The carbon cycle: processes and human impact
TutorAura guide · Updated
Carbon is in every living thing: in carbohydrates, proteins, fats and DNA. The carbon cycle describes how carbon moves between the atmosphere (as carbon dioxide), living organisms, the oceans, soil and rocks, and fossil fuels. Some processes remove carbon dioxide from the air and others return it, and human activity is upsetting the balance.
Key takeaways
- Photosynthesis removes carbon dioxide from the air.
- Respiration, decomposition and combustion return it.
- Feeding passes carbon along food chains.
- Burning fossil fuels and deforestation increase carbon dioxide in the atmosphere.
The processes
| Process | What happens to carbon |
|---|---|
| Photosynthesis | Plants and algae take carbon dioxide from the air and turn it into glucose |
| Feeding | Carbon in plants passes to animals that eat them, and along the food chain |
| Respiration | Plants, animals and microorganisms release carbon dioxide into the air |
| Decomposition | Microorganisms break down dead organisms and waste, releasing carbon dioxide as they respire |
| Combustion | Burning fuels (wood, coal, oil, gas) releases carbon dioxide |
| Fossilisation | Some dead organisms don't decay fully and, over millions of years, form coal, oil and gas |
The carbon in fossil fuels was taken from the atmosphere by photosynthesis long ago.
Following one carbon atom
A carbon atom in a molecule of carbon dioxide is absorbed by a leaf and becomes part of glucose in photosynthesis. The glucose is used to make starch. A rabbit eats the leaf and the carbon becomes part of its body. The rabbit respires, breathing the carbon out as carbon dioxide, or it dies and decomposers break down its body, releasing the carbon dioxide as they respire. Back in the air, the cycle starts again.
Why decomposers matter
Decomposers (bacteria and fungi) return carbon to the atmosphere as carbon dioxide and return mineral ions to the soil for plants. Decay is fastest when it's warm, moist and there's plenty of oxygen, because these conditions suit the microorganisms' enzymes and respiration. That's why gardeners keep compost heaps warm, damp and turned.
Carbon stores
Carbon is stored in the atmosphere (carbon dioxide), living things (biomass), the oceans (dissolved carbon dioxide, and shells and skeletons made of carbonates), rocks such as limestone, and fossil fuels. Oceans absorb a large amount of carbon dioxide, which makes seawater more acidic.
Human impact
- Burning fossil fuels releases carbon that was locked away for millions of years, much faster than photosynthesis can remove it.
- Deforestation removes trees that would absorb carbon dioxide; if the wood is burned, it releases more.
- The result is a rising concentration of carbon dioxide in the atmosphere. Carbon dioxide is a greenhouse gas, and scientists agree the increase is causing global warming and climate change.
Ways to reduce it include using renewable energy, planting trees, and using fuels and materials more efficiently.
The water cycle connection
Exam questions sometimes pair the carbon cycle with the water cycle: water evaporates, condenses into clouds, falls as precipitation, and is taken up by plants and released by transpiration. Both cycles show how materials are recycled in an ecosystem, so they can be used again by living things.
Common mistakes
- Forgetting that plants respire too: they release carbon dioxide all the time, not just at night.
- Saying decomposers "eat" dead material without explaining that their respiration releases carbon dioxide.
- Leaving out combustion when asked about human impact.
Practice questions
- Name the process that removes carbon dioxide from the atmosphere.
- Name three processes that release carbon dioxide.
- Why does decay happen faster in warm, moist conditions?
- Explain how deforestation increases carbon dioxide in the atmosphere.
Answers: 1. Photosynthesis. 2. Respiration, combustion and decomposition. 3. The microorganisms' enzymes work faster, and they need water to grow and respire. 4. Fewer trees remove carbon dioxide by photosynthesis, and burning or decay of the wood releases carbon dioxide.
Frequently asked questions
What is the carbon cycle?
The carbon cycle is the movement of carbon between the atmosphere, living things, oceans, soil, rocks and fossil fuels, through processes such as photosynthesis, respiration, decomposition and combustion.
What removes carbon dioxide from the air?
Photosynthesis by plants and algae. Oceans also absorb carbon dioxide.
How do humans affect the carbon cycle?
Burning fossil fuels and cutting down forests release carbon dioxide faster than it's removed, increasing the amount in the atmosphere.
Sources
- GCSE Biology (8461) specification: subject content. AQA.
- Cambridge IGCSE Biology 0610 syllabus for 2026, 2027 and 2028. Cambridge International.
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