The carbon cycle
Carbon is constantly recycled between the atmosphere, living organisms and fossil fuels. Only four processes are named in the specification — learn them as the four "engines" of the cycle.
The carbon store in the air
Carbon is present in the air as carbon dioxide. Everything in the cycle either removes CO₂ from the air or releases CO₂ into the air.
| Process | Direction | Carried out by |
|---|---|---|
| Photosynthesis | Removes CO₂ from the atmosphere | Green plants (and algae) |
| Respiration | Releases CO₂ into the atmosphere | Plants, animals and decomposers |
| Decomposition | Releases CO₂ (via the decomposers' respiration) | Bacteria and fungi |
| Combustion | Releases CO₂ | Burning of fossil fuels and wood |
The stages, in order
- Photosynthesis — plants absorb carbon dioxide from the air and use light energy to convert it into glucose. The carbon is now locked into carbohydrates, fats and proteins in the plant.
- Feeding — animals eat plants (or eat other animals), so carbon compounds are passed along the food chain.
- Respiration — plants, animals and microorganisms break down glucose and release carbon dioxide back into the air. This happens all the time, day and night.
- Death and waste — dead organisms, faeces and urine contain carbon compounds.
- Decomposition — decomposers (bacteria and fungi) feed on the dead material and waste. As they respire, they release CO₂ back into the atmosphere.
- Fossilisation — if dead organisms are not decomposed (for example in airless, waterlogged conditions), over millions of years their remains are compressed to form fossil fuels such as coal, oil and natural gas. Carbon is locked away here for a very long time.
- Combustion — burning fossil fuels or wood releases the stored carbon as carbon dioxide.
Getting a description question right
If you are asked to describe the carbon cycle, name the process, name the organism doing it, and name the substance moved. "Plants take in carbon" is too vague. "Plants remove carbon dioxide from the air by photosynthesis and convert it into glucose" scores.
Common mistake: writing about photosynthesis and forgetting to say that it removes carbon dioxide from the atmosphere. Examiners repeatedly report answers that mention photosynthesis but never state the removal of CO₂ — that is where the mark sits.
Examiner tip. Two errors lose marks here every year. (1) Confusing the gases: photosynthesis takes in carbon dioxide and gives out oxygen; respiration takes in oxygen and gives out carbon dioxide. Never write that a leaf 'needs oxygen for photosynthesis', and never say respiration needs light. (2) In data or comprehension questions, don't just repeat phrases from the passage — say explicitly which process removes CO₂ and which releases it.