A compost heap contains kitchen waste such as vegetable peelings and fruit scraps. A gardener notices that the compost heap at the bottom, which is several months old, has been fully broken down into a dark, crumbly material, whilst the freshly added material at the top has not yet decomposed. Explain why the rate of decomposition is greater in the lower part of the compost heap than in the freshly added material at the top.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Compost heaps are used by gardeners to recycle organic waste. Decomposers, including bacteria and fungi, break down dead organic material. The lower layers of a compost heap are older and more compact, whilst the top layer consists of freshly added material exposed to the open air.
Model answer (5 marks)
Decomposers such as bacteria and fungi secrete enzymes that hydrolyse the complex organic molecules in the waste.
The lower layer has been present for several months, giving the microbes more time to act; therefore more material has been broken down.
Heat is produced by microbial respiration in the dense lower part of the heap, raising the temperature and enhancing enzyme activity.
Because the lower layer is compacted, it contains less oxygen and is more anaerobic, conditions that favour the growth of many decomposers.
Moisture is retained in the lower part, providing the water required for enzyme function, whereas the top layer dries out more quickly.
The lower layer has been present for several months, giving the microbes more time to act; therefore more material has been broken down.
Heat is produced by microbial respiration in the dense lower part of the heap, raising the temperature and enhancing enzyme activity.
Because the lower layer is compacted, it contains less oxygen and is more anaerobic, conditions that favour the growth of many decomposers.
Moisture is retained in the lower part, providing the water required for enzyme function, whereas the top layer dries out more quickly.
Examiner tips
- Use the word "secrete" for enzymes, not just "produce"; mention both bacteria and fungi. Show the chain: longer time → more decomposition; heat → higher enzyme rate; anaerobic conditions → more decomposers; moisture retention → suitable conditions. Keep each point brief and directly linked to a mark scheme point.
Common mistakes
- Confusing the top layer as the faster decomposing part; forgetting to mention heat or anaerobic conditions. Using vague terms like "more microbes" without specifying bacteria/fungi or enzyme activity. Omitting the time factor or moisture detail, which are explicit marks.
Mark scheme (5 marks)
- Decomposers (bacteria and/or fungi) break down the organic material by secreting enzymes
- The lower layer has had longer to be acted upon by decomposers, so more decomposition has occurred over time
- The lower layer is warmer due to heat released by microbial respiration, increasing the rate of enzyme activity
- The lower layer has less oxygen / is more anaerobic because it is compacted, whereas the top layer has more oxygen available
- Moisture / water is retained in the lower layer, providing suitable conditions for decomposer activity
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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