A farmer stores freshly cut silage (grass) in sealed, airtight bales. After several months, the silage is opened and the grass material is largely unchanged, with very little decomposition having taken place. Explain why sealing the silage in airtight bales greatly reduces the rate of decomposition.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 5.5 Decomposition and decay · Explain · 5 marks · View as Markdown

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

Silage is a method of preserving grass by packing it tightly into sealed, airtight plastic bales. Farmers rely on this technique to store animal feed for winter months. The sealed bales prevent decomposers from breaking down the grass rapidly.

Model answer (5 marks)

Decomposers such as bacteria and fungi break down plant material.
1. Sealing the bale removes oxygen, creating anaerobic conditions.
2. Most decomposers need oxygen for aerobic respiration to generate ATP.
3. Without oxygen they cannot respire aerobically, so their growth and metabolic activity are greatly reduced.
4. Consequently, the enzymes they normally produce to digest the grass are not released, or are produced at a very low rate.
5. The reduced enzyme activity slows the decomposition of the silage, keeping the grass largely unchanged.

Examiner tips

  • Use the command word ‘explain’ – give a clear cause‑effect chain.
  • Mention both oxygen removal and its effect on decomposer metabolism.
  • Show the link between reduced enzyme release and slower decomposition.
  • Keep each point brief but complete – 5 marks total.

Common mistakes

  • Confusing aerobic with anaerobic respiration; forgetting that anaerobes can still decompose but much slower.
  • Failing to mention that enzymes are not produced or are produced at a lower rate.
  • Writing a long narrative instead of concise, point‑wise explanation.

Mark scheme (5 marks)

  1. Decomposers (bacteria and fungi) are responsible for breaking down the organic material
  2. Sealing the bale removes/excludes oxygen, creating anaerobic conditions
  3. Most decomposers require oxygen for aerobic respiration to release energy for growth and reproduction
  4. Without oxygen, decomposers cannot respire aerobically, so their metabolic activity / growth is greatly reduced
  5. Therefore, the enzymes produced by decomposers to digest the organic material are not released / are produced at a much lower rate, slowing decomposition

Related

More Decomposition and decay questions

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