Explain why the nitrogen cycle is important for plants, and describe how nitrogen from the atmosphere is made available to plants in the soil.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Plants need nitrogen to make proteins (and nucleic acids). Nitrogen gas (N₂) in the atmosphere cannot be used directly by plants. Nitrogen‑fixing bacteria in the soil or in root nodules convert N₂ into ammonia or nitrates, which are then oxidised to usable nitrogen compounds. Plants absorb these nitrates (or ammonium ions) from the soil through their roots.
Examiner tips
- Use the word ‘nitrogen‑fixing bacteria’ to show knowledge of the process.
- Mention that plants absorb nitrates or ammonium ions.
- Show the link between nitrogen compounds and protein synthesis.
- Keep the answer concise and use correct terminology.
Mark scheme (4 marks)
- Plants need nitrogen to make proteins (or nucleic acids / DNA)
- Nitrogen gas in the atmosphere cannot be used directly by plants
- Nitrogen-fixing bacteria (in soil or root nodules) convert nitrogen gas into ammonia / nitrates / nitrogen compounds
- Plants absorb nitrates (or nitrogen compounds / ammonium ions) from the soil through their roots
Key terms in this question
Related
- All WJEC GCSE Biology (Wales) revision notes →
- How to answer a "Explain and Describe" question →
- Decode the mark scheme abbreviations →