Describe how nitrogen-fixing bacteria and nitrifying bacteria contribute to making nitrogen available for plant growth.

Edexcel GCSE Biology (1BI0) — 9.1 Ecosystems · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Nitrogen‑fixing bacteria
1. Convert atmospheric N₂ into ammonia (NH₃) or ammonium (NH₄⁺) in root nodules or soil.

Nitrifying bacteria
2. Oxidise NH₄⁺ to nitrite (NO₂⁻) and then to nitrate (NO₃⁻) in the soil.

Plant uptake
3. Roots absorb NO₃⁻ from the soil.

Use by plants
4. NO₃⁻ is incorporated into amino acids and proteins, supporting plant growth.

Examiner tips

Use the exact terms – ‘nitrogen‑fixing bacteria’, ‘nitrifying bacteria’, ‘ammonia’, ‘nitrite’, ‘nitrate’, ‘root uptake’, ‘amino acids’
Show the sequence of transformations in the correct order
Mention the role of plant roots in absorbing nitrate
Keep each point concise – one sentence per point

Common mistakes

Confusing ammonia with ammonium, or omitting the ammonium step
Failing to mention that plants absorb nitrate, not ammonium
Using vague language such as ‘nitrogen compounds’ instead of specific ions

Mark scheme (4 marks)

Nitrogen-fixing bacteria convert atmospheric nitrogen gas into ammonia (nitrogen-containing compounds)
Nitrifying bacteria convert ammonium ions into nitrite and then nitrate ions (in the soil)
Plants absorb nitrate ions from the soil (through their roots)
Plants use the nitrogen-containing compounds (nitrates) to make proteins/amino acids for growth

Key terms in this question

nitrogen-fixing bacteria · nitrifying bacteria