Ecosystems – pollution and nutrient cycles – WJECThe carbon cycle

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.

Removing carbon dioxide from the atmosphere

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.

Passing carbon from one organism to the next

Animals get their carbon by eating plants or other animals.

Returning carbon dioxide to the atmosphere

Carbon dioxide is released to the atmosphere through respiration by animals, plants and micro-organisms. 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 micro-organisms. Decay can be very slow in cold, dry conditions, and when there is a shortage of oxygen.