4.3.2Ecology & the Environment
Nitrogen Cycle — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions
5 min read 4 parts Lesson 6 of 7 Unit quiz
Section 1 of 4Why Nitrogen Matters
About four fifths of the air is nitrogen gas, N₂, but most organisms cannot use it directly.
Proteins
- Living things need nitrogen to build amino acids and proteins, and also the nitrogenous bases of DNA.
Plants
- Plants absorb nitrate ions from the soil through their roots and use them to make amino acids and proteins.
Animals
- Animals obtain nitrogen by eating plants or other animals, then digesting the proteins they contain.
Bacteria
- Several groups of bacteria carry out the steps that move nitrogen between the air, the soil and living organisms.
Section 2 of 4Stages of the Cycle
Follow nitrogen from the air back to the air.
1
Nitrogen fixation: nitrogen-fixing bacteria in the soil, or in root nodules of legumes such as peas, clover and beans, convert N₂ into ammonium compounds.
2
Lightning and the industrial manufacture of fertilisers also fix nitrogen, adding nitrates to the soil.
3
Plants take in nitrates and make proteins, and the nitrogen then passes to animals along the food chain.
4
Decomposition: decomposers such as bacteria and fungi break down dead organisms and waste, releasing ammonia that forms ammonium ions in soil.
5
Nitrification: nitrifying bacteria convert ammonium ions into nitrites and then into nitrates, which plants can absorb again.
6
Denitrification: denitrifying bacteria convert nitrates back to N₂ gas, which returns to the atmosphere.
Section 3 of 4Bacteria and Farming
Bacteria and Farming
Mutualism in nodules
- Legumes give the bacteria in their root nodules sugars, and the bacteria give the plant fixed nitrogen.
Crop rotation
- Growing a legume crop in a field increases soil nitrates for the crop that follows.
Waterlogging
- Denitrifying bacteria are active in waterlogged, low-oxygen soil, which lowers the nitrate available to crops.
Ploughing
- Ploughing and draining soil adds air and so reduces denitrification, helping to keep nitrates in the soil.
Section 4 of 4Exam Checklist & Tips
4.3.2 – Key Points for the Exam
- Nitrogen fixation The conversion of nitrogen gas into ammonium compounds by bacteria.
- Nitrification The conversion of ammonium ions to nitrites and then nitrates by nitrifying bacteria.
- Denitrification The conversion of nitrates into nitrogen gas by denitrifying bacteria.
- Decomposer A microorganism that breaks down dead material and waste, releasing nutrients.
- Legume A plant with root nodules containing nitrogen-fixing bacteria.
Common Mistakes
- Saying that plants absorb nitrogen gas; they take up nitrate ions.
- Mixing up nitrification, which makes nitrates, with denitrification, which removes them.
- Forgetting that decomposers release ammonia from dead organisms and urea.
- Describing the relationship in root nodules as parasitism; it is mutualism.
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