AQA Biology (8461) 4.7.2.2–4.7.2.3 · Required practical
Decomposition
Dead things rot away. Where do their carbon and their minerals go? And what decides how fast they rot?
Triple Science · Foundation tier
Start here
A compost heap steams on a frosty morning.
The air is below 0 °C, but a thermometer pushed into the middle of the heap of garden waste reads over 50 °C. Nothing is on fire, and there is no heater.
If you had to guess, is the heat coming from sunshine soaked up in the day, or from living things inside the heap?
Dead leaves, dead animals and their waste are broken down by microorganisms, mainly bacteria and fungi, called decomposers. Nothing is lost in the process: every atom in a dead leaf ends up somewhere. Follow one fallen leaf, and predict each step before you watch it.
Follow the leaf
The leaf is digested
Step 1 of 3
Bacteria and fungi digest the leaf and respire. Where does most of its carbon go?
Think again · eating is not the answer
“Decomposers eat dead leaves. That is how the carbon gets back into the air.”
Spot the flaw
ExplainWhat is missing from this answer?
Where does it end up?
Air, soil, or locked away?
Each one starts in a living thing.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Decomposers are living organisms, so how fast decay happens depends on their conditions. Temperature: warmer is faster, up to an optimum. Water: they need it to survive and grow. Oxygen: most decomposers respire aerobically; without oxygen, decay is slower, and anaerobic decay produces methane. Gardeners make compost, a natural fertiliser, by giving decomposers the best conditions. A biogas generator keeps waste anaerobic on purpose and collects the methane as a fuel.
A compost heap
Speeds up decay, or slows it?
A gardener's choices for a heap of garden and kitchen waste.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Required practical · RP10 Biology
Milk decays faster in the warm. Time it.
- In a test tube, mix 5 cm³ of full-fat milk, 7 cm³ of sodium carbonate solution and 5 drops of phenolphthalein. The alkaline mixture turns pink.
- Put 1 cm³ of lipase solution in a second tube. Stand both tubes in a water bath at the test temperature for 5 minutes.
- Add the lipase to the milk, stir, and start the stopwatch.
- Stop the clock when the pink colour has gone. Record the time.
- Repeat at other temperatures, such as 20, 30, 40, 50 and 60 °C, and repeat each to calculate a mean.
Variables. Independent: the temperature. Dependent: the time for the pink to go. Control: the volumes and concentrations of milk, lipase and sodium carbonate, and the number of drops of indicator.
Risks. Phenolphthalein is dissolved in ethanol, which is flammable and irritant: no flames, and wear eye protection. Sodium carbonate solution irritates eyes. Lipase can cause allergic reactions, so avoid skin contact. Never taste the milk.
Run the practical
Call the graph first.
0 of 5 temperatures
Commit first. Plot the time for the pink to go against temperature. Which graph will you get?
Think again · why the pink goes
“The milk turns acidic because bacteria in it make acid.”
Spot the flaw
ExplainIn this practical, why does the pink colour disappear?
Equations · rates of decay
rate of decay = mass lost ÷ time
mass lost = starting mass − final mass. In g per week, the time must be in weeks.
rate = 1 ÷ time
For the milk practical: time in seconds, rate in s⁻¹.
Command words in this lesson
- Explain
- Link each step to the next: break down, respire, carbon dioxide out, mineral ions into the soil.
- Identify
- Pick out the right answer from what you are given. No reason needed.
- Calculate
- Rate of decay = mass lost ÷ time. Convert the time to the unit asked for, and give the unit.
- Describe a method
- What you mix, what you change, what you time and how, what you keep the same, how you stay safe.
- Suggest
- Apply what you know to a new situation, such as an improvement to the method.
Key fact
Microorganisms break down dead material. They respire, returning carbon to the atmosphere as carbon dioxide, and they return mineral ions to the soil.
Key note · AQA 4.7.2.2–4.7.2.3 (8461) · RP10
Decomposition
- Microorganisms respire, returning carbon to the atmosphere as carbon dioxide.
- Breaking down dead material returns mineral ions, such as nitrate, to the soil for plant roots to absorb.
- Decay is faster when it is warm (up to an optimum), moist, and there is oxygen.
- Anaerobic decay produces methane; biogas generators collect it as a fuel. Compost is a natural fertiliser.
- RP10: milk + sodium carbonate + phenolphthalein, then lipase; time until the pink goes. Rate = 1 ÷ time.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask MrBadmus
Not sure where the carbon from a dead leaf goes?
The leaf scene is a model: carbon dioxide and mineral ions are drawn as labelled particles, and the soil animals that also feed on dead material are left out. The practical is simulated: the pink fades at a modelled rate for each temperature, the lipase is denatured at 60 °C, and the times you record depend on when you judge the pink has gone.