The carbon cycle

Ecology and the environment · Cycles within ecosystems · note 1 of 1

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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.

Exam tip:

Name the process at each arrow. Photosynthesis takes carbon dioxide out of the air. Respiration, decomposition and combustion put it back.

Carbon cycle diagram: carbon dioxide in the air is taken in by plants in photosynthesis, passes to animals by feeding, goes to dead material by death and waste, is broken down by decomposers, and returns to the air by respiration of plants, animals and decomposers and by combustion of fuels. — The carbon cycle: **photosynthesis** takes carbon dioxide in; **respiration**, **decomposition** and **combustion** put it back.

Quick check

Which process removes carbon dioxide from the air?
Show answer
Photosynthesis.
Name three processes that return carbon dioxide to the air.
Show answer
Any three of: respiration, decomposition, combustion.
Why are decomposers important in the carbon cycle?
Show answer
They break down dead material and release carbon dioxide by respiration.

Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026

Frequently asked questions

How does carbon move through the carbon cycle?

Carbon moves from the air into plants and algae when they take in carbon dioxide for photosynthesis, making carbon compounds. These pass along food chains as animals feed. Carbon returns to the air as carbon dioxide through respiration by all organisms, decomposition of dead material and waste by decomposers, and combustion of fuels.

What would happen to the carbon cycle without decomposers?

Without decomposers, dead plants, dead animals and waste would pile up and the carbon in them would stay locked away. Decomposers, which are bacteria and fungi, break this material down and release carbon dioxide into the air by respiration. Without them, the supply of carbon dioxide for photosynthesis would eventually run out.

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