Nutrient cycles — Cambridge IGCSE Biology
How carbon is cycled by photosynthesis, respiration, feeding, decomposition and combustion, and how bacteria move nitrogen through the nitrogen cycle.
Revision notes
2 short notes, in the order of the specification. Each one 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.
The nitrogen cycle
SupplementThe nitrogen cycle moves nitrogen between the air, the soil and living things. Decomposers break down proteins and urea into ammonium ions, and nitrifying bacteria change these into nitrate ions that plants absorb to make amino acids. Nitrogen fixation by bacteria and lightning turns nitrogen gas into usable compounds, while denitrifying bacteria turn nitrates back into nitrogen gas.
Exam questions
6 exam-style questions (18 marks), each with its mark scheme.
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The whole of organisms and their 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 add carbon dioxide to the air.
Show answer
Respiration, decomposition and combustion.
How are fossil fuels formed?
Show answer
From dead organisms that were buried and did not decompose fully, over millions of years.
Supplement What do nitrifying bacteria do?
Show answer
They change ammonium ions into nitrate ions.
Supplement Name two ways nitrogen gas is fixed.
Show answer
By lightning and by nitrogen-fixing bacteria.
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.
What is the role of bacteria in the nitrogen cycle?
Supplement Bacteria move nitrogen through the cycle. Decomposers break down proteins and urea to ammonium ions. Nitrifying bacteria change ammonium ions into nitrate ions, which plants absorb. Nitrogen-fixing bacteria change nitrogen gas into nitrogen compounds plants can use. Denitrifying bacteria change nitrates back into nitrogen gas, reducing the nitrate available to plants.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026