BIOLOGY · Stage 7 · Chapter 3: Microorganisms and Decomposers
Biology 3.6 Investigating rotting rates — Factors Affecting Decomposition — Alhussan International Academy - Biology (Stages 7-9) Revision Notes & Topical Questions
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Lesson summary
3.6 Investigating rotting rates — Factors Affecting Decomposition — Alhussan International Academy — Biology (Stages 7–9) notes: Plan and evaluate a fair test on decomposition and explain how temperature, moisture and oxygen affect decay.Elephant in Africa — when it dies, decompo…
- Woolly mammoth
- Hypothesis
- Prediction
- Measurement idea
- Observations
- Conclusion
Section 1 of 6Why investigate decomposition
- Elephant in Africa
- when it dies, decomposers reduce it to a skeleton in weeks or months.
- Woolly mammoth
- these hairy elephant-like animals lived in icy conditions thousands of years ago.
- Their bodies are sometimes found in ice still covered in flesh, skin and hair.
- The very cold conditions stop the decomposers from working.
- Question
- how does a condition such as temperature affect the rate at which food, leaves or other material rots.
Section 2 of 6Shakira's investigation
- Hypothesis
- decomposition happens more slowly at low temperatures.
- Prediction
- identical samples of a vegetable decompose faster if kept warm.
- Measurement idea
- decomposers feed on material and digest it, so the material loses mass.
- Cut 3 cubes of potato that each weigh exactly 50 g.
- Put each piece in a dish.
- Keep one piece at 10 °C, one at 20 °C and one at 30 °C for 10 weeks.
- Weigh each piece every two weeks and record the results in a table.
- Draw a graph of mass against time to show links between the variables.
- Observations
- the potato at 30 °C lost mass much faster than at 20 °C, which lost mass faster than at 10 °C.
- Mould grew more slowly on the potato in the coolest place.
- Conclusion
- the hypothesis was supported because warmer samples decomposed more quickly.
Section 3 of 6Variables in a fair test
- Independent variable
- what is changed, such as the temperature.
- Dependent variable
- what is measured, such as change in mass, area of mould or time for a leaf disc to disappear.
- A good measure uses a number, such as the percentage of surface covered by mould.
- Control variables
- type and mass of food, size of container, moisture, time and the type of bag or dish.
- Reliability
- use several samples at each condition and calculate a mean.
- Milk example
- milk gets more acidic as it turns sour, so a fall in pH shows how quickly decomposers are working.
Section 4 of 6Safety
- Mould
- never open bags where mould is growing, because some spores cause allergies.
- Handling
- label each bag clearly and seal it.
- Clean-up
- wash hands afterwards and dispose of the bags safely as the teacher instructs.
Section 5 of 6Typical results and explanations
| Condition | Rate of decay | Reason |
|---|---|---|
| Low temperature (fridge or freezer) | Very slow | Enzymes in microbes work slowly |
| Warm (25 to 40 °C) | Fast | Enzymes and respiration work well |
| Hot (above 60 °C) | None | Enzymes are denatured and microbes killed |
| Dry | Slow | Microbes need water to grow |
| Plenty of oxygen | Faster | Most decomposers respire aerobically |
| Waterlogged or airless | Slow and partial | Little oxygen, so decay is incomplete and peat forms |
- Graph shape
- rate against temperature rises to a peak and then falls sharply, like any enzyme-controlled reaction.
- Cold
- it only slows microorganisms and does not kill them.
Section 6 of 6Evaluating the investigation
- Limitation of the conclusion
- three temperatures show only a trend, so more temperatures are needed to find the optimum.
- Above about 50 °C the decomposers die and decay stops.
- Subjective measure
- visual estimates of mould are subjective, so counting grid squares covered makes it objective.
- Starting conditions
- different ripeness of fruit between samples causes errors.
- Improvements
- use more repeats, measure at the same times each day and use a thermostat-controlled incubator or water bath.
- Each change must be explained as an improvement.
- Application
- food is preserved by cold storage, drying, canning and the use of salt or sugar, which prevent microbes growing.
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