Cycles within ecosystems — Edexcel International GCSE Science (Double Award)
The carbon cycle: how photosynthesis, respiration, combustion and decomposers move carbon between living things and the air.
Revision notes
1 short note, in the order of the specification. Each one 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.
Exam questions
3 exam-style questions (8 marks), each with its mark scheme.
Answer the questionsFlashcards
4 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of ecology and the environment on one page, so you can see where this subtopic fits.
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Quick check questions
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.
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.
Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026