Carbon is passed from the atmosphere, as carbon dioxide, to living things. It is then passed from one organism to the next in complex molecules, and returned to the atmosphere as carbon dioxide again. This is known as the carbon cycle.
The slideshow shows how the cycle works.
Plants use carbon dioxide from the atmosphere for photosynthesis. The carbon becomes part of complex molecules in the plants, such as proteins, fats and carbohydrates.
When an animal eats a plant, carbon from the plant becomes part of the fats and proteins in the animal. Microorganisms and some animals feed on waste material from animals, and the remains of dead animals and plants. The carbon then becomes part of these organisms.
Carbon dioxide is released to the atmosphere through respiration by animals, plants and microorganisms. It is also released by the combustion of wood and fossil fuels (such as coal, oil and natural gas). The use of fossil fuels is gradually increasing the carbon dioxide levels in the atmosphere.
Decomposition or decay also releases carbon dioxide. This process happens faster in warm, moist conditions with plenty of oxygen because it involves microorganisms. Decay can be very slow in cold, dry conditions, and when there is a shortage of oxygen.