The Nitrogen Cycle — AQA GCSE Biology (Separate)
The nitrogen cycle recycles nitrogen, which organisms need to make proteins and DNA.
Why nitrogen matters
Although the air is about 78% nitrogen gas (N₂), plants and animals cannot use nitrogen gas directly. They need nitrogen in the form of nitrate ions (plants) or proteins (animals). The nitrogen cycle converts nitrogen into usable forms.
The key processes and microorganisms
- Nitrogen-fixing bacteria convert nitrogen gas into nitrogen compounds (nitrates). Some live freely in the soil; others live in root nodules of legumes (peas, beans). Lightning also fixes some nitrogen.
- Decomposers (bacteria and fungi) break down dead organisms and waste (proteins, urea), releasing ammonia.
- Nitrifying bacteria convert ammonia into nitrates (which plants can absorb).
- Denitrifying bacteria convert nitrates back into nitrogen gas (this happens in waterlogged soils and reduces soil fertility).
How nitrogen moves through organisms
Plants absorb nitrates from the soil to make amino acids and proteins. Animals get nitrogen by eating plants or other animals. When organisms die or excrete waste, decomposers return the nitrogen to the soil.
Exam tips
- Plants need nitrates, not nitrogen gas — explain why the cycle is needed.
- Learn the four bacteria: nitrogen-fixing, decomposers, nitrifying, denitrifying and what each does.
- Legume root nodules contain nitrogen-fixing bacteria.
- Nitrogen passes to animals by eating.