Edexcel IGCSE Biology · Spec 4.11B
The Nitrogen Cycle
The nitrogen cycle and the roles of the different types of bacteria within it.
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The Nitrogen Cycle
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Explained
The nitrogen cycle, told through four groups of bacteria
Living things need nitrogen to build amino acids, and therefore proteins, and also DNA. Air is about 78 per cent nitrogen, so it seems as though supply should never be a problem. It is, because nitrogen gas is very unreactive and almost no organism can use it directly.
Everything in this cycle exists to solve that one problem. Learn the bacteria by what they convert, and the cycle assembles itself.
Nitrogen fixing bacteria
These convert nitrogen gas from the air into ammonium compounds, which plants can use. Some live free in the soil and some live in nodules on the roots of legumes such as peas, beans and clover.
The relationship in the root nodules benefits both partners. The plant supplies the bacteria with sugars from photosynthesis, and the bacteria supply the plant with usable nitrogen compounds. That is why farmers grow legumes to restore soil fertility.
Lightning also fixes nitrogen, by supplying enough energy for nitrogen and oxygen to react, but it accounts for a small fraction of the total.
Decomposers
Bacteria and fungi break down dead organisms, urine and faeces, releasing the nitrogen they contain as ammonium compounds. Without decomposition, nitrogen would be locked in dead material and the cycle would stop.
Nitrifying bacteria
These convert ammonium compounds into nitrates. This matters because nitrate is the form plant roots absorb. Nitrifying bacteria need oxygen, which is one reason waterlogged soil is poor soil.
Denitrifying bacteria
These convert nitrates back into nitrogen gas, returning it to the air. From a farming point of view this is a loss, and it happens fastest in waterlogged soils where oxygen is short. Ploughing and draining land reduces it.
What examiners say about this topic
A principal examiner report for Edexcel International GCSE Biology describes a challenging question on denitrifying bacteria and river pollution. Strong answers stated that the bacteria convert nitrates into nitrogen, so there would be less risk of eutrophication in the rivers. Some candidates simply stated that the nitrate runoff would reduce, and the report notes that this phrase was given in the text.
Repeating wording from the question earns nothing, because it demonstrates no knowledge. Name the conversion the bacteria carry out, then name the consequence. The report also records that an example of the pollution, such as algal growth, was wanted.
Putting the cycle together
Plants absorb nitrates through their roots and use them to make proteins. Animals eat plants and use the amino acids to make their own proteins. Both eventually die, and animals produce urea in the meantime, so decomposers release ammonium compounds. Nitrifying bacteria turn those into nitrates, and the plants take them up again. Denitrifying bacteria drain nitrogen out of the loop, and nitrogen fixing bacteria feed it back in.
If a question asks you to describe the cycle, follow the nitrogen atom rather than listing the organisms. Where it starts, what changes it, and where it ends up.
Spec 4.11B
What you need to know
- Explain why organisms need nitrogen
- Explain the roles of microorganisms in the cycle
- Describe the complete nitrogen cycle
Active recall
Quick check
Answer each question before opening the answer.
What do nitrogen-fixing and denitrifying bacteria do?
Nitrogen-fixing bacteria convert nitrogen gas into nitrogen compounds/nitrates; denitrifying bacteria convert nitrates back into nitrogen gas.
What is the role of decomposers in the nitrogen cycle?
They break down proteins and urea, releasing ammonium compounds (which nitrifying bacteria turn into nitrates).
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