AQA GCSE Biology (8461), Higher tier · Ecology › Organisation of an ecosystem
Practise Decomposition. 18 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Required practical:Decay (method, variables and exam tips)
Quick recall
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Anaerobic decay produces a gas that can be used as a fuel. Name this gas.
Methane
Students investigated the effect of temperature on the rate of decay of fresh milk. They mixed milk, sodium carbonate solution and phenolphthalein indicator in a test tube, and then added lipase solution. They timed how long it took for the pink colour to disappear as the pH fell. What is the independent variable in this investigation?
Temperature
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Gardeners make compost from waste plant material.
(a) Which two conditions increase the rate of decay in a compost heap? Tick (✓) two boxes.[2]
A supply of oxygen
Dry conditions
Warm temperature
Very low temperature
No air
(b) What do gardeners use compost for?[1]
(c) Anaerobic decay produces a gas that can be used as a fuel. Name this gas.[1]
(d) Name the equipment used to produce this gas as a fuel from waste material.[1]
Show the answer and mark scheme
(a)Answer: A supply of oxygen; Warm temperature
(b)
as a (natural) fertiliser (for growing plants)
(c)Answer: Methane
methane
(d)Answer: Biogas generator
biogas generator
Question 2Medium7 marks
A company sells a compost bin with these features:
it is made of dark-coloured plastic
it has a lid
there are small holes in the sides
it has no base, so the waste sits directly on the soil.
(a) Explain how each feature helps to increase the rate of decay.[4]
(b) A gardener put a thick layer of grass cuttings into the bin and did not mix it. After a few weeks the grass in the middle of the layer was slimy and smelled unpleasant. Suggest why.[2]
(c) Suggest why gardeners should mix the compost regularly.[1]
Show the answer and mark scheme
(a)
dark colour absorbs more heat / keeps the contents warm, so the enzymes of microorganisms work faster
the lid keeps moisture in / stops the contents drying out or getting waterlogged (and keeps heat in)
the holes let in air / oxygen for aerobic respiration by the decomposers
no base lets decomposers (e.g. microorganisms, earthworms) move in from the soil
(b)
the middle of the layer had no oxygen / was anaerobic (grass cuttings packed tightly together)
so anaerobic decay happened, which is slower and produces smelly gases / methane
(c)
to let air / oxygen in, so aerobic decay continues
Question 3Hard8 marks
Milk contains fat. When milk decays, the enzyme lipase breaks down the fat into fatty acids and glycerol. This lowers the pH. You are given: fresh milk; lipase solution; sodium carbonate solution (alkaline); phenolphthalein indicator, which is pink in alkaline solution and turns colourless when the pH falls; water baths; test tubes; syringes; thermometers and a stop-clock.
(a) Plan an investigation to find the effect of temperature on the rate of decay of milk.[6]
(b) A student also set up a tube containing milk, sodium carbonate solution and phenolphthalein, but added water instead of lipase. Explain why.[2]
Show the answer and mark scheme
(a)
use a syringe to measure a fixed volume of milk and of sodium carbonate solution into a test tube; add a set number of drops of phenolphthalein (the mixture is pink)
put this test tube and a tube of lipase solution into a water bath at the chosen temperature
wait until both reach the temperature of the water bath (check with a thermometer)
add a fixed volume of lipase, start the stop-clock and stir
stop the clock when the pink colour disappears
repeat at a range of temperatures, e.g. 10, 20, 30, 40, 50 °C
repeat each temperature at least three times and calculate a mean
calculate rate = 1000 ÷ time and plot rate against temperature
keep volumes and concentrations of milk, lipase and sodium carbonate the same
Marked with levels of response: the full level descriptors are in the app.
(b)
as a control
to show that the change in pH / colour is caused by the lipase (and not by something else, e.g. microorganisms already in the milk)