Decomposition and decay: GCSE Biology
Decay, factors, compost practical, gardeners and biogas generators
The short version
Decomposers break down dead material; temperature, water and oxygen change the rate; compost and biogas use decay.
Key idea
Decay is a biological process, not magic vanishing
Grade 2–3: Rotting is microbes eating the dead plant or animal.
Grade 5–6: I can name bacteria/fungi and CO2.
Grade 9: I mention extracellular enzymes and mineral recycling, and waterlogged peat as slow decay (carbon locked up).
Go a bit deeper
Grade 2–3: Cold, dry, sealed food lasts. A warm wet compost heap does not.
Grade 9: I use denaturing for overheating a compost heap, not "the microbes die of boredom", and I distinguish aerobic vs anaerobic.
Question: Explain why food lasts longer in a fridge. [2 marks] [2 marks]
Worked example
Worked example
Grade 2–3: Rotting is microbes eating the dead plant or animal. Grade 5–6: I can name bacteria/fungi and CO2. Grade 9: I mention extracellular enzymes and mineral recycling, and waterlogged peat as slow decay (carbon locked up).
Lessons in this topic
- Decay
- Factors affecting decay
- Required practical: decay
- Compost and gardeners
- Biogas
In the app each lesson is video (when we have a real YouTube id) then intro, learn and summary steps.
What the examiner wants
Explain factors affecting decay, describe a rate-of-decay practical, and apply composting and biogas to resource questions.
Try these questions
Cover the answers and have a go first. Worked solutions are underneath.
The main decomposers are
Decomposers return carbon to the air mainly as
Decay is fastest when conditions are
Turning a compost heap adds
Common mistakes
- Decay is not "the smell". Smell can happen; the biology mark is microbes + respiration + recycling.
- Boiling kills microbes (cooking); refrigeration only slows them. Do not say a fridge sterilises.
- Do not incubate unknown rotting food at 37 °C as if it were the microorganism RP. If you culture microbes, school incubators are 25 °C, not 37 °C.
- Compost is not fertiliser made in the Haber process. It recycles what was already in the plants.
Answers and working
Frequently asked questions
Is GCSE Decomposition and decay on Combined Science and Triple?
Decomposition and decay is Triple-only Biology. Combined Science students can skip it.
Which exam board is this Decomposition and decay page for?
The ideas are board-agnostic for AQA, Edexcel, OCR, WJEC and CCEA. Paper lengths and question numbers differ — check your specification.
Is Decomposition and decay Foundation or Higher?
The core ideas appear on both tiers. Higher papers add extra application and some HT-only detail, marked in lessons with a Higher badge.
Learn, try, get feedback, fix the mistake
Open the stepped lesson, then the quiz with hints. Required-practical and calculation questions use the same method you will need in the paper.
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