The carbon cycle

Carbon is an essential element for life on Earth and most biological molecules include carbon atoms. Carbon is present in:

  • carbon dioxide gas in the atmosphere
  • carbon dioxide dissolved in the oceans
  • calcium carbonate in the shells of sea creatures
  • calcium carbonate in the form of limestone
  • hydrocarbon compounds in fossil fuels
  • carbohydrate molecules in living organisms

The carbon cycle shows how atoms of this element are cycled between different compounds within the biotic and abiotic components of an ecosystem 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 below.

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

Almost all living things depend on photosynthesis in producers (eg green plants). This process converts carbon dioxide into carbohydrates which support other living things. Animals feed on plants, so the carbon compounds are passed along food chains. Animals and plants respire, releasing carbon dioxide back into the atmosphere.

When dead plants and animals are decomposed by microorganisms, carbon dioxide is released to the atmosphere and mineral ions are returned to the soil.