You are viewing a free preview of this lesson.
Subscribe to unlock all 9 lessons in this course and every other course on LearningBro.
A fallen apple in autumn is gone by spring; a compost heap turns kitchen scraps into rich soil; a forgotten loaf grows mould within days. All of these are decomposition — the breaking down of dead material by decomposers. You have already met decomposers as the recyclers of the carbon and nitrogen cycles; this lesson, part of Topic B4 of OCR Gateway Science A, looks at decomposition in its own right. It examines the factors that affect how fast decay happens, sets out the B4 required practical on the rate of decay (with example data), works through the rate-of-decay calculation, and explains why these ideas matter in composting, sewage treatment and food preservation.
By the end of this lesson you should be able to describe how decomposers break down dead material, explain how temperature, water, oxygen and the number of decomposers affect the rate of decay, carry out a rate-of-decay calculation, and explain how this knowledge is used in everyday life.
Decomposers are microorganisms — mainly bacteria and fungi — that break down dead organisms and waste (dead plants and animals, fallen leaves, faeces). As they feed, they digest the dead material and respire, releasing carbon dioxide and returning mineral ions (such as nitrate) to the soil. This is how the nutrients locked in dead bodies are made available again to plants — decomposition is the step that completes the recycling of carbon and nitrogen. Some animals, such as earthworms and woodlice, called detritivores, help by feeding on dead material and breaking it into smaller pieces, which gives the decomposers a larger surface area to work on and speeds up decay.
Decomposition is the step that completes the cycles of materials you met in the previous lessons. Without it, the carbon and the mineral ions locked up in dead bodies and waste would never be returned: dead material would simply pile up, and the supply of CO₂ and nitrates that producers depend on would slowly run out. By breaking dead matter down, decomposers release carbon dioxide back to the air (through their respiration) and return mineral ions such as nitrate to the soil, so that plants can absorb them and the whole community can keep going. This is why decomposition is sometimes called nature's recycling — and why the rate at which it happens matters so much, both in nature and in the ways humans use or control it.
Because decomposers are living microorganisms whose activity depends on enzymes (recall Topic B1), the rate of decay depends on the conditions they live in. Four factors are key.
Decomposers work fastest in warm conditions. As temperature rises towards an optimum, their enzymes work faster (more kinetic energy, more frequent collisions), so decay speeds up. But if it gets too hot, the decomposers' enzymes denature and the microorganisms are killed, so decay slows or stops. In cold conditions the enzymes work slowly, so decay is very slow — which is exactly why a fridge or freezer keeps food fresh.
Decomposers need water (moisture). Decay is faster in moist conditions because the microorganisms need water to live and to carry out their reactions. In dry conditions, decay is very slow — which is why drying food (for example, dried fruit or dried pasta) preserves it.
Most decomposers respire aerobically, so they need oxygen. Decay is faster when there is plenty of oxygen. Where oxygen is absent, only anaerobic decomposers can work, and decay is slower (and produces different products — see the Higher section).
The more decomposers there are, the faster the rate of decay, because more microorganisms are breaking the material down at once.
| Factor | Faster decay when… | Why |
|---|---|---|
| Temperature | Warm (up to an optimum) | Decomposers' enzymes work faster; too hot denatures them |
| Water / moisture | Moist | Decomposers need water to live and react |
| Oxygen | Plenty of oxygen | Most decomposers respire aerobically |
| Number of decomposers | Many present | More microorganisms breaking material down |
Exam Tip: Almost every "rate of decay" answer comes back to the decomposers and their enzymes: warm + moist + oxygen-rich conditions let the microorganisms grow and their enzymes work fast, so decay is rapid; cold, dry or oxygen-poor conditions slow them down. Frame your answer around what the microorganisms need.
A common B4 required practical investigates how a factor (such as temperature) affects the rate of decay. One standard approach measures the change in pH as fresh milk decays: as microorganisms break down the milk, they produce acids, so the pH falls — and the faster the pH falls, the faster the decay. The investigation might compare milk kept at different temperatures.
A reliable method:
Example readings (illustrative data showing the pattern, not real measurements):
| Temperature (°C) | Time for pH to fall from 7.0 to 5.0 (hours) |
|---|---|
| 5 | 48 |
| 15 | 24 |
| 25 | 12 |
| 35 | 8 |
These example readings show the expected pattern: as the temperature rises, the time taken falls, so the rate of decay increases — the warmer the conditions (up to the optimum), the faster the decomposers work. Control variables include the volume of milk, the amount of culture/indicator, and the starting pH, so that only temperature affects the result.
Exam Tip: In a decay practical, the rate of decay can be measured as 1 ÷ time taken for a change to happen (for example, the time for the pH to reach a set value). A shorter time means a faster rate. Identify the independent variable (e.g. temperature), the dependent variable (e.g. pH or time) and the control variables for easy marks.
Higher tier only: you may be asked to calculate a rate of decay from data. The general rate equation is the same one you used in B1:
rate=timechange
In a decay experiment the pH of milk falls from 7.0 to 5.2 over 6 hours. Calculate the rate of decay in pH units per hour.
Step 1 — find the change in pH:
change in pH=7.0−5.2=1.8
Step 2 — divide the change by the time:
rate=6 hours1.8=0.3 pH units per hour
Answer: the rate of decay is 0.3 pH units per hour.
A pile of compost has a mass of 800 g. After 40 days of decay its mass has fallen to 560 g. Calculate the rate of decay in grams per day.
Step 1 — find the change in mass:
change in mass=800 g−560 g=240 g
Step 2 — divide by the time:
rate=40 days240 g=6 g per day
Answer: the rate of decay is 6 g per day.
Subscribe to continue reading
Get full access to this lesson and all 9 lessons in this course.