Decomposition and the Rate of Decay

When a plant or animal dies, its body does not simply vanish. It is broken down by decomposers — microorganisms, specifically bacteria and fungi, that feed on dead and decaying matter.

Decomposers do not have mouths or stomachs. Instead they carry out external digestion: they secrete enzymes out onto the dead material. These enzymes break large, insoluble molecules into small, soluble ones, which the microorganism then absorbs.

Decomposer — a microorganism (bacterium or fungus) that breaks down dead plant and animal matter by secreting enzymes onto it and absorbing the soluble products. Digestion happens outside the organism's body.

This breakdown is called decay or decomposition. It is essential to ecosystems because it releases the elements locked inside dead organisms — particularly carbon and mineral ions such as nitrates — back into the soil and atmosphere so that living plants can reuse them. Without decomposers, dead material would pile up and nutrients would never be recycled.

The faster the decomposers work, the faster decay happens. The rate of decay depends on the conditions the decomposers are living in.

Because decay is carried out by living microorganisms using enzymes, anything that affects how fast those microorganisms grow and how fast their enzymes work will change the rate of decay. The AQA specification requires three factors: temperature, water (moisture) and the availability of oxygen.

Factor	Effect on rate of decay	Why
Temperature (up to optimum)	Higher temperature increases the rate	Warmth speeds up enzyme reactions and microbial growth; molecules collide more often
Temperature (too high)	Above the optimum, rate falls sharply	Enzymes denature — their shape changes so they can no longer break down the material
Water / moisture	More moisture increases the rate	Microorganisms need water to grow, and enzyme reactions need a moist environment to work
Oxygen availability	More oxygen increases the rate of aerobic decay	Most decomposers respire aerobically and need oxygen to release the energy they need to grow

A useful detail often tested: increasing the surface area of the dead material (for example by shredding garden waste) also speeds up decay, because it gives the decomposers and their enzymes more area to act on at once.

Decay is fastest in warm, moist, well-aerated (oxygen-rich) conditions — exactly why food spoils quickly on a kitchen counter but lasts in a dry, cold, sealed environment.

This is also why we slow decay to preserve food: a fridge lowers the temperature, freezing nearly stops enzyme activity, drying removes water, and canning or vacuum-packing removes oxygen.

Required practical activity 10 investigates how temperature affects the rate of decay of fresh milk by measuring the change in pH.

Fresh milk contains bacteria. As they decompose the milk, they produce lactic acid, which lowers the pH — the milk turns sour. A faster fall in pH means faster decay. You can also run the practical using the enzyme lipase, which breaks down the fats in milk to fatty acids, again lowering the pH.

Method:

1. Measure equal volumes of fresh milk into several tubes.
2. Add a fixed volume of a pH indicator (such as phenolphthalein, which is pink in alkali) plus a little sodium carbonate to make the milk slightly alkaline, or use a pH probe.
3. Place each tube in a water bath at a different temperature (for example 10 °C, 20 °C, 30 °C, 40 °C). Temperature is the independent variable.
4. Add the same volume of lipase to each tube and start a stopwatch.
5. Record the time taken for the indicator to change colour (or for the pH to fall to a set value). This is the dependent variable.

Control variables: volume of milk, volume of indicator, volume and concentration of lipase, and the starting pH must all be kept the same so that only temperature affects the result.

The result you measure is a time. A shorter time to change colour means a faster rate of decay, so the rate is found by comparing $\frac{1}{\text{time}}$ across the different temperatures.

The specification expects you to be able to calculate rate changes in the decay of biological material. Rate is simply how much something changes in a given time:

$\text{rate}=\frac{\text{change}}{\text{time}}$

When the measurement is pH (as in Required practical 10), the rate of decay is the change in pH per unit time, in pH units per minute (or per hour).

(Worked example — calculating rate of decay.) A sample of milk has a starting pH of $7.0$. After 25 minutes the bacteria have soured it to a pH of $4.5$. Calculate the rate of decay.

$\text{change in pH}=7.0-4.5=2.5$

$\text{rate}=\frac{\text{change in pH}}{\text{time}}=\frac{2.5}{25}=0.1\text{ pH units per minute}$

You can use this to compare decay at different temperatures. If a second sample at a higher temperature falls by the same $2.5$ pH units in only 10 minutes, its rate is $\frac{2.5}{10}=0.25$ pH units per minute — two and a half times faster — confirming that the warmer conditions decayed the milk more quickly.

(Extra context — not required by AQA 8461.) Some teachers mention the "$Q_{10}$" rule of thumb, that reaction rate roughly doubles for every 10 °C rise. AQA 8461 only asks you to calculate rate as change ÷ time and to compare rates, so you do not need $Q_{10}$ for this paper.

Temperature does not increase the rate of decay forever. Understanding the shape of the temperature graph is a common exam requirement.

As temperature rises from cold towards an optimum, the rate of decay increases. The decomposers' enzymes and the molecules they act on gain kinetic energy, so they collide more frequently and react faster. The microorganisms also grow and reproduce more quickly when warm.

Worked interpretation — reading the temperature graph:

• From $0{}^{\circ }\text{C}$ up to about $30$–$40{}^{\circ }\text{C}$ the rate climbs steeply: warmth speeds the enzyme reactions.
• At the optimum temperature the rate reaches its maximum.
• Above the optimum the rate falls sharply, and at high temperatures decay almost stops.

The fall happens because the enzymes secreted by the decomposers denature. Heat changes the shape of the enzyme's active site, so the substrate (the dead material) no longer fits and can no longer be broken down. The microorganisms themselves are also killed by the heat. This is why a graph of rate against temperature is not a straight line — it rises to a peak, then drops.

Always explain a fall in rate at high temperature with the word denature and a reason: the active site changes shape, so the enzyme can no longer break down the material. "The enzymes died" does not gain credit — enzymes are not alive.

Decay is not just something to prevent — gardeners, farmers and engineers deliberately speed it up to produce useful products.

Compost. Gardeners and farmers heap up grass cuttings, vegetable peelings and other waste biological material to make compost. They provide the optimum conditions for rapid decay — they keep the heap warm, moist and aerated (turning it lets in oxygen and shredding the waste increases surface area). The decomposers break the waste down quickly, and the compost produced is used as a natural fertiliser that returns mineral ions to the soil for growing garden plants or crops.

Biogas. Where decay happens without oxygen — anaerobic decay — different microorganisms break the material down and produce methane gas. A biogas generator is a sealed tank with no oxygen, fed with waste such as manure, sewage or plant material. The biogas it produces is mainly methane and can be burned as a fuel for heating, cooking or generating electricity.

Process	Conditions	Useful product
Making compost	Warm, moist, aerobic (oxygen present)	Compost — a natural fertiliser
Biogas generator	Warm, moist, anaerobic (no oxygen)	Biogas (mainly methane) — a fuel

The single most-tested contrast here: compost needs oxygen; biogas (methane) is made by anaerobic decay with no oxygen.

1. Saying decomposers "eat" the dead material

Decomposers do not ingest food. They secrete enzymes onto the dead matter and digest it externally, then absorb the soluble products. Always describe the enzymes and external digestion.

2. Forgetting that rate falls at high temperature

A graph of rate against temperature rises to an optimum and then falls. It does not keep rising. The fall is caused by enzymes denaturing — the active site changes shape so the material can no longer be broken down.

3. Writing "the enzymes died" or "the bacteria died of heat"

Enzymes are not living, so they cannot die — they denature. Explain the loss of activity in terms of the active site changing shape.

4. Mixing up compost and biogas conditions

Compost is made in aerobic (oxygen-rich) conditions. Biogas (methane) is produced by anaerobic decay with no oxygen. Swapping these loses marks.

5. Quoting a rate without units

Rate of decay from a pH experiment must carry units such as pH units per minute. A bare number is not a complete rate answer.

6. Ignoring the control variables in Required practical 10

Only temperature should change. Keeping the volume of milk, the volume and concentration of lipase, and the volume of indicator constant is what makes the comparison of decay rates valid.