This topic summary covers Knowledge Organiser within Decomposition for GCSE Biology. Topic 4: Decomposition It is section 11 of 11 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 11 of 11
Practice
15 questions
Recall
20 flashcards
Knowledge Organiser
Key Terms
- Decomposer: A microorganism (bacterium or fungus) that breaks down dead organic matter by secreting enzymes externally and absorbing the soluble products
- Detritivore: An animal that feeds on dead and decaying organic material (e.g. earthworm, maggot) — physically breaks material into smaller pieces, increasing surface area for decomposers
- Saprotrophic nutrition: Feeding by secreting digestive enzymes onto food outside the body, then absorbing the soluble products — used by all fungi and many bacteria
- Compost: Decomposed organic matter used as a soil improver — adds mineral ions and improves soil structure
- Biogas: A mixture of methane (CH₄) and carbon dioxide produced by anaerobic decomposition of organic waste
TOMP Factors (RPA6/RPA10)
- Temperature: Rate increases with temperature up to ~37°C optimum; above this, enzymes in decomposers begin to denature and rate falls
- Oxygen: Most decomposers are aerobic — they need oxygen for respiration; waterlogged (anaerobic) conditions severely slow decomposition
- Moisture: Water is needed for enzyme activity and for decomposers to survive; dry conditions slow decomposition
- pH: Neutral pH is optimal; acidic or alkaline conditions reduce enzyme activity
- Peat bogs: anaerobic + acidic → both factors combine to give extremely slow decomposition → ancient carbon stored for thousands of years
- Grade 7+ separator: In the required practical (RPA10), as temperature increases, the rate of decomposition follows an enzyme-activity curve — not a straight line. Students who describe a linear increase throughout will lose the mark; the correct answer describes an increase then a peak then a decrease (denaturation).
Must-Know Facts
- Both bacteria AND fungi act as decomposers
- Decomposers use saprotrophic nutrition — external enzyme secretion then absorption
- Decomposition returns mineral ions (e.g. nitrates) to the soil for plant uptake
- Compost heap: warm + moist + well-aerated = fastest decomposition
- Biogas generator: sealed, anaerobic conditions needed to produce methane
- Without decomposers, dead matter would accumulate and mineral ions would not be recycled
Common Mistakes
- Naming only bacteria as decomposers: Both bacteria AND fungi are decomposers. When asked to name examples of decomposers, a single-word answer of "bacteria" scores 1 mark; "bacteria and fungi" scores both marks. Never omit fungi.
- Writing "decomposers eat dead material": Decomposers do not ingest food. They secrete enzymes onto dead material externally (outside their body), then absorb the soluble digestion products. The key mark-scheme phrase is "external enzyme secretion" or "saprotrophic nutrition."
- Saying decomposition always produces CO₂: Aerobic decomposition releases CO₂. Anaerobic decomposition (e.g. in a biogas generator or waterlogged soil) releases methane (CH₄). If the question specifies anaerobic conditions, the answer must be methane, not CO₂.
- Describing temperature effect as always "faster with more heat": At very high temperatures, the decomposers' enzymes denature — the active site changes shape and the rate falls to zero. The correct description is: rate increases with temperature up to an optimum, then decreases sharply as enzymes denature. A straight-line answer will not score full marks.
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Practice Questions for Decomposition
Which organisms are the main decomposers?
Explain how temperature affects the rate of decomposition.
Quick Recall Flashcards
What is decomposition and who carries it out?
Decomposition is the breakdown of dead organisms and waste into simpler substances.
Carried out by decomposers: mainly bacteria and fungi.
This releases nutrients back into the soil for plants to use.
How do decomposers feed? (saprotrophic nutrition)
Decomposers use saprotrophic nutrition:
1. Secrete enzymes onto dead material (extracellular digestion)
2. Enzymes break down large molecules
3. Decomposer absorbs the small soluble products
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