DecompositionSpec 4.7.2.3
In short
Decay is the breakdown of dead biological material by microorganisms. It is faster in warm conditions, up to an optimum, in moist conditions and when oxygen is available, because the microorganisms' enzymes and respiration work faster. If it is too hot, the enzymes denature. Gardeners use these conditions to make compost, and anaerobic decay produces methane gas for fuel.
Decay is the breakdown of dead biological material by microorganisms. The rate of decay depends on the conditions the decomposers are in.
| Factor | Effect on the rate of decay |
|---|---|
| Temperature | Warmer conditions speed up decay, because the enzymes and the respiration of the microorganisms work faster, until an optimum. If it is too hot, the enzymes denature and decay slows. In cold conditions, decay is slow |
| Water | Microorganisms need water to grow and be active. More water (moist conditions) means faster decay. Dry material decays slowly |
| Availability of oxygen | Most decomposers need oxygen for aerobic respiration, so more oxygen means faster decay. With little or no oxygen, decay is slower |
Compost and biogas
Gardeners and farmers try to provide optimum conditions for rapid decay of waste biological material. The compost produced is used as a natural fertiliser for growing garden plants or crops.
Anaerobic decay (decay without oxygen) produces methane gas. Biogas generators can be used to produce methane gas as a fuel.
Calculating rate changes
Rate of decay
The mass of dead leaves in a compost heap falls from 80 g to 56 g in 12 days. Calculate the mean rate of decay.
- Change in mass = 80 − 56 = 24 g.
- Rate = change in mass ÷ time = 24 g ÷ 12 days = 2 g per day.
Answer: 2 g per day
You may also be asked to translate information between numerical and graphical form, and to plot a graph, for example mass of material or pH against time. Choose a scale that uses more than half the grid and label the axes with units.
Required practical 10: investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change. The method is below.
- Put a set volume of milk into a test tube. Add a set volume of sodium carbonate solution, which makes it alkaline, and a few drops of phenolphthalein indicator, which is pink in alkaline conditions.
- Put this tube, and a separate tube of lipase solution, into a water bath at the chosen temperature. Leave them until they reach that temperature.
- Add a set volume of lipase to the milk, stir and start a stopwatch.
- Stop the stopwatch when the pink colour disappears. Lipase breaks down the fat in milk into fatty acids, which lower the pH.
- Repeat at a range of temperatures, for example 20, 30, 40 and 50 °C, and repeat each temperature.
- Calculate the rate for each temperature as 1 ÷ time taken.
Another version leaves fresh milk at different temperatures and measures its pH at regular intervals with a pH probe. As bacteria in the milk decay it, acid is produced and the pH falls.
The independent variable is the temperature. The dependent variable is the time taken for the colour to change (or the change in pH). Control variables: the volume of milk, the volume and concentration of lipase and sodium carbonate, and the number of drops of indicator.
A typical result is that the rate increases as the temperature rises, up to an optimum. Above the optimum the rate falls, because the enzymes denature.
Safety: wear eye protection, do not taste the milk, wash your hands afterwards and take care with hot water baths.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026