The carbon cycleSpec 4.10
In short
The carbon cycle is the movement of carbon between the atmosphere and living things. Plants and algae remove carbon dioxide from the air by photosynthesis and make carbon compounds, which pass along food chains when animals feed. Respiration, decomposition of dead material and waste by bacteria and fungi, and combustion of fuels all return carbon dioxide to the air.
All living things are made of carbon compounds. The carbon cycle returns carbon to the atmosphere as carbon dioxide, so that it is available for plants to use in photosynthesis again. Carbon is also stored in fossil fuels, which are the remains of long-dead organisms.
- Plants and algae take carbon dioxide from the air and use it in photosynthesis to make carbon compounds such as glucose, starch and proteins.
- The carbon compounds pass along the food chain when animals feed.
- All living organisms (plants, animals and decomposers) release carbon dioxide into the air by respiration.
- When plants and animals die, and when they produce waste, decomposition by bacteria and fungi breaks down the material. The decomposers release carbon dioxide into the air by respiration.
- Burning fuels such as wood, coal, oil and natural gas (combustion) releases carbon dioxide into the air.
The role of decomposers
Decomposers are bacteria and fungi. They break down dead plants, dead animals and waste, and they respire as they do so, releasing carbon dioxide. Without decomposers, dead material would pile up and the carbon in it would be locked away. The supply of carbon dioxide for photosynthesis would eventually run out.
Name the process at each arrow. Photosynthesis takes carbon dioxide out of the air. Respiration, decomposition and combustion put it back.
(opens full size in a new tab)Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026