Explain how nitrogen from the atmosphere becomes available to plants, and why this process is essential for plant growth.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants need nitrogen to make proteins, but they cannot use nitrogen gas directly from the atmosphere. Certain bacteria play a key role in making nitrogen available to plants.
Model answer (4 marks)
Nitrogen‑fixing bacteria in the soil convert atmospheric N₂ into ammonia (NH₃) or ammonium (NH₄⁺) through the enzyme nitrogenase. The ammonia is oxidised to nitrite (NO₂⁻) and then to nitrate (NO₃⁻), which plants absorb via their roots. The nitrate is incorporated into amino acids, the building blocks of proteins, which are essential for cell structure, enzyme function and growth. Without this conversion, plants would lack the nitrogen needed for protein synthesis, resulting in stunted or poor growth.
Examiner tips
- Use the word "nitrogen‑fixing bacteria" and state the conversion to ammonia or nitrate.
- Show the step of plant uptake via roots and the use of nitrate in amino acid synthesis.
- Explain the consequence for plant growth if the process fails.
Common mistakes
- Confusing nitrogen gas with nitrate directly; failing to mention the bacterial conversion step.
- Omitting the role of nitrate in protein synthesis or the impact on plant growth.
- Using vague terms like "nitrogen" instead of specific compounds such as ammonia, ammonium or nitrate.
Mark scheme (4 marks)
- Nitrogen-fixing bacteria convert nitrogen gas into nitrates (or ammonia/ammonium compounds)
- These nitrates are absorbed by plants through their roots
- Plants use nitrates to make amino acids / proteins
- Without sufficient nitrates, plants cannot make enough protein, leading to poor/stunted growth
Related
- All Eduqas GCSE Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →