The carbon cycle

Organisms and their environment · Nutrient cycles · note 1 of 2

Spec 19.3.1Download all Nutrient cycles notes (PDF)Download all notes (PDF)5 pages

The carbon cycleSpec 19.3.1

In short

The carbon cycle moves carbon between the air, living organisms, dead material and fossil fuels. Photosynthesis removes carbon dioxide from the air, and feeding passes carbon compounds along food chains. Respiration by plants, animals and decomposers returns carbon dioxide, as does combustion of fossil fuels and wood. Some buried dead organisms form coal, oil and natural gas.

All living things are made of carbon compounds, such as carbohydrates, proteins and fats. The carbon cycle moves carbon between the air, living organisms, dead material and fossil fuels. It makes sure carbon dioxide is available for plants to use in photosynthesis again.

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 by feeding, when animals eat plants and other animals.
Plants, animals and decomposers all release carbon dioxide into the air by respiration.
When organisms die, and when they produce waste, decomposers (bacteria and fungi) break down the material. They release carbon dioxide into the air by respiration.
Over millions of years, some dead organisms are buried and do not decompose fully. They form fossil fuels (coal, oil and natural gas).
When fossil fuels, wood or other fuels are burned, combustion releases carbon dioxide into the air.

Processes in the carbon cycle
Process	Effect on carbon dioxide in the air
Photosynthesis	Removes carbon dioxide from the air
Respiration (plants, animals, decomposers)	Adds carbon dioxide to the air
Decomposition	Adds carbon dioxide to the air, because decomposers respire
Combustion	Adds carbon dioxide to the air
Feeding	Passes carbon compounds from one organism to the next; no change in the air
Formation of fossil fuels	Locks carbon away in coal, oil and gas

Carbon cycle flow diagram: carbon dioxide in the air is taken in by plants by photosynthesis, passes to animals by feeding, and returns to the air by respiration of plants, animals and decomposers; death and waste pass to dead material, which decomposers break down (decomposition) or which forms fossil fuels, and combustion of fossil fuels releases carbon dioxide. — The carbon cycle. Photosynthesis removes carbon dioxide from the air; **respiration**, **decomposition** and **combustion** return it.

Exam tip:

Name the process at every arrow. Photosynthesis takes carbon dioxide in. Respiration, decomposition and combustion put it back. Do not write 'breathing' for respiration.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

How does carbon move through the carbon cycle?

Carbon moves through the carbon cycle as plants and algae take in carbon dioxide for photosynthesis and make carbon compounds. Feeding passes these compounds along food chains. Respiration by plants, animals and decomposers returns carbon dioxide to the air. Some dead organisms form fossil fuels, and combustion of fuels releases carbon dioxide again.

Which processes add carbon dioxide to the air?

Respiration, decomposition and combustion add carbon dioxide to the air. Plants, animals and decomposers all release carbon dioxide by respiration, and decomposition adds it because decomposers respire as they break down dead material and waste. Burning fossil fuels, wood or other fuels releases carbon dioxide by combustion. Photosynthesis is the process that removes it.

What is nitrogen fixation?

Supplement Nitrogen fixation is the change of nitrogen gas into nitrogen compounds that plants can use. It is done by nitrogen-fixing bacteria in the soil and in root nodules, and by lightning. The energy of lightning makes nitrogen combine with oxygen to form nitrogen oxides, which dissolve in rain and reach the soil as nitrates.

All 4 questions on Nutrient cycles