AQA GCSE Biology Foundation (8461), Foundation tier · Ecology › Organisation of an ecosystem
Practise Decomposition. 10 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.
How temperature, water and oxygen affect the rate at which microorganisms decay dead material, how gardeners make compost and how biogas generators make methane. It includes the decay of milk required practical, so expect rate calculations and graph work. This subtopic is in GCSE Biology only, not Combined Science.
Key facts
Faster decay: warm, moist, plenty of oxygen
Too hot → decomposer enzymes denature → decay slows
Compost = decayed waste used as a natural fertiliser
Lipase: fat → fatty acids + glycerol, so the pH falls
Notes
Decomposers
Decomposers are microorganisms, such as bacteria and fungi, that break down dead plants, dead animals and waste.
They secrete enzymes onto the material. The small soluble molecules produced diffuse into their cells.
Their respiration releases carbon dioxide, and mineral ions are returned to the soil.
Factors affecting the rate of decay
diagram
Temperature: warmth speeds up the decomposers' enzyme-controlled reactions, so decay is faster. If it is too hot, the enzymes are denatured and decay slows or stops. Cold slows decay, which is why a fridge keeps food fresh for longer.
Decay speeds up as it gets warmer, until the decomposers' enzymes denature.
Water: decomposers need moist conditions for their reactions and to digest their food. Dry material decays very slowly.
Oxygen: most decomposers respire aerobically, which releases more energy, so they grow and multiply faster. With little oxygen, decay is slower.
Compost and biogas
Gardeners and farmers try to give waste the best conditions for rapid decay: warm, moist and with plenty of oxygen (heaps are turned to let air in).
The finished compost is spread on gardens and fields as a natural fertiliser.
Anaerobic decay (decay without oxygen) produces methane gas.
Biogas generators use the anaerobic decay of waste, such as animal manure or food waste, to produce methane, which is used as a fuel.
Required practical: decay of milk
diagram
Mix milk, sodium carbonate solution (an alkali) and phenolphthalein indicator. The mixture is pink.
Add lipase. It breaks down the fat in milk into fatty acids and glycerol, and the fatty acids lower the pH.
Time how long the pink colour takes to disappear. Use water baths to repeat at different temperatures, warming the milk mixture and the lipase to the test temperature before mixing.
Warm both tubes to the test temperature, mix them and time how long the pink colour takes to go.
Rate = 1 ÷ time (or 1000 ÷ time). A pH probe gives a clearer end point than judging a colour change by eye.
Keep the volumes and concentrations of milk, lipase and sodium carbonate the same.
Don’t lose marks
Saying high temperatures 'kill' enzymes. Enzymes are not alive: they are denatured.
Writing 'biogas' when asked to name the gas. Name methane.
Forgetting to bring the lipase and the milk mixture to the test temperature before mixing them.
Leaving out units in rate answers (e.g. g per day).
More tips
Memory tricks
Decay needs WOW: Warmth, Oxygen, Water.
No oxygen, methane made: anaerobic decay gives a gas you can burn as fuel.
Sense check a rate: a decay rate is a positive number describing how fast mass is lost.
Exam technique
For each factor, name the organisms (decomposers or microorganisms) and the process (enzymes or respiration) in your answer.
In rate questions, check the time units: hours, days or weeks.
The colour change is gradual, so a common improvement is to use a pH probe or data logger.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State what decomposers doMicroorganisms such as bacteria and fungi break down dead organisms and waste.
Grade 4
Name the three factors affecting decay rateTemperature, water (moisture) and the availability of oxygen.
Grade 5
Describe how compost and biogas are producedWaste decays in warm, moist, well-aerated heaps to make compost; anaerobic decay in a biogas generator makes methane.
Required practical:Decay (method, variables and exam tips)
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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