Cycles within ecosystems — Edexcel International GCSE Biology
The carbon cycle with photosynthesis, respiration, decomposition and combustion, and the roles of bacteria in the nitrogen cycle.
Revision notes
2 short notes, 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.
The nitrogen cycle
Biology onlyThe nitrogen cycle is how nitrogen moves between the air, soil and living things. Plants need nitrates to make proteins but cannot use nitrogen gas. Decomposers break down proteins and urea into ammonium ions, nitrifying bacteria convert these into nitrates, nitrogen-fixing bacteria convert nitrogen gas into nitrogen compounds, and denitrifying bacteria turn nitrates back into nitrogen gas.
Exam questions
6 exam-style questions (17 marks), each with its mark scheme.
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8 cards: flip them, mark what you knew, and practise the rest.
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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.
Biology only What do nitrogen-fixing bacteria do?
Show answer
Convert nitrogen gas from the air into nitrogen compounds.
Biology only What do denitrifying bacteria do?
Show answer
Convert nitrates in the soil into nitrogen gas.
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.
What is the role of bacteria in the nitrogen cycle?
Biology only Bacteria make nitrates available to plants. Decomposers turn proteins and urea in dead material and waste into ammonium ions, nitrifying bacteria convert ammonium ions into nitrates, and nitrogen-fixing bacteria in soil or legume root nodules convert nitrogen gas into nitrogen compounds. Denitrifying bacteria turn nitrates back into nitrogen gas.
Why can plants not use nitrogen from the air?
Biology only Plants cannot use nitrogen gas directly, even though about 78% of the air is nitrogen. They need nitrates, absorbed from the soil through their roots, to make amino acids and proteins. Nitrogen-fixing bacteria, in the soil or in the root nodules of legumes, convert nitrogen gas into nitrogen compounds, which become nitrates that plants can absorb.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026