Decomposers play an essential role in the recycling of nutrients in ecosystems. Explain how decomposers break down dead organic matter and describe three environmental conditions that affect the rate of decomposition.

Pearson Edexcel A-Level Biology A (9BI0) — 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).

In a woodland ecosystem, fallen leaves and dead organisms accumulate on the forest floor. Over time, these materials are broken down and their nutrients are returned to the soil.

Model answer (5 marks)

Decomposers such as bacteria and fungi secrete extracellular enzymes onto dead organic matter, breaking down complex molecules into smaller soluble compounds.
These soluble products are then absorbed by the decomposers, while the released nutrients (e.g. nitrogen, phosphorus) become available to plants.
Three environmental conditions that influence the rate of decomposition are:
1. Temperature – higher temperatures (up to an optimum) increase enzyme activity and therefore the rate of decomposition.
2. Moisture – sufficient water is required for metabolic reactions; moist conditions accelerate decomposition.
3. Oxygen – aerobic decomposers are more active when oxygen is available; higher oxygen levels increase the rate of decomposition.

Examiner tips

  • Use the term ‘extracellular enzymes’ and mention bacteria/fungi; link enzyme activity to temperature. Show the three conditions in separate points and explain why each affects the rate. Remember to mention nutrient release into the soil.
  • common_mistakes
  • :
  • Confusing intracellular digestion with extracellular enzyme secretion. Failing to name all three conditions or mixing up oxygen with anaerobic conditions. Using vague phrases like ‘water helps’ without explaining the metabolic link.

Mark scheme (5 marks)

  1. Decomposers (bacteria and fungi) secrete enzymes onto dead organic matter / digest it extracellularly
  2. Soluble products are absorbed by the decomposers / nutrients are released into the soil
  3. Temperature affects rate of decomposition — higher temperatures (up to an optimum) increase the rate because enzyme activity increases
  4. Moisture / water availability affects decomposition — moist conditions increase the rate as decomposers need water for metabolic reactions / enzyme activity
  5. Oxygen / aerobic conditions affect decomposition — more oxygen increases the rate as aerobic decomposers respire aerobically and are more active

Key terms in this question

decomposers · decomposition · nutrients

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