The carbon cycle
Biology · Key Stage 3 · The School
Carbon moves, it does not disappear
Plants take in carbon dioxide from the air during PHOTOSYNTHESIS. They build it into sugars, which become part of the plant's own body. Animals get their carbon by eating plants or other animals. Both plants and animals release some carbon back into the air, as carbon dioxide, through RESPIRATION.
Decomposition returns carbon too
When a plant or animal dies, DECOMPOSERS break it down. These are bacteria and fungi. As they feed, they respire, and this releases the carbon stored in the dead body back into the air as carbon dioxide. Without decomposition, that carbon would stay locked in place. Living things later on would never get it back.
Combustion and fossil fuels
Over millions of years, some dead plants and animals did not fully decay. They got buried and squeezed into FOSSIL FUELS — coal, oil and natural gas. This locks their carbon away for a very long time. Burning fossil fuels is called COMBUSTION. It releases that long-buried carbon back into the air, far faster than it was first stored.
Take one carbon atom, once part of an old plant. It can return to the air in different ways. If an animal ate the plant and later respired, the carbon returns within days. If the plant died and rotted down normally, it returns within months or years. If the plant got buried instead, and slowly turned into coal, that same atom could stay locked away for millions of years. Only if the coal is dug up and burned does it finally return to the air, all at once — released by burning, not by any living process at all. Same starting point. Wildly different timescales. All part of one cycle.