Describe how nitrogen from a dead animal is returned to the atmosphere as nitrogen gas.

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)

1. Decomposers such as bacteria and fungi break down the protein in the dead animal, releasing ammonium ions (NH₄⁺).
2. Nitrifying bacteria oxidise ammonium to nitrite ions (NO₂⁻) and then to nitrate ions (NO₃⁻).
3. In anaerobic conditions, denitrifying bacteria reduce nitrate to nitrogen gas (N₂).
4. The nitrogen gas is released into the atmosphere, completing the nitrogen cycle.

Examiner tips

  • Use the exact terms: decomposers, ammonium, nitrifying, denitrifying, anaerobic conditions. Show the stepwise conversion from protein to N₂. Mention the environment (anaerobic) for denitrification.

Common mistakes

  • Confusing ammonium with ammonia gas. Omitting the anaerobic requirement for denitrification. Using vague terms like "bacteria" without specifying nitrifying/denitrifying.

Mark scheme (4 marks)

  1. Decomposers (bacteria/fungi) break down protein in the dead animal, producing ammonium ions
  2. Nitrifying bacteria convert ammonium ions into nitrites, then nitrates
  3. Denitrifying bacteria convert nitrate ions to nitrogen gas
  4. Denitrification occurs in anaerobic conditions (without oxygen)

Related

More Nutrient cycles questions

▶ Try answering this question with AI marking (free) →