The carbon cycle

It is reasonably likely that you have some carbon atoms in your body that were once inside Charles Darwin or Marie Curie or any other long dead individual. Atoms exist in different forms or compounds at different times in history and cycle between them.

Carbon is an essential element for life on Earth and parts of the cells in our bodies, including carbohydrates, proteins, lipids and DNA, 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 below.

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