Explain how decomposers and nitrifying bacteria work together to make nitrogen available for plant uptake.

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

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

Plants require nitrogen-containing compounds to synthesise proteins. Although nitrogen gas makes up approximately 78% of the atmosphere, plants cannot use it directly.

Model answer (4 marks)

Decomposers break down dead plant and animal material, releasing ammonia or ammonium ions into the soil.
Nitrifying bacteria then convert the ammonium ions first into nitrite ions and subsequently into nitrate ions.
Plants absorb the nitrate ions through their roots from the soil.
The absorbed nitrate is used by the plant to synthesise proteins and other nitrogen‑containing compounds.

Examiner tips

Use the exact terms: decomposers, nitrifying bacteria, ammonium, nitrite, nitrate, plant roots, proteins.
Show the sequence: decomposition → ammonium → nitrite → nitrate → plant uptake.
Mention that nitrate is the form taken up by plants.
Keep the answer concise and directly linked to the marks.

Common mistakes

Confusing ammonium with ammonia; not specifying the conversion to nitrate.
Omitting the role of nitrifying bacteria or the step from nitrite to nitrate.
Using vague terms like "nitrogen" instead of "nitrate" for plant uptake.

Mark scheme (4 marks)

Decomposers break down dead plant/animal material (proteins and urea) into ammonia / ammonium ions
Nitrifying bacteria convert ammonium ions into nitrite ions and then nitrate ions (in the soil)
Nitrate ions are absorbed by plant roots (from the soil)
Plants use nitrate ions to synthesise proteins / nitrogen-containing compounds

Key terms in this question

decomposers · nitrifying bacteria