Describe how nitrogen from the atmosphere is made available to plants, and explain why plants need nitrogen.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Nitrogen gas (N₂) in the atmosphere is fixed by nitrogen‑fixing bacteria into ammonia (NH₃) or ammonium (NH₄⁺) and then into nitrates (NO₃⁻). The bacteria live either free in the soil or in root nodules of leguminous plants. The nitrates are absorbed by plant roots from the soil. Once inside the plant, nitrogen is incorporated into amino acids, which are assembled into proteins. Nitrogen is also a key component of nucleic acids (DNA and RNA), so it is essential for growth, development and reproduction.
Examiner tips
- Use the term "nitrogen‑fixing bacteria" and mention root nodules or soil; link nitrates to root uptake; state proteins and nucleic acids as the main uses; keep answer concise and use correct terminology.
- Show the sequence: atmospheric N₂ → fixation → nitrates → root uptake → proteins/DNA.
- Include both protein synthesis and DNA synthesis as reasons plants need nitrogen.
Common mistakes
- Confusing nitrogen fixation with nitrification; writing only "nitrogen is fixed" without specifying bacteria or nitrates.
- Omitting that nitrates are taken up by roots or that nitrogen is needed for DNA.
- Using vague phrases like "nitrogen is important" without specifying proteins or nucleic acids.
Mark scheme (5 marks)
- Nitrogen gas in the atmosphere is fixed / converted into nitrates (or ammonia/ammonium compounds) by nitrogen-fixing bacteria
- Nitrogen-fixing bacteria live in the soil or in root nodules of leguminous plants
- Nitrates are absorbed by plant roots (from the soil)
- Plants use nitrogen to make / synthesise proteins
- Plants also use nitrogen to make DNA / genetic material / nucleic acids
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →