91�ȱ�

A balanced carbon cycle is essential.

Carbon is a major component in carbohydrates, fats and proteins.

The carbon cycle involves the exchange of carbon between living organisms (biotic) and their atmosphere (abiotic).

In the carbon cycle, carbon is constantly removed from, and returned to, the environment.

Processes involved in the carbon cycle are:

• Photosynthesis – plants absorb carbon dioxide from the atmosphere and form it into sugar, starch and other organic compounds. This is the only process in the cycle that decreases the level of carbon dioxide in the atmosphere.
• Feeding – moves carbon in the form of biological molecules along the food chain.
• Respiration – when living organisms (plants, animals and decomposers) respire they release carbon dioxide into the atmosphere (this is a form of excretion).
• Fossilisation – if conditions are not favourable for the process of decomposition, dead organisms decay slowly or not at all. These organisms build up and, if compressed over millions of years, can form fossil fuels (coal, oil or gas).
• Combustion – the burning of fossil fuels releases stored carbon dioxide into the atmosphere.
• Excretion – when waste is removed from the body (urine). This excreted material can be broken down during the process of decomposition.
• Egestion – the removal of faeces from an animal that will contain carbon. This egested material can be broken down during the process of decomposition.
• Decomposition – when complex, carbon compounds in dead organisms, urine and faeces are broken down into simpler carbon compounds by bacteria or fungi.