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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

📚 Edexcel International GCSE Biology 4BI1 Hub / Topical Revision
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Part 1 of 4Why Nitrogen Matters

4.3.2 Nitrogen Cycle

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.

Part 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.

Part 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.

Part 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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