The nitrogen cycle Cambridge IGCSE Biology (9–1) revision
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In plain words
Air is nearly four fifths nitrogen, and every protein needs nitrogen, yet plants and animals can't use the gas at all. Bacteria are the go-betweens: some turn nitrogen gas into a form plants can take up, some recycle it from dead things, and some send it back to the air.
Three things to know
- Plants take in nitrate ions from the soil and use them to make amino acids and proteins. Animals get their nitrogen by eating plants or other animals.
- Decomposers break down dead organisms and waste into ammonium ions. Nitrifying bacteria turn ammonium ions into nitrites and then nitrates.
- Nitrogen-fixing bacteria (in the soil and in the root nodules of peas and beans) turn nitrogen gas into compounds plants can use; lightning does a little of this too. Denitrifying bacteria turn nitrates back into nitrogen gas.
Tips and tricks
- Nitrifying bacteria make nitrates. Denitrifying bacteria destroy them. Nitrogen-fixing bacteria use nitrogen gas.
- Plants take in nitrates, never nitrogen gas.
It lands in your notebook with its questions as flashcards.
The nitrogen cycle: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
In what form do plants take in nitrogen?
They absorb nitrates from the soil through their roots.
What do nitrogen-fixing bacteria do?
Some live in the root nodules of peas and beans.
What do denitrifying bacteria do?
They remove nitrates from the soil.
What do nitrifying bacteria produce?
They convert ammonium ions to nitrites and then nitrates.
What do plants use nitrate ions to make?
Proteins contain nitrogen.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
The nitrogen cycle
Cambridge IGCSE Biology (9–1) 0970 · 7 marks · papermunch.org
Name ______________________________ Date ______________
Explain why plants cannot use the nitrogen gas in the air, and how they get their nitrogen.[2]
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Nitrogen gas is unreactive and plants cannot absorb it. They take in nitrate ions from the soil through their roots.
Describe the role of bacteria in turning the protein in a dead animal into nitrate ions.[3]
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Decomposers break down the protein into ammonium ions. Nitrifying bacteria convert the ammonium ions into nitrites and then nitrates.
Explain why farmers grow peas or beans in a field every few years.[2]
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Their root nodules contain nitrogen-fixing bacteria, which add nitrogen compounds to the soil, so less fertiliser is needed.
Answers: The nitrogen cycle
- 1. Nitrogen gas is unreactive and plants cannot absorb it. They take in nitrate ions from the soil through their roots.
- 2. Decomposers break down the protein into ammonium ions. Nitrifying bacteria convert the ammonium ions into nitrites and then nitrates.
- 3. Their root nodules contain nitrogen-fixing bacteria, which add nitrogen compounds to the soil, so less fertiliser is needed.