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.

All cells - whether animal, plant or bacteria - contain carbon, because they all contain proteins, fats and carbohydrates. Plant cell walls, for example, are made of cellulose - a carbohydrate.

Carbon is passed from the atmosphere, as carbon dioxide, to living things, 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

Green plants remove carbon dioxide from the atmosphere by photosynthesis. The carbon becomes part of complex molecules such as proteins, fats and carbohydrates in the plants.

Returning carbon dioxide to the atmosphere

Organisms return carbon dioxide to the atmosphere by respiration. It is not just animals that respire. Plants and microorganisms do, too. Carbon dioxide is also released by combustion. The burning of fossil fuels releases large quantities into the atmosphere.

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. Decomposers and some animals, called detrivores, feed on waste material from animals, and the remains of dead animals and plants. The carbon then becomes part of these organisms.

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 three key processes and how they convert carbon are shown in the table below:

ProcessCarbon starts asCarbon ends as
Photosynthesis Carbon dioxideGlucose
Respiration GlucoseCarbon dioxide
Combustion (burning)Fuel (eg methane or wood)Carbon dioxide