Decomposition (AQA GCSE Biology): Revision Notes
Decomposition
What is decomposition?
Decomposition happens when dead materials break down and rot. This process is carried out by tiny living things called microorganisms. When plants and animals die, decomposers break them down into simple substances that can be used by new plants.
This natural recycling process is essential for life on Earth, as it returns nutrients to the soil and makes them available for new plant growth.
Factors that affect the rate of decay
Three main factors control how quickly things decompose:
Temperature
- Warmer conditions speed up decay
- Heat makes chemical reactions happen faster inside microorganisms
- This means they break down materials more quickly
- Cold conditions slow decomposition down
Temperature affects the metabolic rate of decomposer organisms. Just like humans are more active in warm weather, microorganisms work faster when it's warm.
Water content
- Moist conditions help decomposition
- Microorganisms need water to survive and carry out their life processes
- Very dry conditions slow down decay because decomposers cannot function properly
- Too much water can also slow decay by preventing air from reaching decomposers
Oxygen availability
- Most decomposers need oxygen to respire
- When oxygen is present, microorganisms use aerobic respiration
- This type of respiration is much faster than without oxygen
- Without oxygen, some microorganisms use anaerobic respiration, which is much slower
Understanding these three factors is crucial because they explain why food spoils faster in warm, moist conditions and why refrigeration and drying are effective preservation methods.
Biogas generators
Some decomposers can work without oxygen. When they break down waste in anaerobic conditions, they produce methane gas. This process happens in biogas generators.
How biogas generators work:
- Animal waste (manure) is put into a sealed container
- Microorganisms break down the waste without oxygen present
- This produces methane gas and biogas
- The methane can be burned for cooking and heating
- The leftover material makes good fertiliser
Biogas generators are an excellent example of sustainable technology, turning waste into both useful fuel and fertiliser while reducing environmental pollution.
Making compost
People use decomposition in useful ways:
Gardeners:
- Mix kitchen scraps and garden waste in compost bins
- Decomposers break down the plant material
- This creates natural fertiliser for growing plants
Farmers:
- Make compost and manure on a much larger scale
- Use animal waste and crop remains
- Spread the finished compost on fields to help crops grow
Measuring decomposition rates
Scientists can measure how fast things decay by weighing samples over time.
Rate of decay calculation:
Worked Example: Calculating Decay Rate
If grass clippings lose 45g in 6 days, the rate of decay is:
Key findings from experiments:
- Damp conditions cause fastest decay
- Dry conditions slow decay because microorganisms lack water
- Disinfectant kills microorganisms, so decay becomes very slow
These experimental findings prove that living organisms (microorganisms) are responsible for decomposition, not just chemical processes alone.
Key Points to Remember:
- Decomposition breaks down dead materials using microorganisms
- Three factors speed up decay: warmth, moisture, and oxygen
- Biogas generators use anaerobic decay to make methane fuel
- Composting turns waste into useful fertiliser
- Rate of decay = change in mass ÷ time taken