Chapter 2 — Respiration – The Energy Releasing System

Every activity of a living cell, from building a protein to contracting a muscle, needs energy. The food made by photosynthesis or eaten as a meal contains that energy in its chemical bonds, but a cell cannot use the bonds directly. The energy has to be released in small, controlled steps and packed into a molecule the cell can spend, ATP (adenosine triphosphate). The process that does this is respiration. In most organisms it involves the oxidation of glucose by oxygen with the release of carbon dioxide and water, so respiration is often described as the reverse of photosynthesis.

Students frequently mix up two words. Breathing is the physical process of taking air rich in oxygen into the lungs and pushing air rich in carbon dioxide out. It involves muscles, ribs and the diaphragm, it is mechanical, and it produces no energy; it actually uses a little. Cellular respiration is the chemical process inside every living cell in which food is oxidised step by step and its energy is trapped in ATP. It involves enzymes, it happens in the cytoplasm and mitochondria, and it releases energy. Breathing supplies the oxygen that cellular respiration needs and removes the carbon dioxide it produces, so the two are linked, but they are not the same. Plants respire in every living cell but do not breathe in the sense of moving air with muscles; they rely on diffusion through stomata and lenticels.

The overall equation of aerobic respiration is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (about 2900 kJ per mole of glucose, of which roughly 38 ATP are formed)

Respiration differs from burning, even though the equation looks the same. Burning releases all the energy at once as heat and light; respiration releases it in about twenty small enzyme-controlled steps at body temperature, and captures a large part of it in ATP instead of losing it all as heat. Some heat is still released, which is why active animals are warm and why a heap of germinating seeds becomes warm.

Respiration in a multicellular animal can be thought of in three stages: external respiration, the exchange of gases between the environment and the blood at the respiratory surface; transport of gases by the blood; and internal or cellular respiration, the exchange of gases between blood and cells and the oxidation of food in the cells. This chapter takes them in that order.

The human respiratory system consists of a series of air passages leading to two lungs, together with the rib cage and diaphragm that ventilate them. Air passes in order through the nostrils, nasal cavity, pharynx, larynx, trachea, bronchi and bronchioles, and finally reaches the alveoli where gas exchange occurs.

Nostrils and nasal cavity. Air enters through the two nostrils into the nasal cavity, which is divided by a septum and lined with a moist mucous membrane bearing fine hairs and ciliated cells. The hairs filter dust, the mucus traps dust and germs, the cilia sweep the mucus towards the throat, and the rich blood supply warms and moistens the air. Breathing through the nose is therefore healthier than breathing through the mouth. The nasal cavity also has the olfactory cells for smell.

Pharynx. The nasal cavity opens at the back into the pharynx, the throat, which is a common passage for both food and air. From the pharynx, food goes down the oesophagus behind and air goes down the larynx in front.

Larynx. The larynx, or voice box, is a box of cartilage at the top of the windpipe; its front can be felt as the Adam's apple. It contains two folds, the vocal cords, which vibrate when air passes over them and produce the voice; the pitch is changed by muscles that tighten or loosen the cords. The opening of the larynx, the glottis, is covered during swallowing by a leaf-like flap of cartilage, the epiglottis, so that food does not enter the windpipe.

Trachea. The windpipe is a tube about 12 cm long and 2 cm wide, lying in front of the oesophagus. Its wall is supported by 16 to 20 C-shaped rings of cartilage, incomplete at the back, which keep it open at all times so that it does not collapse when the pressure inside falls during inspiration; the gap at the back allows the oesophagus to expand when a bolus passes. The trachea is lined by ciliated epithelium with mucus-secreting goblet cells; the mucus traps particles and the cilia beat upward, moving it to the throat where it is swallowed or coughed out.

Bronchi and bronchioles. At its lower end the trachea divides into two bronchi (singular bronchus), one entering each lung. Inside the lung each bronchus divides again and again into smaller and smaller tubes, the bronchioles, like the branches of a tree, until the finest bronchioles are less than 1 mm wide. The larger branches have cartilage plates; the smallest have only smooth muscle, which can contract to narrow them, as happens in asthma. Each terminal bronchiole ends in a cluster of tiny air sacs, the alveoli, which are the working surface of the lung.

The whole system of passages from the nose to the bronchioles is called the conducting zone; no gas exchange takes place in it, so the air in it at the end of inspiration, about 150 mL, is called dead space air.

The two lungs are soft, spongy, pink, cone-shaped organs filling most of the chest cavity (thorax), one on each side of the heart. The right lung has three lobes and the left has two, leaving room for the heart. Each lung is enclosed in a double membrane, the pleura; the thin film of pleural fluid between the two layers reduces friction as the lungs expand and contract, and the membranes keep the lungs stuck to the chest wall so that when the chest expands the lungs must expand too. Infection of the pleura is pleurisy.

Inside, each lung is a mass of branching bronchioles ending in about 300 million alveoli (singular alveolus), grape-like air sacs about 0.2 mm across. The alveoli give the lungs their spongy texture and provide an enormous surface for gas exchange, roughly 70 to 100 square metres in an adult, about forty times the area of the skin. Each alveolus is surrounded by a dense net of blood capillaries carried by the pulmonary artery.

A good respiratory surface has four features, and the alveoli show all of them. (1) It is thin: the wall of an alveolus and the wall of the capillary pressed against it are each a single flattened cell, so gases diffuse across less than one thousandth of a millimetre. (2) It is moist: the alveolar wall is coated with a film of fluid in which oxygen dissolves before it diffuses; the fluid contains a surfactant that prevents the alveoli collapsing. (3) It has a large surface area, achieved by the vast number of tiny sacs. (4) It has a rich blood supply so that oxygen is carried away as fast as it enters and the concentration gradient is maintained. The same four features are found in the gills of a fish, the skin of a frog and the tracheoles of an insect.

The lungs have no muscle of their own and cannot expand or contract by themselves. They are stretched and released by changes in the volume of the chest cavity, produced by the diaphragm, a dome-shaped sheet of muscle separating the thorax from the abdomen, and by the intercostal muscles between the ribs. This is described in the next topic.

Lung capacity gives a useful set of numbers. At rest a person takes in and gives out about 500 mL of air with each breath, the tidal volume. By breathing in as hard as possible after a normal inspiration, a further 2500 to 3000 mL can be drawn in (inspiratory reserve), and by breathing out as hard as possible about 1000 to 1100 mL more can be forced out (expiratory reserve). The largest volume that can be exhaled after the deepest inspiration, about 4500 mL in a healthy adult male, is the vital capacity; it is larger in athletes and smaller in smokers and in those with lung disease. About 1200 mL, the residual volume, always remains in the lungs and can never be breathed out, which is why the lungs of a person who has once breathed will float in water.

Breathing is a pair of movements, inspiration (inhalation, taking air in) and expiration (exhalation, letting air out), repeated 12 to 16 times a minute in a resting adult, about 20 to 30 times in a child and up to 60 times in a newborn. It depends on a simple physical principle: air flows from a region of higher pressure to a region of lower pressure. The chest cavity is a closed box; if its volume increases the pressure inside falls below atmospheric pressure and air rushes in; if its volume decreases the pressure rises and air is pushed out. The volume is changed by two sets of muscles.

Inspiration is an active process. (1) The diaphragm, which at rest is domed upward into the chest, contracts and flattens, moving downward and pushing the abdominal organs down and out; this increases the volume of the chest from top to bottom. (2) At the same time the external intercostal muscles between the ribs contract and pull the rib cage upward and outward, like a bucket handle being lifted; this increases the volume from front to back and side to side. The chest cavity enlarges, the lungs, stuck to the chest wall through the pleura, are pulled open, the pressure inside the alveoli drops about 1 to 3 mm of mercury below atmospheric, and air flows in through the nose and air passages until the pressures are equal.

Expiration at rest is largely passive. The diaphragm relaxes and springs back into its dome, the external intercostals relax and the ribs fall back by their own weight, and the stretched elastic tissue of the lungs recoils. The volume of the chest decreases, the pressure in the alveoli rises above atmospheric, and air is pushed out. During forced expiration, as in blowing out a lamp or during exercise, the internal intercostal muscles and the abdominal muscles contract to squeeze the chest further.

The mechanism is usually demonstrated with a bell-jar model: a bell jar with a Y-tube through its cork carrying two balloons, and a rubber sheet tied across the open bottom. The bell jar is the chest wall, the Y-tube the trachea and bronchi, the balloons the lungs and the rubber sheet the diaphragm. Pulling the sheet down increases the volume inside, lowers the pressure and inflates the balloons; releasing it deflates them. The model does not show the movement of the ribs, which is its main limitation.

Breathing is controlled automatically by the respiratory centre in the medulla oblongata of the brain. The centre is sensitive to the concentration of carbon dioxide in the blood: during exercise the muscles produce more carbon dioxide, the blood becomes slightly more acidic, and the centre responds by increasing the rate and depth of breathing. This is why you pant after running. Breathing can be controlled voluntarily for a short time, as in speaking, singing or holding the breath, but the rising carbon dioxide soon forces a breath. Yawning, sneezing, coughing and hiccups are reflex modifications of breathing.

Gas exchange takes place by diffusion, the movement of a substance from where it is more concentrated to where it is less concentrated. No energy is spent by the body on it; the body only maintains the concentration differences by breathing and by circulating the blood. Exchange happens at two places, the alveoli and the tissues, and in opposite directions.

At the alveoli (external respiration). Air in the alveoli, freshly renewed by inspiration, has a high concentration of oxygen (about 14 percent, partial pressure about 100 mm Hg) and a low concentration of carbon dioxide (partial pressure about 40 mm Hg). The blood arriving in the alveolar capillaries through the pulmonary artery has come from the body tissues; it is deoxygenated blood, poor in oxygen (about 40 mm Hg) and rich in carbon dioxide (about 46 mm Hg). Oxygen therefore dissolves in the film of moisture on the alveolar wall and diffuses across the two thin cell layers into the blood, where it binds with haemoglobin in the red cells. Carbon dioxide diffuses the other way, from blood into the alveolar air, from where it is expired. In less than a second the blood is converted from deoxygenated to oxygenated and leaves the lungs by the pulmonary veins for the heart.

At the tissues (internal respiration). The oxygenated blood is pumped by the heart through the arteries to the capillaries of every organ. The cells of the tissues are constantly using oxygen in respiration and producing carbon dioxide, so around them the oxygen concentration is low and the carbon dioxide concentration is high. Haemoglobin releases its oxygen, which diffuses through the capillary wall and the tissue fluid into the cells; carbon dioxide diffuses out of the cells into the blood. The blood, now deoxygenated, returns through the veins to the heart and then to the lungs, and the cycle repeats.

The differences that drive diffusion at both sites are summarised as: in the lungs, oxygen moves from alveolus to blood and carbon dioxide from blood to alveolus; in the tissues, oxygen moves from blood to cell and carbon dioxide from cell to blood. Each exchange depends on the previous one: the lungs keep the arriving blood low in oxygen, and the tissues keep it high in carbon dioxide.

The efficiency of exchange depends on the steepness of the concentration gradient and the thinness of the barrier. Anything that thickens the alveolar wall (fluid in pneumonia, scarring in fibrosis), destroys alveoli (emphysema from smoking) or reduces the oxygen in the air (high altitude, a closed room with a charcoal fire) reduces the oxygen reaching the blood and produces breathlessness. At high altitude the body compensates over weeks by making more red blood cells, which is why climbers acclimatise gradually and why people living in mountain regions have a higher red-cell count.

Oxygen dissolves only poorly in water: a litre of plasma at body temperature holds about 3 mL of oxygen, far too little to keep the body alive. The problem is solved by haemoglobin (Hb), the red iron-containing protein packed into the red blood cells. Each haemoglobin molecule has four iron atoms and each iron can bind one oxygen molecule, so a fully loaded molecule carries four O₂. A litre of blood holds about 150 g of haemoglobin and can carry nearly 200 mL of oxygen, seventy times what plasma alone could.

In the lungs, where oxygen is plentiful, haemoglobin combines with oxygen to form bright red oxyhaemoglobin. In the tissues, where oxygen is scarce and carbon dioxide and acidity are higher, oxyhaemoglobin breaks down and releases its oxygen, leaving purplish-red deoxygenated haemoglobin. The reaction is easily reversible, which is exactly what a carrier needs: it loads where oxygen is high and unloads where oxygen is low.

Hb + 4O₂ ⇌ Hb(O₂)₄ (forward in the lungs, reverse in the tissues)

A person with too little haemoglobin, from iron deficiency or blood loss, has anaemia: tissues receive less oxygen, the person tires easily and looks pale. Carbon monoxide from incomplete burning of fuel, in vehicle exhaust or a charcoal fire in a closed room, binds haemoglobin about 250 times more strongly than oxygen and does not let go; the blood can then carry no oxygen and the person dies in sleep without any warning. This is why charcoal angithis must never be burnt in a closed room.

Carbon dioxide is carried in three ways. About 7 percent dissolves directly in the plasma. About 23 percent combines loosely with the amino groups of haemoglobin to form carbaminohaemoglobin. The largest share, about 70 percent, travels as bicarbonate ions. Inside the red cells the enzyme carbonic anhydrase combines carbon dioxide with water very rapidly to form carbonic acid, which splits into hydrogen ions and bicarbonate ions; the bicarbonate diffuses out into the plasma and is carried to the lungs. There the reactions reverse: bicarbonate re-enters the red cells, reforms carbonic acid and then carbon dioxide, which diffuses into the alveoli and is breathed out.

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

The bicarbonate system also acts as the main buffer of the blood, keeping its pH close to 7.4. If carbon dioxide accumulates, as in shallow breathing, the blood becomes more acidic; if it is blown off too fast, as in hyperventilation, the blood becomes alkaline and the person feels dizzy and tingly. The respiratory centre monitors this acidity and adjusts breathing to hold it steady.

The oxygen delivered by the blood is used inside the cells to oxidise food, chiefly glucose, and release energy. This is cellular respiration, a sequence of about twenty enzyme-controlled reactions that together achieve what the single equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy summarises. It is convenient to divide the sequence into three stages: glycolysis, the Krebs cycle and the electron transport chain. The first stage happens in the cytoplasm and needs no oxygen; the second and third happen inside the mitochondria and need oxygen.

Glycolysis (glyco = sugar, lysis = splitting) is the breakdown of one molecule of glucose, a six-carbon sugar, into two molecules of pyruvic acid (pyruvate), each with three carbons. It takes place in the cytoplasm of every cell, whether animal, plant, fungus or bacterium, and it is common to both aerobic and anaerobic respiration. Because it is so widespread it is thought to be the oldest energy-releasing pathway, one that evolved before there was oxygen in the atmosphere.

Glycolysis has ten steps, each catalysed by its own enzyme, but the pattern is simple. In the first steps the cell invests energy: two molecules of ATP are spent to add phosphate groups to glucose, making it reactive. The phosphorylated six-carbon sugar is then split into two three-carbon pieces. In the later steps each three-carbon piece is oxidised, hydrogen is removed and handed to the carrier NAD (forming NADH), and enough energy is released to make two ATP from each piece, four in all. Since two ATP were spent and four were made, the net gain of glycolysis is two ATP per glucose, plus two NADH that carry hydrogen to the mitochondria.

Glucose (6C) + 2ATP + 2NAD → 2 pyruvic acid (3C) + 4ATP + 2NADH

Glycolysis releases only about 5 percent of the energy of glucose; most of the energy still remains in the two pyruvic acid molecules. What happens next depends on whether oxygen is available. If it is, pyruvic acid enters the mitochondria and is completely oxidised by the Krebs cycle and the electron transport chain, yielding a further 34 to 36 ATP. If oxygen is absent, pyruvic acid stays in the cytoplasm and is converted by fermentation into lactic acid or into ethanol and carbon dioxide, with no further ATP. Glycolysis is therefore the common starting point, and the fate of pyruvic acid is the branching point between aerobic and anaerobic respiration.

The energy of respiration is not released as a burst but is stored in the phosphate bonds of ATP. ATP is often called the energy currency of the cell: it is made in respiration and spent wherever energy is needed, in muscle contraction, active transport, protein synthesis, cell division and the transmission of nerve impulses. When a cell spends ATP it breaks off the terminal phosphate, releasing about 30.5 kJ per mole and leaving ADP, which is recharged to ATP in respiration.

When oxygen is available, the two pyruvic acid molecules formed by glycolysis pass from the cytoplasm into a mitochondrion. Mitochondria are rod-shaped organelles about 1 to 2 micrometres long, bounded by two membranes; the inner membrane is folded into shelves called cristae, which enclose a fluid matrix. The matrix holds the enzymes of the Krebs cycle and the cristae hold the electron transport chain, so a cell that needs a great deal of energy, such as a muscle fibre or a liver cell, has thousands of mitochondria with many cristae. Because they make most of the cell's ATP, mitochondria are called the powerhouses of the cell.

Entry of pyruvic acid. In the matrix each three-carbon pyruvic acid loses one carbon as carbon dioxide and the remaining two-carbon acetyl group is attached to a carrier, coenzyme A, forming acetyl-CoA. Hydrogen released here is taken up by NAD.

Krebs cycle (citric acid cycle, named after Hans Krebs who worked it out in 1937). The two-carbon acetyl group combines with a four-carbon compound (oxaloacetic acid) to form six-carbon citric acid. In a series of eight reactions the citric acid is progressively oxidised: two carbons are removed as two molecules of carbon dioxide, hydrogen atoms are stripped off and handed to the carriers NAD and FAD, one ATP is formed directly, and the four-carbon compound is regenerated to accept the next acetyl group. This is why it is a cycle. For each glucose the cycle turns twice, producing in all 4 CO₂, 2 ATP, 6 NADH and 2 FADH₂. Notice that the Krebs cycle itself does not use oxygen; its job is to complete the removal of carbon as carbon dioxide and to load hydrogen on to the carriers.

Electron transport chain and oxidative phosphorylation. The NADH and FADH₂ from glycolysis, the entry step and the Krebs cycle now deliver their hydrogen to a series of carrier proteins embedded in the inner mitochondrial membrane. The hydrogen is split into protons and electrons; the electrons pass down the chain from carrier to carrier, releasing energy at each step, and that energy is used to make ATP from ADP and phosphate. At the end of the chain the electrons and protons combine with oxygen to form water. This is the only step in which oxygen takes part, and it is the reason oxygen is needed at all: without oxygen to accept the electrons, the chain stops, NADH cannot be recycled to NAD, and the Krebs cycle and all ATP production in the mitochondrion halt. Each NADH yields about 3 ATP and each FADH₂ about 2, so the chain yields about 34 ATP per glucose.

Adding the stages together, complete aerobic respiration of one glucose gives 2 ATP from glycolysis, 2 from the Krebs cycle and about 34 from electron transport, a total of roughly 38 ATP (about 36 in many cells because of the cost of moving NADH into the mitochondrion). The six oxygen molecules consumed all end up in water, and the six carbon atoms of glucose all end up in carbon dioxide, which diffuses out of the cell into the blood.

Respiration is called aerobic (aer = air) when it uses oxygen and anaerobic when it does not. Both begin with glycolysis; they differ in what happens to pyruvic acid afterwards, in the products, and above all in the amount of energy released.

Aerobic respiration occurs in most plants and animals. Glucose is completely oxidised to carbon dioxide and water; the process runs through glycolysis in the cytoplasm and the Krebs cycle and electron transport chain in the mitochondria; and it yields about 38 ATP per glucose, roughly 2900 kJ per mole, about 40 percent of which is captured in ATP and the rest released as heat.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP

Anaerobic respiration occurs in the absence of oxygen. It takes place entirely in the cytoplasm; the mitochondria are not involved. Glucose is only partly broken down, so the products still contain much energy, and the yield is only the 2 ATP of glycolysis, about 5 percent of the aerobic yield. There are two common forms. In yeast and many bacteria, and in plant tissues deprived of oxygen (roots in waterlogged soil, seeds in a sealed container), pyruvic acid is converted into ethanol and carbon dioxide; this is alcoholic fermentation.

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2 ATP (yeast)

In animal muscle during strenuous exercise, and in some bacteria such as Lactobacillus that curdle milk, pyruvic acid is converted into lactic acid with no carbon dioxide released.

C₆H₁₂O₆ → 2C₃H₆O₃ (lactic acid) + 2 ATP (muscle)

Some organisms, called obligate anaerobes, respire only anaerobically and are actually killed by oxygen; the tetanus bacterium Clostridium is one, which is why deep puncture wounds where air cannot reach are dangerous. Others, called facultative anaerobes, such as yeast, respire aerobically when oxygen is present and switch to fermentation when it is not. Humans are essentially aerobic but their muscles can borrow anaerobic respiration for short bursts.

The comparison can be tabulated. Site: aerobic in cytoplasm and mitochondria, anaerobic in cytoplasm only. Oxygen: required, not required. Breakdown of glucose: complete, incomplete. Products: carbon dioxide and water; ethanol and carbon dioxide or lactic acid. Energy: about 38 ATP, 2 ATP. Occurrence: most organisms; yeast, some bacteria, muscles under strain, parasites of the gut. Anaerobic respiration is wasteful of food but has the advantage of continuing when oxygen fails, and it is far faster per second, which is what a sprinting muscle needs.

Fermentation is anaerobic respiration by microorganisms, and humans have used it for thousands of years without knowing the chemistry. Louis Pasteur showed in the 1850s that living yeast, not a mere chemical change, turns sugar into alcohol, and described fermentation as life without air.

Yeast is a single-celled fungus. In the presence of sugar and the absence of oxygen it carries out alcoholic fermentation, converting each glucose into two molecules of ethanol and two of carbon dioxide and gaining two ATP. Both products are useful. The carbon dioxide is used in baking: yeast added to dough ferments the sugar, the gas bubbles are trapped in the sticky gluten of wheat flour and the dough rises, giving bread its spongy texture; the ethanol boils off during baking. The ethanol is used in brewing and wine-making: grape juice or malted barley fermented by yeast gives wine or beer, and distillation of a fermented mash gives spirits and industrial alcohol. Fermentation of molasses is the main source of ethanol in India, and ethanol is now blended with petrol as a fuel.

Bacteria ferment in other ways. Lactobacillus converts the lactose of milk into lactic acid; the acid curdles the milk protein and gives curd (dahi) its sour taste and keeping quality. The same bacteria are used for making cheese, pickles, sauerkraut and the batter of idli and dosa, which becomes light and sour overnight as bacteria and wild yeast ferment it. Acetobacter converts ethanol into acetic acid, giving vinegar. Fermentation by bacteria in the rumen of cattle digests the cellulose of grass that the animal's own enzymes cannot, and fermentation in sealed pits produces biogas (methane) from cattle dung. In each case the microorganism gains a little energy and we harvest its by-products.

Lactic acid in muscles. Human muscles normally respire aerobically. During vigorous exercise, however, the muscle fibres need ATP faster than the blood can bring oxygen. The fibres then supplement aerobic respiration with anaerobic respiration: pyruvic acid from glycolysis is converted to lactic acid, which lets glycolysis continue and provides ATP quickly, though inefficiently. Lactic acid accumulates in the muscle and diffuses into the blood, and it is this accumulation that causes the burning sensation, fatigue and the cramps felt during and after hard exercise. The lowered pH also reduces the power of contraction, which is why a sprinter cannot keep up full speed beyond a few hundred metres.

After the exercise stops, the person continues to breathe deeply for some minutes. The extra oxygen taken in is used to oxidise part of the lactic acid to carbon dioxide and water, and the energy so released converts the rest back into glucose and glycogen in the liver. The amount of oxygen needed to clear the lactic acid is the oxygen debt, and the panting after a race is the body paying it back. A trained athlete has more mitochondria, a better blood supply and a higher tolerance of lactic acid, so the debt builds up more slowly.

Plants respire just as animals do, in every living cell, day and night, by the same glycolysis-Krebs-electron transport pathway. The difference is only in how the gases reach the cells: plants have no lungs, blood or breathing movements, and rely entirely on diffusion. This works because a plant's metabolic rate is low compared with an animal's, because every living part of a plant is close to a surface or an air space, and because the loose packing of cells leaves a continuous system of intercellular air spaces through which gases move.

In leaves and green stems, oxygen enters and carbon dioxide leaves through the stomata, and the spongy mesophyll's air spaces carry the gases to every cell. In woody stems the bark is impermeable, but it has small loose-celled openings called lenticels, visible as raised spots on the twig, through which gases diffuse to the living tissues underneath. Roots take in oxygen from the air held between soil particles, through the root hairs and the epidermis by diffusion. This is why plants die in waterlogged soil: the water fills the air spaces, the roots cannot obtain oxygen, they respire anaerobically for a while, produce ethanol, and then rot. It is also why the soil around plants is loosened by hoeing, why earthworms are welcome in a garden, and why over-watering kills a pot plant.

Respiration and photosynthesis together. A green leaf carries out both processes. In daylight photosynthesis is much faster than respiration, typically ten to twenty times, so the net movement of gases is carbon dioxide in and oxygen out, and the carbon dioxide released by respiration is used inside the leaf before it can escape. At night there is no photosynthesis, so the leaf takes in oxygen and gives out carbon dioxide like any animal. At the dim light of dawn and dusk there is a point where the two rates are equal and no net gas exchange takes place; this is the compensation point. The idea that plants give out carbon dioxide only at night is therefore wrong: they respire all the time, but by day the photosynthesis masks it. The old advice not to sleep under a tree at night is based on this small release of carbon dioxide, though in the open the amount is far too small to matter.

Over a whole day a healthy plant makes much more food than it respires, and the surplus is what becomes its growth, its stored starch and the harvest we take. A plant kept in continuous dim light near the compensation point does not grow.

Plant respiration is also economically important. Germinating seeds respire rapidly, which is why stored grain must be kept dry and cool: moist grain begins to respire, heats up, and is spoiled by fungi. Fruits respire during ripening, and cold storage slows the respiration of apples and potatoes so they keep for months. Fresh vegetables wilt and lose sweetness after harvest partly because their sugars are being respired away.

Two simple experiments, both standard for the board, demonstrate the products of respiration in germinating seeds. Germinating seeds are used because they respire very actively and are not photosynthesising, so there is no risk of the carbon dioxide being taken up again.

Experiment 1: germinating seeds give out carbon dioxide. Take a conical flask and place in it about 50 g of moist germinating gram or bean seeds on damp cotton. Fit the flask with a two-holed cork. Through one hole pass a thistle funnel whose stem dips below the surface of some water at the bottom of the flask; through the other pass a delivery tube whose free end dips into a beaker or test tube of lime water (calcium hydroxide solution), or fresh KOH solution. Make the joints airtight with wax or vaseline and leave the apparatus for a few hours. Then pour water through the thistle funnel; as the water level rises in the flask it pushes the air from the flask through the delivery tube into the lime water. The lime water turns milky, showing that the air in the flask contains carbon dioxide. A control flask set up in the same way with boiled (dead) seeds, or with dry seeds, does not turn the lime water milky. Hence living, respiring seeds release carbon dioxide. The thistle funnel is used because it drives the gas out without opening the flask; if KOH is used in a bent tube instead of lime water, the water level in the tube rises as the gas is absorbed, showing the same thing by a different sign.

An alternative for animals: a small animal such as a cockroach or a mouse is placed in a bell jar over a dish of lime water, which turns milky after some time; the lime water in a control jar without the animal stays clear.

Experiment 2: germinating seeds give out heat. Take two thermos flasks. Fill one with germinating seeds that have been surface-sterilised by washing in dilute formalin or a mild disinfectant (to kill bacteria, which would also respire and produce heat) and the other with an equal quantity of boiled and cooled seeds, also disinfected. Plug the mouth of each with cotton wool through which a thermometer passes into the seeds. Record the temperature in both flasks at intervals over 24 hours. The temperature in the flask with living seeds rises by several degrees while the flask with dead seeds shows no change. A thermos flask is used because it prevents the heat escaping. Hence respiration releases heat, and dead seeds do not respire.

Experiment 3: respiration uses oxygen. Germinating seeds are placed in a conical flask with a small tube of KOH hanging inside to absorb the carbon dioxide produced. The flask is corked with a delivery tube dipping into coloured water. As the seeds use up oxygen and the carbon dioxide they produce is absorbed, the volume of gas in the flask decreases and the coloured water rises in the tube. A control with dead seeds shows no rise. A related classic is the Ganong's respiroscope, in which the mercury or coloured water in a graduated tube rises as seeds in the bulb consume oxygen with the carbon dioxide absorbed by KOH.

In each experiment note the three things an examiner asks: the aim, the role of the control, and the precaution that makes the result reliable (airtight joints, sterilised seeds, boiled seeds as control).

Different animals have evolved different respiratory surfaces, but all of them are thin, moist, extensive and well supplied with blood or body fluid. In Amoeba, Hydra and other very small animals, the whole body surface serves for gas exchange by simple diffusion; no special organ is needed because no cell is far from the outside. The earthworm and the leech breathe through the moist skin, which is kept wet by mucus and has a dense network of blood capillaries just beneath; an earthworm dies if its skin dries, and comes to the surface after rain because the water in its burrow has no oxygen. The frog uses three surfaces: its moist skin at all times and especially under water and during hibernation, the lining of its mouth (buccal cavity), and a pair of simple sac-like lungs on land. Fish use gills: feathery filaments with thin lamellae, richly supplied with blood, over which water is pumped by the mouth and the gill covers (operculum). Because water holds far less oxygen than air, the fish must pass a great deal of water over the gills, and the blood flows through the lamellae in the direction opposite to the water flow (counter-current) so that it can extract most of the oxygen. Insects such as the cockroach and grasshopper have a system of branching air tubes, the tracheae, opening on the sides of the body through small pores called spiracles; the tubes branch into fine tracheoles that deliver air directly to every cell, so insect blood carries no oxygen and has no haemoglobin. Birds and mammals have lungs, those of birds supplemented by air sacs that give them a one-way flow of air and an efficiency that lets them fly at heights where a mammal could not survive.

Keeping the respiratory system healthy. The system is exposed to the air we breathe, and most of its diseases come from that air. Smoking is the single greatest cause of damage: tar paralyses and destroys the cilia, so mucus and dirt accumulate; the irritation causes chronic bronchitis; the destruction of alveolar walls causes emphysema, in which the lungs lose their surface area and the person is breathless at the least exertion; and the carcinogens in tar cause lung cancer, the commonest cancer death in men. The carbon monoxide in smoke reduces the oxygen-carrying power of the blood. Passive smoking harms children in the same house. Air pollution from vehicle exhaust, factory smoke, dust and the burning of crop residue causes and aggravates asthma and bronchitis; masks and clean fuels reduce the exposure. Infections such as the common cold, influenza, pneumonia and tuberculosis spread by droplets from coughing and sneezing; covering the mouth, not spitting in public, ventilating rooms and taking the BCG and other vaccines reduce their spread, and tuberculosis is fully curable with a complete course of DOTS medicines. Asthma is narrowing of the bronchioles by contraction of their muscle and swelling of the lining, triggered by dust, pollen, smoke or cold air, and is controlled with inhalers. Regular exercise, breathing exercises such as pranayama, a dust-free home and a diet rich in vitamin A and C all help keep the lungs efficient.