Describe how nitrogen in the proteins of a living plant becomes available to other plants as nitrate ions in the soil.

Cambridge International IGCSE Biology (0610) — 19.3 Nutrient cycles · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Decomposers such as bacteria and fungi break down the proteins of dead plant material, releasing ammonium ions (NH4+). Nitrifying bacteria then oxidise the ammonium first to nitrite ions (NO2−) and subsequently to nitrate ions (NO3−). The nitrate ions are taken up by plant roots from the soil and incorporated into amino acids, which are used to synthesise new proteins.

Examiner tips

Use the sequence: decomposers → ammonium → nitrifying bacteria → nitrate → plant uptake → protein synthesis
Mention the key organisms (bacteria/fungi, nitrifying bacteria) and the ions involved
Keep the answer concise and use the exact terms (ammonium, nitrite, nitrate, amino acids)

Common mistakes

Confusing ammonium with nitrate as the final product
Omitting the role of nitrifying bacteria
Using vague terms like ‘decomposition’ without specifying the ions produced

Mark scheme (4 marks)

Decomposers (bacteria/fungi) break down proteins in dead plant material, producing ammonium ions
Nitrifying bacteria convert ammonium ions to nitrite ions, and then to nitrate ions
Nitrate ions are absorbed by plant roots from the soil
Plants use nitrate ions to make amino acids / proteins

Key terms in this question

nitrate ions