A compost heap contains dead plant material, food scraps and animal waste. Explain why the rate of decomposition in a compost heap is much faster in warm, moist conditions than in cold, dry conditions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Gardeners often add water to their compost heaps and position them in sunny spots to speed up the breakdown of organic material.
Model answer (5 marks)
Decomposers such as bacteria and fungi break down the organic material in the heap.
They use enzymes to hydrolyse complex molecules into simpler compounds.
Enzyme activity rises with temperature, so in warm conditions the reactions occur more rapidly and decomposition speeds up.
Moisture is required for the enzymes to function and for the diffusion of substrates into the cells and products out of them; therefore, a moist heap allows decomposers to work more efficiently.
In cold, dry conditions enzyme activity is low and water is unavailable for diffusion, so decomposer activity is reduced and decomposition is much slower.
They use enzymes to hydrolyse complex molecules into simpler compounds.
Enzyme activity rises with temperature, so in warm conditions the reactions occur more rapidly and decomposition speeds up.
Moisture is required for the enzymes to function and for the diffusion of substrates into the cells and products out of them; therefore, a moist heap allows decomposers to work more efficiently.
In cold, dry conditions enzyme activity is low and water is unavailable for diffusion, so decomposer activity is reduced and decomposition is much slower.
Examiner tips
- Use the exact terms – decomposers, enzymes, temperature, moisture. Explain the link between temperature and enzyme activity. Mention both enzyme function and diffusion. Show the cause–effect chain: warm/moist → faster activity → faster decomposition.
Common mistakes
- Confusing the role of decomposers with that of the plant material. Forgetting to mention moisture’s role in diffusion. Using vague terms like "more active" without linking to enzymes or temperature.
Mark scheme (5 marks)
- Decomposers (bacteria / fungi) are responsible for breaking down the organic material
- Decomposers use enzymes to break down organic material
- Warmer temperatures increase the rate of enzyme activity, so decomposition is faster
- Moist conditions allow decomposers to be more active / allow enzymes to function / allow diffusion of materials into and out of decomposers
- In cold, dry conditions decomposer activity is reduced / enzymes are less active, so decomposition is slower
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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