7.9 Decomposition
6 exam questions worth 13 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Give three conditions that increase the rate of decay.
Mark scheme
- warm temperature (1)
- moisture / water (1)
- oxygen (1)
Model answer
Warmth, moisture and oxygen.
Explain why turning a compost heap speeds up decay.
Mark scheme
- lets in oxygen / air (1)
- for aerobic respiration of decomposers / microorganisms (1)
Model answer
Turning the heap lets in oxygen, which the decomposers need for aerobic respiration, so they work faster.
Explain why food lasts longer in a fridge.
Mark scheme
- low temperature (1)
- slows enzymes / growth / reproduction of microorganisms / decomposers (1)
Model answer
The cold slows the enzymes and growth of the microorganisms that cause decay.
What gas is produced in a biogas generator, and what is it used for?
Mark scheme
- methane (1)
- fuel / burned for heating, cooking or electricity (1)
Model answer
Methane, which is burned as a fuel.
A bag of leaves had a mass of 120 g. After 20 days, its mass was 84 g.
Calculate the mean rate of decay in g per day.
Mark scheme
- 120 − 84 = 36 (1)
- 36 ÷ 20 = 1.8 (g per day) (1)
Model answer
Change in mass = 120 − 84 = 36 g. Rate = 36 ÷ 20 = 1.8 g per day.
Explain why decay is very slow at 70 °C.
Mark scheme
- enzymes (of decomposers) are denatured (1)
- so microorganisms cannot break down the material / are killed (1)
Model answer
At 70 °C the decomposers' enzymes are denatured, so the microorganisms cannot break the material down.
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