The carbon cycle

Carbon is an essential element for life on Earth and parts of each of the cells in our bodies are made from it. The carbon cycle shows how atoms of this element can exist within different compounds at different times.

Stage one of the carbon cycle. Overnight, CO2 from burning fuel (combustion) and respiration by cows, birds and plants pass into the atmosphere.

Stage one of the carbon cycle

Carbon enters the atmosphere as carbon dioxide from respiration and combustion.

The carbon cycle is easiest to understand in terms of its processes and the conversion of carbon that they undertake. The three key processes and their conversions are shown in the table.

ProcessCarbon starts asCarbon ends as
PhotosynthesisCarbon dioxideGlucose
RespirationGlucoseCarbon dioxide
Combustion (burning)Fuel (eg methane or wood)Carbon dioxide

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

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.

Returning carbon dioxide to the atmosphere

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 into the atmosphere.

Decomposition or decay also releases carbon dioxide to the atmosphere. 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.