Decay Detectives
Nothing goes to waste in nature. Meet the microbes that break down the dead, and learn what makes them work fast or slow, from a compost heap to a biogas generator.
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Decay Detectives
A fallen leaf. A dead animal. Why does the world not slowly pile up with them? Because an army of **decomposers** takes the dead apart and puts the pieces back into the soil. **Decomposition** is the breakdown of dead organisms and waste by **microorganisms**, mainly **bacteria** and **fungi**. They cannot swallow anything, so they work from the outside in: they **secrete enzymes** onto the dead material, digest it where it lies, and then **absorb** the products, releasing carbon dioxide and mineral ions back into the ecosystem as they go. That last part is why decay matters. Without it the nutrients locked inside every dead thing would stay locked, and the producers would eventually run out.
Which is not decomposition?
Three of these are decomposition. Which one is something else?
- A fox eating a dead rabbit it has found
- Mould spreading across a slice of forgotten bread
- Bacteria breaking down a fallen leaf on the woodland floor
- Fungi digesting the wood of a dead tree stump
What sets the pace
Three factors decide how fast decay happens, and in the exam you get the mark for the **reason**, not for the word:
Why has this not rotted?
- A chicken breast kept in a freezer for a year
- Dried apricots sealed in a packet in a cupboard
- A body preserved for two thousand years in a peat bog
- Milk left out on a warm kitchen worktop overnight
- Too cold: the decomposers' enzymes work far too slowly
- Too dry: without water the decomposers cannot live or respire
- Too little oxygen: waterlogged ground excludes the air that aerobic decomposers need
- Nothing is limiting it, so this one decays fast
Speed the heap up
A compost heap is decaying very slowly. Select ALL THREE changes that would speed it up.
- Turning the heap regularly with a fork
- Watering it during a dry spell
- Moving it to a sunnier corner of the garden as spring arrives
- Sealing the whole heap inside an airtight bag
- Keeping it under a cold frame through the winter to protect it
- Covering it to keep every drop of rain off it
RP10: the milk test
Required practical 10 asks how **temperature** affects the rate at which fresh milk turns acidic, measured by a change in **pH**. Milk is mixed with **sodium carbonate solution** and a few drops of **cresol red** indicator, which is **purple** while the mixture is alkaline. The tube is brought up to the test temperature, and then **lipase** is added and the stopwatch started. The lipase digests the **fat** in the milk into **fatty acids**, the pH falls, and the indicator turns **yellow**. The time to that colour change is what you measure, and a shorter time means a faster reaction. The link to decay is the enzymes. Decomposers work by secreting enzymes too, and every enzyme responds to temperature the same way, which is what this practical is really showing you.
Run the experiment
Put the steps of RP10 into the order you would carry them out.
- Measure milk and sodium carbonate solution into a tube and add cresol red, which starts purple
- Stand the tube in a water bath until it reaches the temperature being tested
- Add the lipase and start the stopwatch
- Time how long the mixture takes to turn yellow
- Repeat at a range of temperatures and work out the rate at each one
Turning a time into a rate
The stopwatch gives you a **time**, and the graph needs a **rate**. They run in opposite directions: a longer time means a slower reaction, so you cannot plot the time and call it a rate. The fix is to divide one by it: **rate = 1 ÷ time** At 20 °C the mixture took **4 minutes** to turn yellow, so the rate is 1 ÷ 4 = **0.25** per minute. At 40 °C it took **2 minutes**, so the rate is 1 ÷ 2 = **0.5** per minute: twice as fast, and the number now goes up as the reaction gets quicker, which is what a graph needs. Watch the units. If you time in seconds the rate is per second, and the two are not interchangeable.
Work out the rate
At 30 °C the mixture took 5 minutes to turn yellow. What is the rate of reaction, in per minute (min⁻¹)?
Plot the rate
A class pooled its results. The x-axis is temperature in units of 10 °C (so x = 3 means 30 °C) and the y-axis is the rate. Plot all five points: 10 °C → 1, 20 °C → 4, 30 °C → 6, 40 °C → 5, 50 °C → 1.
Why does it fall?
Look at the shape of the graph you have just plotted. After the peak, the rate drops away sharply. Why?
- The enzyme denatures, so its active site no longer fits the substrate
- Reactions always slow down once they have been going for a while
- The enzyme runs out of fat to digest at high temperatures
- The milk evaporates, so there is less of it to react
Two kinds of decay
Decay does not stop when the oxygen runs out. It changes into something else, and the difference is worth a mark every time:
Why sealed?
A biogas generator is built as a sealed tank, kept warm, with waste fed in continuously. Why must it be sealed?
- To keep oxygen out, because it is the anaerobic microorganisms that produce methane
- To keep the heat in, which is the only reason it works
- To stop the smell escaping into the surrounding area
- To stop rainwater getting in and drying the waste out
Case closed
Decomposition is carried out by bacteria and _____, which secrete enzymes onto dead material and absorb what results. The rate depends on temperature, water and _____. It rises with temperature to an optimum and then falls, because the enzymes _____. Decay without oxygen is anaerobic, and a biogas generator uses it to produce _____ as a fuel.