Inheritance and ecology · GCSE Biology
Decay and decomposition
Explain decay for GCSE Biology: bacteria and fungi respire dead material, releasing minerals and CO₂ fastest when it is warm, moist and aerobic, and apply that to compost, food storage and the carbon cycle.
Decomposers are microorganisms. Fast decay: warm (enzyme optimum), moist (so enzymes can work in solution), oxygen (aerobic respiration). Compost heaps are managed decay. Fridges slow it.
The important bits
What you need to know
- 1
Decomposition is the breakdown of dead organisms and waste by microorganisms (bacteria and fungi) that secrete enzymes, digest the material, and absorb the products.
- 2
Decomposers respire: aerobically they produce CO₂ and water and transfer more energy; anaerobically they may produce methane (landfill, waterlogged soil). This returns carbon to the cycle.
- 3
Mineral ions such as nitrate are released into the soil and can be taken up again by plants (active transport in roots) — recycling that keeps producers growing.
- 4
Rate of decay increases with temperature up to the enzyme optimum (often around 30–40 °C for compost microbes), then falls if enzymes denature. This is an enzyme graph in an ecological setting.
- 5
Moisture is needed so enzymes can diffuse and cells can function. Dry conditions preserve (dried food, mummies, seeds). Waterlogging can cut oxygen and slow aerobic decay, or shift it to anaerobic.
- 6
Oxygen speeds aerobic decay. Turning a compost heap aerates it, reduces methane, and raises temperature as respiration is exothermic. A packed, wet heap goes anaerobic and smelly.
- 7
Food storage slows decay: fridge (low temperature, slow collisions), freezer (water unavailable, very slow), salt or sugar (low water potential, osmosis draws water out of microbes), canning (heat denatures enzymes, seal keeps oxygen and microbes out).
- 8
The decay required practical (AQA): often milk and lipase, or compost temperature, investigating how temperature affects the rate of decay — independent variable temperature, dependent a colour change or pH, controls volumes and concentrations.
Quotations worth analysing
Short evidence. Real method.
“Decay is fastest in warm, moist, aerobic conditions.”
Three conditions, three reasons: enzymes, solution/osmosis, oxygen for aerobic respiration. Do not list them without reasons.
“Decomposers release mineral ions and carbon dioxide.”
Ions to soil for plants; CO₂ to air for photosynthesis. Decay joins ecology to bioenergetics.
“Refrigeration slows decay by slowing enzyme-controlled reactions.”
Cold does not usually denature the enzymes; it slows collisions. Freezing also makes water unavailable.
Go deeper
How do I explain a compost-heap temperature curve?
At the start, mesophilic bacteria respire the easy sugars; the heap warms because respiration is exothermic. If it reaches 50–60 °C, different microbes (thermophiles) take over; weed seeds and some pathogens die — useful. If it goes hotter still, enzymes denature and the rate can fall until the heap is turned and oxygen returns. Moisture must stay like a wrung-out sponge: too dry, enzymes cannot work; too wet, air spaces flood and the heap goes anaerobic (methane, acids, smell). Carbon-rich brown material mixed with nitrogen-rich green material feeds a balanced microbial diet. In the exam, treat the heap as a large enzyme practical: IV could be turning (oxygen), water, or insulation; DV is temperature or the time for a mass of waste to disappear. Link heat to respiration, not to “the compost being alive” as a vague phrase.
Go deeper
How does decay join the carbon cycle to mineral cycles?
Carbon in cellulose and protein is respired to CO₂ (or methane). Nitrogen in proteins is released as ammonium then converted by other bacteria to nitrate (nitrification) — Triple may want the names; Combined wants “minerals returned to the soil”. Plants then take up nitrate by active transport and rebuild amino acids with glucose from photosynthesis. If decay stops (frozen soil, desert), nutrients stay locked in dead matter and growth slows. Rainforests recycle so fast that the soil itself is poor — nutrients are in the living biomass. Cut the forest, heavy rain leaches the last ions, and the system does not recover quickly. That is a biodiversity plus decay plus active-transport story in one.
Go deeper
What should I say in the milk-decay or lipase practical?
If you use lipase on milk with an indicator (phenolphthalein going colourless as fatty acids lower pH), the independent variable is often temperature. Dependent: time to a colour change, then rate = 1/time. Controls: milk volume, lipase volume, pH buffer if used, indicator volume. Water bath for temperature. Expect a peak near the enzyme optimum and a crash when lipase denatures — the same graph as the amylase practical, now labelled decay. Evaluation: colour change is subjective; a pH probe is better; milk batches differ. Safety: do not taste; water baths scald. If the practical is burying bags of leaf litter, mass loss over weeks is the DV, and you must retrieve bags from comparable sites. State that microorganisms caused the mass loss, not “the soil ate it”.
See the idea in action
Garden waste in a turned, moist heap at about 35 °C decays quickly: bacterial and fungal enzymes digest cellulose and proteins, aerobic respiration releases CO₂ and heat, and nitrates return to the soil for plants. The same waste in a sealed, waterlogged bin decays slowly and anaerobically, producing methane. Milk in a fridge at 4 °C lasts longer because lipase and microbial enzymes collide less often — they are not denatured, so warming the milk later lets decay resume. Boiling then sealing (canning) denatures enzymes and excludes new microbes, so decay stays stopped.
Exam technique
Turn knowledge into marks
Always name microorganisms and enzymes. Give warm, moist, aerobic with a reason each. Compost: turn for oxygen. Fridge: slow collisions. Link decay to CO₂ (carbon cycle) and to nitrate (plant growth). Rate = 1/time if you time an end-point.
Common mistakes
Do not give these marks away
- 01
Saying decay just “happens” without microorganisms or enzymes.
- 02
Claiming cold denatures compost enzymes, or that a fridge kills all bacteria.
- 03
Forgetting that anaerobic decay still occurs and can produce methane in landfill.
Which conditions make decay fastest?
ACold, dry and no oxygen
BWarm, moist and aerobic
CBoiling water with no microbes
DFrozen and salty
Show the answer
Warm, moist and aerobic. Warmth speeds enzyme collisions (up to the optimum), moisture lets enzymes work, and oxygen allows aerobic respiration of decomposers. Cold, dry, frozen or sterile conditions slow or stop decay.
Quick questions
If this is the bit you searched
What causes decay in GCSE Biology?
Microorganisms (bacteria and fungi) secrete enzymes that digest dead material. They respire the products and release carbon dioxide and mineral ions.
Why does a compost heap get warm?
Decomposer respiration is exothermic, so energy is transferred as heat. Turning the heap supplies oxygen for faster aerobic respiration.
How does a fridge slow food decay?
Low temperature slows enzyme-controlled reactions in microbes. It does not usually denature the enzymes, which is why food still spoils if left out later.
How does decay help plants?
It releases mineral ions such as nitrate into the soil so roots can take them up by active transport and make amino acids. It also returns CO₂ for photosynthesis.