The carbon cycle

Carbon is an essential element for life on Earth. Every living organism has carbon compounds inside each of its cells, such as fats and proteins. The carbon cycle shows how atoms of carbon can exist within different compounds at different times and be recycled between living organisms and the environment.

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

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

Processes in the carbon cycle

The carbon cycle is easiest to understand in terms of its processes and how carbon is converted. The three key processes and the conversions are shown in the table below.

ProcessCarbon starts asCarbon ends as
PhotosynthesisRespirationCombustion (burning)
Carbon dioxideGlucoseFuel (eg methane or wood)
GlucoseCarbon dioxideCarbon dioxide
  1. Carbon enters the atmosphere as carbon dioxide from respiration and combustion.
  2. Carbon dioxide is absorbed by producers to make glucose in photosynthesis.
  3. Animals feed on the plant passing the carbon compounds along the food chain. Most of the carbon they consume is exhaled as carbon dioxide that was formed during aerobic respiration. The animals and plants eventually die.
  4. Decomposers break down the dead organisms and return the carbon in their bodies to the atmosphere as carbon dioxide by respiration. In some conditions, decomposition is blocked. The plant and animal material may then be available as fossil fuel in the future for combustion.