Decomposition
Practise Decomposition for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Microorganisms decompose material fastest in warm, moist and oxygen-rich conditions — the basis of composting and biogas production.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Decomposition is the breakdown of dead organisms and waste by microorganisms, and it releases the materials locked in them back into the ecosystem.
Three factors affect the rate. Temperature: warmer conditions speed up the enzymes involved, but very high temperatures denature them. Water: moist conditions speed up decay because microorganisms need water. Oxygen: most decomposers respire aerobically, so more oxygen means faster decay.
Gardeners and farmers use these conditions deliberately in compost heaps, providing optimum warmth, moisture and aeration so waste breaks down quickly into compost that returns mineral ions to the soil. Anaerobic decay produces methane gas, which can be used as a fuel in biogas generators.
Revision notes
Factors affecting the rate
Temperature: warmer conditions increase the rate because enzyme-controlled reactions speed up, but very high temperatures denature the enzymes.
Water: moist conditions speed up decay because microorganisms need water. Oxygen: most decomposers respire aerobically, so a good oxygen supply increases the rate.
Compost
Gardeners and farmers provide optimum conditions in compost heaps — warmth, moisture and aeration.
The waste breaks down quickly, and the compost returns mineral ions to the soil, which plants can absorb for healthy growth.
Anaerobic decay and biogas
Anaerobic decay happens without oxygen and produces methane gas.
Biogas generators use this to produce methane as a fuel. They must be kept at a constant warm temperature to keep the microorganisms respiring efficiently.
Key points
- Decomposition is the breakdown of dead material by microorganisms.
- Warmer temperatures increase the rate, up to a point.
- Moist conditions increase the rate.
- Oxygen increases the rate for aerobic decomposers.
- Compost returns mineral ions to the soil.
- Anaerobic decay produces methane gas.
Worked examples
Example 1
Explain why decay is faster in warm conditions. [2 marks]
Model answer
Decomposition is controlled by enzymes in microorganisms (1 mark), and warmer temperatures increase the rate of enzyme-controlled reactions, so the material breaks down more quickly (1 mark).
Example 2
Explain why compost heaps are turned regularly. [2 marks]
Model answer
Turning the heap allows more air to reach the microorganisms (1 mark), providing the oxygen they need for aerobic respiration, which increases the rate of decay (1 mark).
Example 3
Name the gas produced by anaerobic decay and state one use for it. [2 marks]
Model answer
Methane (1 mark), which can be used as a fuel in a biogas generator (1 mark).
Common mistakes
Saying decay is always faster when hotter.
Very high temperatures denature the enzymes, so the rate falls.
Forgetting that decomposers need water.
Moisture is one of the three key factors.
Saying anaerobic decay produces carbon dioxide.
It produces methane, which is why biogas is useful as a fuel.
Not linking turning a compost heap to oxygen.
The reason is aeration for aerobic respiration.
Exam tips
- Learn the three factors and explain each through the microorganisms.
- Mention that very high temperatures denature enzymes.
- Name methane as the product of anaerobic decay.
- Link compost to returning mineral ions to the soil.
Key terms
- Decomposition
- The breakdown of dead organisms by microorganisms.
- Decomposer
- A microorganism that breaks down dead material.
- Compost
- Decayed organic material returning nutrients to soil.
- Anaerobic decay
- Decomposition without oxygen, producing methane.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.