Explain how the nitrogen found in a dead animal's body is eventually made available to plants growing in the soil.
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 in the dead animal, releasing ammonia (NH₃) or ammonium (NH₄⁺) into the soil. Nitrifying bacteria then convert the ammonia/ammonium into nitrate (NO₃⁻) through nitrification. Plants absorb the nitrate from the soil through their roots, making the nitrogen available for growth.
Examiner tips
- Use the exact terms: decomposers, ammonia/ammonium, nitrifying bacteria, nitrate, roots.
- Show the sequence clearly: decomposition → ammonia → nitrification → plant uptake.
- Mention the role of bacteria in each step.
- Keep the answer concise to fit the 4‑mark limit.
Common mistakes
- Confusing ammonium with nitrate as the first product of decomposition.
- Omitting the role of nitrifying bacteria.
- Using vague terms like ‘microbes’ instead of ‘decomposers’ or ‘bacteria’.
Mark scheme (4 marks)
- Decomposers / bacteria / fungi break down the proteins in the dead animal
- This releases ammonia / ammonium compounds into the soil
- Nitrifying bacteria convert ammonia / ammonium compounds into nitrates
- Plants absorb the nitrates from the soil through their roots
Related
- All WJEC GCSE Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →