A farmer stores bales of hay outdoors over winter. By spring, the bales that were left uncovered and in contact with the wet ground have largely decomposed, whereas bales stored on raised wooden pallets under a waterproof cover remain intact. Explain why the uncovered bales decomposed much faster than the covered bales.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hay bales are made of dried grass. Decomposers such as bacteria and fungi break down organic material by secreting enzymes onto it and absorbing the products.
Model answer (5 marks)
Decomposers such as bacteria and fungi are responsible for breaking down the organic material in the hay.
The uncovered bales were wet or moist, providing the water required for decomposer activity.
The decomposers secrete extracellular enzymes onto the hay, which hydrolyse the complex polymers into soluble products that the organisms then absorb.
The covered bales were kept dry, so decomposer activity was greatly reduced.
Raising the bales on pallets also reduced contact with soil microorganisms and prevented waterlogging, further limiting decomposition.
The uncovered bales were wet or moist, providing the water required for decomposer activity.
The decomposers secrete extracellular enzymes onto the hay, which hydrolyse the complex polymers into soluble products that the organisms then absorb.
The covered bales were kept dry, so decomposer activity was greatly reduced.
Raising the bales on pallets also reduced contact with soil microorganisms and prevented waterlogging, further limiting decomposition.
Examiner tips
- Use the exact terms ‘decomposers’, ‘extracellular digestion’, and ‘moisture’ to match the mark scheme.
- Show the causal chain: moisture → enzyme activity → decomposition.
- Mention the role of pallets in reducing soil contact and waterlogging.
- Keep the answer concise – 5 short points will earn full marks.
Common mistakes
- Confusing ‘decomposers’ with ‘predators’ or ‘herbivores’. Failing to mention the importance of moisture for enzyme activity. Using vague phrases like ‘more bacteria’ without linking to water or pallets.
Mark scheme (5 marks)
- Decomposers (bacteria and fungi) are responsible for breaking down the organic material in the hay
- The uncovered bales were wet / moist, providing the water needed for decomposer activity
- Decomposers secrete enzymes onto the hay and absorb the soluble products (extracellular digestion)
- The covered bales were kept dry, so decomposer activity was greatly reduced
- Raising the bales on pallets also reduced contact with soil microorganisms / prevented waterlogging, further limiting decomposition
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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