released comes from the splitting of water, not from carbon dioxide; this was proved by experiments using a heavy isotope of oxygen as a tracer. Glucose is not stored as such; it is quickly converted into starch, which is insoluble and can be stored in the leaf, stem, root or fruit without disturbing the water balance of the cell. This is why the standard test for photosynthesis is a test for starch, not for glucose.

Photosynthesis takes place in three broad steps: (1) absorption of light energy by chlorophyll, (2) conversion of light energy into chemical energy and splitting of water into hydrogen and oxygen, and (3) reduction of carbon dioxide to carbohydrate using that hydrogen. The first two steps need light and are called the light reaction; the third does not need light directly and is called the dark reaction. Both take place inside the chloroplast.

The rate of photosynthesis is affected by external factors such as light intensity, carbon dioxide concentration, temperature and water supply. Increasing any one of these increases the rate up to a point, after which some other factor becomes limiting. A farmer who provides adequate water and a gardener who keeps a greenhouse warm are both exploiting this principle.

The presence of starch in a leaf is taken as evidence that photosynthesis has taken place. So every experiment on photosynthesis ends with the starch test, and you must know its steps and the reason for each.

Destarching. Before any experiment the plant is kept in complete darkness for 48 to 72 hours. In the dark no new starch forms and the existing starch is used up in respiration or moved out of the leaf. A destarched plant is one whose leaves give a negative starch test, so that any starch found later must have been made during the experiment.

Testing a leaf for starch. (1) Pluck the leaf and boil it in water for a minute. This kills the cells and stops all enzyme activity, and it softens the leaf. (2) Boil the leaf in ethyl alcohol (methylated spirit) on a water bath. Alcohol dissolves the chlorophyll and the leaf turns pale white or cream. A water bath is used because alcohol is highly inflammable and must never be heated over a naked flame. (3) Dip the leaf in warm water to soften it, since alcohol makes it brittle. (4) Spread the leaf on a white tile and add a few drops of iodine solution. Parts containing starch turn blue-black; parts without starch remain the brown colour of iodine.

Experiment to show that light is necessary. Take a destarched potted plant. Cover a part of one leaf on both sides with a strip of black paper or aluminium foil, held with clips, so that no light reaches that part. Leave the plant in sunlight for four to six hours. Remove the leaf, take off the paper, and perform the starch test. The uncovered parts turn blue-black, showing starch; the covered part remains pale. Since the covered and uncovered parts belonged to the same leaf and received the same carbon dioxide, water and chlorophyll, the only difference was light. Hence light is necessary for photosynthesis.

A variation is the Ganong's light screen, a clip carrying a metal plate with a cut-out design; after the experiment the design appears in blue-black on the leaf. Another variation keeps one whole plant in a dark cupboard and another in light; the leaf from the dark plant gives no starch.

In examinations you may be asked why the plant is destarched, why alcohol is heated on a water bath, and what would happen if the paper were fixed on one side only (light would enter from the other side, so the whole leaf would show starch). The answer to each comes from the reasons given above.

Experiment to show that chlorophyll is necessary. Some plants, such as croton, coleus and variegated money plant, have variegated leaves in which only some patches are green while the rest are white, yellow or red. The non-green patches lack chlorophyll. Take a destarched variegated plant, draw an outline of a leaf marking the green and non-green regions, and expose the plant to sunlight for four to six hours. Then remove the leaf and perform the starch test. Only the regions that were green turn blue-black; the regions that had no chlorophyll remain pale. All parts of the leaf had the same light, water and carbon dioxide, so the only difference was chlorophyll. Therefore chlorophyll is necessary for photosynthesis.

Experiment to show that carbon dioxide is necessary (the KOH experiment). Take a destarched potted plant. Insert one healthy leaf, still attached to the plant, into a wide-mouthed bottle containing a little potassium hydroxide (KOH) solution, and seal the mouth of the bottle with a split cork and vaseline so that it is airtight. The leaf must not touch the KOH. Potassium hydroxide absorbs carbon dioxide from the air inside the bottle, so this leaf gets light and water but no carbon dioxide. Keep the whole plant in sunlight for four to six hours. Then test both the leaf inside the bottle and a leaf outside the bottle for starch. The outside leaf turns blue-black; the leaf from the bottle does not. Hence carbon dioxide is necessary for photosynthesis.

For a fair test a control is set up: a second bottle containing water or sodium bicarbonate solution instead of KOH, arranged in exactly the same way. Sodium bicarbonate actually releases carbon dioxide, so the leaf in that bottle gives a strong positive result. The control proves that it was the absence of carbon dioxide, and not the bottle or the cork, that stopped photosynthesis.

Historically the study of photosynthesis grew from a series of famous investigations. Van Helmont (1648) grew a willow in a weighed tub of soil for five years and found the tree gained about 74 kg while the soil lost only 57 g; he concluded, wrongly, that the plant was built from water alone, but he showed that soil is not the source of plant material. Joseph Priestley (1770s) showed that a mint plant restored air that a burning candle or a mouse had spoiled, demonstrating that plants release something we now call oxygen. Jan Ingenhousz showed that this happens only in light and only in green parts. Julius von Sachs showed that starch is formed in the illuminated parts of a leaf. The three school experiments above are the modern versions of these discoveries.

Photosynthesis takes place in the chloroplasts, disc-shaped green organelles found in the mesophyll cells of the leaf and in the green cells of young stems. A typical mesophyll cell has 30 to 100 chloroplasts. Each chloroplast is bounded by a double membrane and contains a colourless fluid, the stroma, in which lie stacks of flattened membrane sacs. Each sac is a thylakoid and a stack of them is a granum (plural grana). The grana are connected by membrane tubes called stroma lamellae. Chlorophyll and the other pigments are embedded in the thylakoid membranes, so the light reaction takes place in the grana; the enzymes that fix carbon dioxide float in the stroma, so the dark reaction takes place there.

Chlorophyll is the green pigment that absorbs light. It absorbs mainly the blue-violet and red parts of the spectrum and reflects green, which is why leaves look green. There are two main forms, chlorophyll a and chlorophyll b, and accessory pigments such as carotenoids (orange-yellow) that capture other wavelengths and pass the energy to chlorophyll a. Chlorophyll contains an atom of magnesium at the centre of its molecule, and a plant short of magnesium develops yellow leaves, a condition called chlorosis.

Carbon dioxide enters and oxygen and water vapour leave the leaf through the stomata (singular stoma), tiny pores in the epidermis, most numerous on the lower surface of a dorsiventral leaf. Each stoma is bounded by two bean-shaped guard cells, which are the only epidermal cells to contain chloroplasts. The inner wall of each guard cell (the wall facing the pore) is thick and the outer wall is thin.

Opening and closing of stomata. In light the guard cells make sugar by photosynthesis and take up potassium ions; their cell sap becomes more concentrated, water enters by osmosis, and the guard cells become turgid. Because the thin outer wall stretches more than the thick inner wall, the cells bend outwards and the pore opens. In darkness or when the plant is short of water the guard cells lose water, become flaccid and straighten, and the pore closes. Thus stomata open by day for gas exchange and close at night, and close in dry conditions to reduce water loss by transpiration. A large amount of water, sometimes 90 percent of what the roots absorb, is lost through open stomata, so the plant has to balance its need for carbon dioxide against its need to conserve water.

A leaf is beautifully adapted for photosynthesis: it is thin and flat to expose a large area to light and let gases diffuse quickly; the upper palisade layer packs chloroplasts close to the light; the spongy layer has air spaces for gas movement; the veins bring water and carry away sugar; and the stomata regulate the gases.

Photosynthesis is not a single reaction but a chain of many steps, grouped into two phases. The light reaction (light-dependent phase) requires light and occurs in the grana of the chloroplast. The dark reaction (light-independent phase) does not use light directly and occurs in the stroma. The word dark does not mean it happens only at night; it goes on in daylight too, using the products of the light reaction.

Light reaction. When light falls on a chlorophyll molecule, electrons in it are raised to a higher energy level. This excitation energy is used in two ways. First, it splits water molecules into hydrogen and oxygen, a process called photolysis of water: 2H₂O → 4H⁺ + 4e⁻ + O₂. The oxygen is released into the air through the stomata; this is the oxygen we breathe. Second, the energy is used to make an energy-rich compound, ATP (adenosine triphosphate), from ADP and inorganic phosphate; this is called photophosphorylation. The hydrogen from water is picked up by a carrier molecule called NADP, forming NADPH. So the products of the light reaction are ATP, NADPH and oxygen; the first two carry energy and hydrogen into the next phase.

Dark reaction. In the stroma, carbon dioxide taken in through the stomata is combined with a five-carbon sugar and, using the energy of ATP and the hydrogen of NADPH, is reduced step by step to a three-carbon sugar and finally to glucose. This series of reactions is called the Calvin cycle, after Melvin Calvin who worked it out using radioactive carbon and received the Nobel Prize in 1961. The process of turning carbon dioxide into carbohydrate is called carbon fixation. The glucose formed is used at once in respiration, converted to sucrose for transport, or built into starch for storage; it is also the starting material for cellulose, fats and, with nitrogen from the soil, proteins.

The two phases are linked: the light reaction produces ATP and NADPH, and the dark reaction consumes them and returns ADP and NADP. Without light there is soon no ATP or NADPH and the dark reaction stops; without carbon dioxide the ATP and NADPH pile up and the light reaction slows. This is why both light and carbon dioxide were found necessary in the experiments.

The energy stored in glucose is released again in respiration, when the glucose is oxidised back to carbon dioxide and water. Photosynthesis and respiration are therefore reverse processes in their overall chemistry, one storing solar energy and the other releasing it, and together they keep the carbon and oxygen of the atmosphere in balance.

Organisms that cannot make their own food are heterotrophs. They must obtain ready-made organic food, directly or indirectly, from autotrophs. According to how the food is obtained and taken in, heterotrophic nutrition is of three main kinds.

Saprophytic nutrition (sapros = rotten). Saprophytes feed on dead and decaying organic matter such as fallen leaves, dead animals, dung, rotting fruit and bread. They do not take solid food inside; instead they secrete digestive enzymes on to the food outside their body, the food is digested externally into simple soluble substances, and these are absorbed through the body surface. Fungi such as bread mould (Rhizopus), mushrooms and yeast, and many bacteria, are saprophytes. They are the decomposers of nature; without them dead matter would pile up and the minerals locked in it would never return to the soil.

Parasitic nutrition. A parasite lives on or inside the body of another living organism, the host, and takes its food from the host, usually causing it harm. Parasites that live on the outside of the host are ectoparasites (lice, ticks, leeches, bed bugs); those living inside are endoparasites (tapeworm, roundworm, Plasmodium which causes malaria). Among plants, Cuscuta (dodder, amarbel) is a leafless yellow twining stem parasite that sends sucking roots called haustoria into the host to draw ready-made food; it has no chlorophyll and is a total parasite. Mistletoe is a partial parasite, green but taking water and minerals from the host tree. Some fungi such as rust and smut are parasites of crops.

Holozoic nutrition (holo = whole, zoon = animal). This is the mode of most animals, including humans: solid or liquid food is taken into the body as such and is then digested inside. Holozoic nutrition has five steps. Ingestion is the taking in of food. Digestion is the breaking down of complex insoluble food into simple soluble molecules, partly by mechanical action (chewing, churning) and mainly by enzymes. Absorption is the passage of the digested food through the wall of the gut into blood or body fluid. Assimilation is the use of absorbed food by the cells for energy, growth and repair. Egestion is the removal of undigested food from the body. Amoeba, Hydra, earthworm, frog, birds and mammals all show holozoic nutrition, though the organs differ enormously in complexity.

A few organisms combine modes. Euglena has chlorophyll and photosynthesises in light but absorbs organic food in darkness. Insectivorous plants such as pitcher plant, Venus fly-trap and bladderwort photosynthesise normally but trap and digest insects to obtain nitrogen, since they grow in nitrogen-poor soils. Lichens are a partnership (symbiosis) of an alga that photosynthesises and a fungus that provides water, minerals and shelter.

Amoeba is a microscopic single-celled animal found in fresh-water ponds and on the mud at the bottom. It has no fixed shape; its body is a mass of cytoplasm with a nucleus, a contractile vacuole and several food vacuoles, bounded by a thin plasma membrane. It moves by pushing out finger-like projections of the cytoplasm called pseudopodia (false feet), and it uses the same pseudopodia to capture food. Amoeba shows holozoic nutrition, and because everything happens inside one cell it is the simplest system in which the five steps can be seen.

Ingestion. Amoeba feeds on tiny algae, bacteria, diatoms and other protozoa. When a food particle touches or comes near the surface, the cytoplasm flows out on either side of the particle as two pseudopodia. The pseudopodia surround the particle and their tips fuse, enclosing it together with a drop of water in a food vacuole. This engulfing of solid food by the cell is called phagocytosis, and the cup formed by the pseudopodia during capture is the food cup. Amoeba has no mouth; ingestion can take place at any point on the surface.

Digestion. Digestive enzymes from the cytoplasm are secreted into the food vacuole. The contents first become acidic, which kills the prey, then alkaline, and the enzymes break the proteins, carbohydrates and fats of the food into amino acids, simple sugars and fatty acids. Digestion is thus intracellular, within the cell, unlike the extracellular digestion in the gut of higher animals. The food vacuole moves around in the streaming cytoplasm and gradually becomes smaller as digestion proceeds.

Absorption. The digested soluble food diffuses out of the food vacuole into the surrounding cytoplasm. Since the cell is tiny, no transport system is required; diffusion reaches every part.

Assimilation. The absorbed molecules are used to build new cytoplasm, so the Amoeba grows, and are oxidised in respiration to release energy for movement and other activities. Excess food may be stored as glycogen and fat droplets.

Egestion. The undigested remains in the food vacuole are thrown out of the cell. The vacuole moves to the surface, the membrane ruptures and the waste is expelled. There is no fixed anus; egestion may occur at any point, though it usually occurs at the trailing end as the animal moves forward.

Amoeba illustrates that even a single cell can carry out all the steps of nutrition. Multicellular animals divide the work among specialised organs, but the chemistry of digestion, breaking large molecules into small ones with enzymes, is the same.

The human digestive system consists of the alimentary canal, a muscular tube about 9 metres long running from the mouth to the anus, and the digestive glands (salivary glands, liver and pancreas) that pour their secretions into it. The parts of the canal in order are the mouth (buccal cavity), pharynx, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum, colon, rectum) and anus.

Mouth. Food is ingested through the mouth. The teeth cut, tear and grind it into small pieces, a mechanical digestion that increases the surface area for enzymes. Humans have 32 permanent teeth in four kinds: 8 incisors for cutting, 4 canines for tearing, 8 premolars and 12 molars for grinding. The tongue tastes the food, mixes it with saliva and rolls it into a soft ball called a bolus for swallowing.

Saliva. Three pairs of salivary glands (parotid below the ears, submandibular below the jaw, sublingual below the tongue) secrete about 1 to 1.5 litres of saliva a day. Saliva is 99 percent water with mucus and the enzyme salivary amylase (ptyalin). The water moistens the food, the mucus lubricates it, and the amylase begins the digestion of cooked starch, converting some of it into the sugar maltose. Saliva works best at the slightly acidic to neutral pH of the mouth, and its enzyme is inactivated by the acid in the stomach. The sweet taste that develops when a piece of chapati is chewed for a long time is due to this conversion of starch to sugar.

Pharynx and swallowing. The bolus is pushed by the tongue into the pharynx, which is the common passage for food and air. During swallowing a flap of cartilage called the epiglottis closes the opening of the windpipe (glottis) so that food goes into the oesophagus and not into the lungs. Talking or laughing while eating can interfere with this and cause choking.

Oesophagus (food pipe). This is a 25 cm muscular tube with no digestive function; it carries the bolus to the stomach. Its wall has two layers of smooth muscle, circular and longitudinal. The muscles behind the bolus contract and those ahead relax, producing a wave that pushes the food along. This rhythmic wave of contraction and relaxation is called peristalsis, and it occurs throughout the alimentary canal. Peristalsis is why food reaches the stomach even if you eat lying down or standing on your head.

At the lower end of the oesophagus a ring of muscle, the cardiac sphincter, relaxes to let the bolus enter the stomach and then closes to prevent the acidic contents from flowing back. When it fails to close properly the acid irritates the oesophagus, causing the burning sensation called heartburn.

The stomach is a J-shaped, thick-walled muscular bag lying in the upper left of the abdomen, just below the diaphragm. It has three parts: the cardiac part near the entrance, the fundus and body in the middle, and the pyloric part near the exit into the duodenum. The exit is guarded by the pyloric sphincter, a ring of muscle that opens from time to time to release small amounts of partly digested food into the small intestine. An empty stomach is about the size of a fist, but it can stretch to hold 1.5 to 2 litres after a meal.

The inner lining of the stomach is thrown into folds and contains millions of tiny gastric glands. They secrete gastric juice, about 2 to 3 litres a day, which has three main components. (1) Hydrochloric acid (HCl) makes the contents strongly acidic, about pH 1.5 to 2. The acid kills most bacteria that enter with the food, stops the action of salivary amylase, softens the fibrous parts of the food and, most importantly, converts the inactive enzyme pepsinogen into the active enzyme pepsin. (2) Pepsin is the protein-digesting enzyme of the stomach. It works only in an acidic medium and breaks long protein chains into shorter chains called peptones and proteoses. In infants the stomach also secretes rennin, which curdles milk so that its protein casein stays in the stomach long enough to be digested. (3) Mucus, secreted by the goblet cells of the lining, forms a slippery protective coat over the wall so that the acid and pepsin do not digest the stomach itself. When the mucus layer is damaged, by infection with the bacterium Helicobacter pylori, by stress, by alcohol or by certain pain-killers, the acid attacks the wall and produces a peptic ulcer.

The muscular wall of the stomach has three layers of muscle running in different directions. Their contractions churn the food and mix it thoroughly with the gastric juice. After three to four hours the food is converted into a semi-liquid, acidic paste called chyme. The pyloric sphincter then releases the chyme in small squirts into the duodenum, so that the small intestine is not overwhelmed and has time to neutralise the acid.

Note what the stomach does not do: it does not digest carbohydrates (the acid stops amylase) or fats (there is no fat-digesting enzyme of importance), and it absorbs very little except water, some salts, alcohol and certain drugs such as aspirin. Its main jobs are to store food, to begin protein digestion and to sterilise the meal.

The secretion of gastric juice is controlled both by nerves and by a hormone called gastrin, released by the stomach wall when food arrives. The sight, smell and even thought of food start the flow, which is why the mouth waters and the stomach rumbles at meal time.

The small intestine is the longest part of the alimentary canal, about 6 to 7 metres long and 2.5 cm wide, coiled in the abdomen. It is called small because of its narrow diameter. Its first 25 cm is the C-shaped duodenum, followed by the jejunum and the ileum. Most digestion and almost all absorption take place here. Three digestive juices act in the small intestine: bile from the liver, pancreatic juice from the pancreas, and intestinal juice from the intestinal wall itself. Together they make the medium alkaline, which is needed for their enzymes to work.

Liver and bile. The liver is the largest gland of the body, weighing about 1.5 kg, lying in the upper right of the abdomen. Its cells secrete bile, a yellow-green alkaline fluid stored and concentrated in the gall bladder and released into the duodenum through the bile duct when fatty food arrives. Bile contains no enzymes. It contains bile salts, which break large drops of fat into tiny droplets, a process called emulsification that gives fat-digesting enzymes a much larger surface to act on; and it contains bile pigments (bilirubin and biliverdin), waste products from the breakdown of old red blood cells, which give faeces their colour. The alkali in bile also neutralises the acidic chyme. Besides making bile, the liver stores glucose as glycogen, makes blood proteins, removes poisons and converts ammonia into urea.

Pancreas and pancreatic juice. The pancreas is a leaf-shaped gland lying below the stomach in the curve of the duodenum. It has two functions: as an endocrine gland it secretes the hormones insulin and glucagon into the blood, and as a digestive gland it pours pancreatic juice into the duodenum through the pancreatic duct. Pancreatic juice is alkaline (it contains sodium bicarbonate) and carries three enzymes that act on all three classes of food: trypsin breaks proteins and peptones into peptides; pancreatic amylase breaks starch into maltose; and lipase breaks emulsified fats into fatty acids and glycerol. Trypsin is secreted as the inactive trypsinogen and activated in the intestine, which protects the pancreas from digesting itself.

Intestinal juice (succus entericus). Glands in the wall of the small intestine secrete an alkaline juice that completes digestion. It contains peptidases (erepsin) that break peptides into amino acids; maltase, sucrase and lactase that break maltose, sucrose and lactose into simple sugars (glucose, fructose, galactose); and intestinal lipase that completes the digestion of fats. The end products of digestion, ready for absorption, are therefore glucose and other monosaccharides, amino acids, and fatty acids and glycerol.

Food is moved along by peristalsis and mixed by segmenting movements. It stays in the small intestine for about four to eight hours, during which the finely digested, milky fluid, now called chyle, is absorbed.

Absorption is the passage of the products of digestion through the wall of the small intestine into the blood and lymph. The small intestine is superbly adapted for it. It is very long, giving time for absorption. Its inner lining is folded, and the folds carry millions of tiny finger-like projections called villi (singular villus), each about 1 mm long; each villus in turn has cells whose surfaces bear microscopic microvilli. Together these increase the internal surface area to about 250 square metres, the area of a tennis court. Each villus has a thin wall only one cell thick, a rich network of blood capillaries, and a central lymph vessel called a lacteal. The villi also keep moving, which brings them into contact with fresh chyle.

Glucose, other simple sugars, amino acids, minerals and water-soluble vitamins pass into the blood capillaries of the villi and are carried by the hepatic portal vein to the liver before going to the rest of the body. The liver stores excess glucose as glycogen, deals with any harmful substances, and regulates the level of amino acids. Fatty acids and glycerol are recombined into tiny fat globules inside the cells of the villi and pass into the lacteals; the lymph carries them to the blood stream near the heart. This is why the lymph draining the intestine after a meal looks milky.

Assimilation is what the cells do with the absorbed food. Glucose is oxidised in respiration to release energy, or stored as glycogen in the liver and muscles, or converted into fat. Amino acids are built into the proteins of new protoplasm, enzymes and hormones; those in excess are broken down in the liver, the nitrogen part becoming urea. Fats provide energy and are stored under the skin and around organs. Nothing is absorbed by the body until it is assimilated; a person can eat well yet be undernourished if assimilation is poor.

Large intestine. The undigested residue, mostly cellulose fibre, water and bacteria, passes through the ileo-caecal valve into the large intestine, about 1.5 metres long and wider than the small intestine. Its parts are the caecum with the finger-like appendix, the ascending, transverse and descending colon, and the rectum. The large intestine has no villi and secretes no enzymes; its work is to absorb water and some salts from the residue, converting it into semi-solid faeces, and to house helpful bacteria that make vitamin K and some B vitamins. Faeces are stored in the rectum and expelled through the anus, whose sphincter is under voluntary control; this removal of undigested food is egestion. It is not excretion, because the material was never part of the body's cells.

If the residue moves too fast, too little water is absorbed and diarrhoea results, which can cause dangerous dehydration in children; oral rehydration solution (ORS) of salt and sugar replaces the lost water and salts. If it moves too slowly, too much water is absorbed and constipation results; a diet with plenty of fibre and water, and regular exercise, prevents it.

The digestive system works best when it is given the right food in the right way. A balanced diet contains carbohydrates, fats, proteins, vitamins, minerals, water and roughage in the proportions the body needs. Of these, roughage and water are the two most directly connected with the health of the alimentary canal itself.

Roughage or dietary fibre is the indigestible part of plant food, mainly cellulose from the cell walls of whole grains, pulses, vegetables and fruit, and the bran of unpolished rice and whole wheat. Humans have no enzyme to digest cellulose, so it passes through unchanged. Yet it is essential. It adds bulk to the food, which stimulates peristalsis and moves the contents smoothly along; it holds water, keeping the faeces soft; it helps remove cholesterol and slows the absorption of sugar. A diet low in fibre, made of polished rice, refined flour and fried snacks, leads to constipation, piles and, over the years, a greater risk of cancer of the colon. Simply eating more vegetables, fruit with skin, whole grains and sprouted pulses solves most of this.

Water is needed for the secretion of about 8 litres of digestive juices every day, for the movement of food, and for the absorption of nutrients. Most of this water is reabsorbed in the large intestine, but two to three litres must still be drunk daily, more in hot weather.

Good habits. Chewing food well breaks it into small pieces, mixes it with saliva and starts starch digestion; food swallowed in lumps is digested poorly. Meals at regular times train the secretion of juices. Overeating stretches the stomach and delays digestion; very spicy, fried or very hot food irritates the lining. Washing hands before eating and drinking clean water keep out the bacteria and worms that cause diarrhoea, typhoid and intestinal worms. Hookworm and roundworm infections, common where sanitation is poor, rob the body of food and blood; regular deworming of children is part of the national school health programme for this reason. Smoking and alcohol damage the lining of the stomach and the liver.

Common disorders. Indigestion (dyspepsia) is discomfort after eating, often from overeating or eating too fast. Acidity or heartburn is acid entering the oesophagus; antacids such as milk of magnesia neutralise the excess acid. Peptic ulcer is a sore in the lining of the stomach or duodenum. Jaundice is yellowing of the skin and eyes when bile pigment accumulates in the blood because the liver is damaged, usually by the hepatitis virus spread through contaminated water. Appendicitis is inflammation of the appendix. Gallstones form when bile in the gall bladder crystallises. Most of these can be prevented by clean food, a fibre-rich diet and sensible habits, and the rest respond to timely treatment.

Malnutrition is the condition that results when the diet does not provide the right amount of nutrients: too little of some, or too much of others. It includes both undernutrition and overnutrition, and it is one of the largest health problems in India, especially among children under five.

Protein-energy malnutrition (PEM) is the most serious form. It appears in two main patterns. Kwashiorkor occurs in children between one and three years of age whose diet has enough calories, mostly from cereals or starchy roots, but too little protein. Typically it strikes when a child is weaned off breast milk on to a thin cereal gruel. The child has a swollen belly and swollen legs and face due to accumulation of fluid (oedema), a moon-like face, thin limbs, sparse and discoloured hair, patchy skin, poor appetite and retarded growth; the child is irritable and listless. The name comes from a West African word meaning the disease of the child displaced by the next baby. Marasmus occurs in infants below one year whose diet is short of both protein and calories, that is, of food altogether. The body uses its own fat and muscle; the child is extremely thin with wrinkled, loose skin, prominent ribs, an old-looking face and sunken eyes, but there is no oedema. Both conditions are treated by a diet rich in energy and good-quality protein such as milk, eggs, pulses and groundnuts, given gradually, together with treatment of any infection.

Obesity is overnutrition: the intake of energy is more than the body uses, and the excess is stored as fat. It results from a diet rich in fried food, sweets and soft drinks combined with lack of physical activity. Obesity increases the risk of diabetes, high blood pressure, heart disease and joint problems. A person is considered overweight when the body mass index, weight in kilograms divided by the square of the height in metres, exceeds 25, and obese above 30. Regular exercise and a diet with more vegetables and fewer refined foods are the remedy.

Vitamin deficiency diseases. Vitamins are needed in tiny amounts but their absence causes specific diseases. Vitamin A (in carrot, papaya, green leafy vegetables, milk, liver): night blindness and xerophthalmia, which can end in permanent blindness. Vitamin B₁ thiamine (whole grains, pulses): beriberi, with weak muscles and nerve damage. Vitamin B₂ riboflavin: cracks at the corners of the mouth. Vitamin B₃ niacin: pellagra, with dermatitis and diarrhoea. Vitamin B₁₂: pernicious anaemia. Vitamin C (citrus fruit, amla, tomato): scurvy, with bleeding gums and slow healing of wounds. Vitamin D (sunlight on skin, fish oil, milk): rickets in children, with bow legs and deformed bones, and osteomalacia in adults. Vitamin K (green vegetables): delayed clotting of blood.

Mineral deficiencies are also common: lack of iron causes anaemia, lack of iodine causes goitre, and lack of calcium weakens bones and teeth. Government programmes such as iodised salt, mid-day meals, vitamin A drops and iron tablets for adolescent girls exist to fight these deficiencies, and a varied diet of locally available foods prevents almost all of them.