Respiration Main Index

Respiration, breathing and photosynthesis


All animals and plants get the energy they need to live from their food. The process by which they obtain their energy from their food is called respiration. Respiration is a chemical change. Typically the chemical reaction involved in respiration is of the form

glucose + oxygen -----> carbon dioxide + water (+ energy)

Respiration is carried out by all animals and all plants, all through the day and all through the night. Respiration takes place in every cell of the animal or plant. In mammals (and many other animals) the sugar and oxygen needed for respiration are carried round the body to the cells in the bloodstream, and the bloodstream also removes the carbon dioxide and water produced by the cells' respiration. The carbon dioxide produced by respiration is a waste product and must be removed from the bloodstream, but the water produced by respiration is called metabolic water and this can be used by the body.

Some animals such as the jerboa live entirely on metabolic water and never need to drink any water at all. The water they need is produced by their own respiration and is not contained in any moisture in the food they eat - they can live on food which has been oven-dried to remove all water from it.

Camels are very greedy animals and eat and drink as much as they can whenever they have the opportunity. This makes them enormously fat, but unlike other animals they store all their fat in one place, in a hump, rather than all over their body. This enables them to keep cool in the hot desert, because fat all over their body would act as a very good insulator.

A camel's hump contains not water but fat, but as the camel uses the fat to give it energy its respiration produces metabolic water, and a kilogram of fat produces a lot more than a litre of metabolic water. The fatter the camel is, that is, the larger its hump, the longer it can go without both food and water.

We are used to seeing pictures of well-fed camels in zoos and in countries where camels are no longer used for very long journeys across the desert - these camels always have big humps. But in some of the very poor countries in the Sahara desert camels are still used in the old way, and after a long journey across the desert the camels' humps are little more than an empty bag of folded skin falling down over their sides. The camel drivers will not be able to start the next part of the journey until all the camels have got fat enough again.


The process by which mammals and many other animals obtain the oxygen needed for respiration and get rid of the carbon dioxide produced by respiration is called breathing. Many animals get the oxygen they need from the air, and mammals and many other air-breathing animals have lungs.

The air contains about 80% nitrogen and only about 21% oxygen, with a little carbon dioxide and other gases. The air that we breathe out contains only slightly less oxygen and only slightly more carbon dioxide than the air we breathe in, but is still mostly nitrogen - it is wrong to say we breathe in oxygen and breathe out carbon dioxide. To see a page about the gases in the atmosphere please click here scarab

Mouth to mouth resuscitation (the kiss of life) is possible only because the air we breathe out contains almost as much oxygen as the air we breathe in.

In humans and most other mammals breathing is involuntary, that is, we breathe without needing to think about it. We cannot go for very long without breathing. But whales have total control over their breathing, and can stay under water for up to half an hour, until almost all of the oxygen in their lungs has been used and replaced by carbon dioxide.

When a whale eventually surfaces it must empty and refill its lungs completely and very rapidly. The air it blows out has been in its lungs long enough to get very warm and very moist: the whale's spout as it surfaces is the water vapour in its breath condensing as it mixes with the cold air.

Fishes and many animals which live in water get their oxygen from the oxygen dissolved in the water which surrounds them, and fishes have gills to enable them to do this. What we often call the gills are actually the gill covers. When a fish breathes it first opens its mouth and shuts its gill covers, and sucks in water. It then shuts its mouth and opens its gill covers and blows the water out, and oxygen is taken out of the water and carbon dioxide is put into it as it passes over the gills. Sharks do not have gill covers so they can only pass water over their gills by swimming continuously with their mouths open, thus pushing water over their gills.

Cold water contains much more dissolved oxygen than warm water so there is more marine life in cold water than in warm water.


Animals and most other organisms can only obtain the food they need for respiration by eating plants or other animals, but plants (and only plants) can make their own food by photosynthesis.

(energy +) carbon dioxide + water ---> glucose + oxygen

This process gets the energy it needs from sunlight and therefore takes place only by day. Photosynthesis takes place in the leaves, and other green parts, of the plant. As soon as the glucose has been produced it is converted into starch, so photosynthesising leaves contain starch not glucose.

Plants respire by day and by night, but photosynthesise only by day. By night therefore they are using glucose and oxygen but not producing them; by day, although they are using glucose and oxygen they are also producing them, and are producing far more than they are using, enough not only for their own needs but also for the needs of all other living things for the day and for the night.

During photosynthesis plants take in carbon dioxide and produce oxygen, but it is quite wrong to say that plants breathe in carbon dioxide and breathe out oxygen.

Photosynthesis takes carbon dioxide out of the atmosphere. Some of the carbon in this carbon dioxide is returned to the atmosphere in the form of carbon dioxide by the plant's respiration, and some of it goes into the structure of the plant - all living organisms have structures made mainly of compounds of carbon and other elements. If the plant is eaten by an animal some of the carbon may be returned to the atmosphere as carbon dioxide by the animal's respiration and some incorporated into the animal's body. This animal may in turn get eaten by another animal. Ultimately all animals and plants will either be eaten or decompose. When an animal decomposes the carbon in it is returned to the atmosphere (or soil or water). Eventually most of the carbon dioxide removed from the atmosphere by photosynthesis will be returned to it. Carbon dioxide is also returned to the atmosphere when a plant is burnt - it makes no difference to the oxygen and carbon dioxide levels in the atmosphere whether a plant is cut down and burnt or eaten by animals or just dies and decays. Burning plants does not contribute to the greenhouse effect: the only cause of the greenhouse effect is the burning of fossil fuels such as coal and oil. There is more about the greenhouse effect on the page on Atmospheric pollution - click here to see it scarab

There is much more about the carbon cycle on the Carbon Cycle Page.

If all the plants were to die it is readily apparent that all the animals would soon die because they would have no food or oxygen. It is less readily apparent that if all the animals were to die so would all the plants. This is because in the same way that a photosynthesising plant is producing far more oxygen than it needs for its own respiration (the excess being used by animals) it is also using far more carbon dioxide than is produced by its own respiration, and so without animal respiration the carbon dioxide level in the atmosphere would quickly be reduced to a level at which photosynthesis could not take place.

Anaerobic respiration

The above Note is concerned mainly with aerobic respiration, in which free oxygen is used and carbon dioxide produced. Many simple organisms can respire without the use of free oxygen - this is called anaerobic respiration. The bacteria which live in the mud at the bottom of ponds, in our intestines and in cows' stomachs, and in food decomposing on rubbish tips etc. respire anaerobically - the waste products of their respiration may include methane and hydrogen sulphide.

Yeast is a microscopic fungus which can respire aerobically or anaerobically. In bread-making it respires mainly aerobically, but in wine-making it respires anaerobically.

When we make bread the bubbles of carbon dioxide produced by the yeast's respiration makes the bread rise. Once the bread is put into the oven the heat kills the yeast and the bread will not rise any more. Note that this is not the same as making a cake: when we make a cake we use baking powder which gives off carbon dioxide when it it heated, so the cake does not rise until it is put into the oven.

We make wine and beer and other alcoholic drinks in the same way

sugar + water ---> carbon dioxide + ethanol (+ energy)

We call this fermentation. There is more about this on the Alcohols page - to read it please click scarab

When we start to exercise our muscles need more oxygen and produce more carbon dioxide. Our bodies sense the increase in carbon dioxide in our blood and the heart starts to pump the blood around the body faster and we start to breathe more deeply. Until the effect of these changes take effect our muscles may respire anaerobically and one of the products of anaerobic respiration in our muscles is lactic acid. This produces discomfort or cramp. This is why we must warm up before taking part in sports.

We may also get cramp if we take a lot more exercise than usual when we are not very fit, or if we are very fit but pushing our muscles to their limits, but one of the commonest cause of cramp is when we exercise after a meal, when we are digesting our food and blood is being diverted from our muscles to our stomach and intestines. Cramp while running round the school playground after lunch is unpleasant; cramp when swimming in the sea after a meal could be life-threatening.

There is more about digestion on another page: to see it click here scarab .

Producers and consumers

Food gives us energy and the energy in our food must come from somewhere.

Plants produce food by photosynthesis, using the energy of sunlight, and animals consume it. We say that plants are producers and animals are consumers. At one time it was thought that plants were the only producers, but now we have discovered other producers, for example near hot mineral springs deep in the oceans and in sealed cave systems. These producers get their energy in ways not involving sunlight.

© Barry Gray May 2011