Explain how nitrogen from the atmosphere becomes available to a plant as nitrates in the soil.

Pearson Edexcel International GCSE Biology (4BI1) — 4.3 Cycles within ecosystems · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. Atmospheric nitrogen (N₂) is fixed by nitrogen‑fixing bacteria.
2. These bacteria live in the soil or in root nodules of legumes.
3. They convert N₂ into ammonia (NH₃) or ammonium (NH₄⁺).
4. Nitrifying bacteria then oxidise NH₄⁺ to nitrate (NO₃⁻), which the plant roots absorb.

Examiner tips

Use the exact terms: nitrogen‑fixing bacteria, nitrifying bacteria, ammonia, nitrate, root absorption.
Show the sequence of steps in order.
Mention both soil and root‑nodule habitats.
Keep each point brief but complete.

Common mistakes

Confusing nitrogen fixation with nitrification; mixing up ammonia and nitrate.
Omitting the role of nitrifying bacteria.
Using vague language such as ‘bacteria’ without specifying nitrogen‑fixing or nitrifying.

Mark scheme (4 marks)

Nitrogen-fixing bacteria convert atmospheric nitrogen gas into nitrates (or nitrogen-containing compounds)
Nitrogen-fixing bacteria may live in the soil or in the root nodules of plants
Nitrifying bacteria convert ammonia (in the soil) into nitrates
Nitrates are absorbed by (the roots of) the plant

Key terms in this question

nitrates