Decomposition (AQA GCSE Biology): Revision Note
Exam code: 8461
Rate of decay
Temperature, water and availability of oxygen affect the rate of decay of biological material
Decomposition (also known as decay or rotting) is the breaking down and digestion of biological material (waste products and dead organisms) by organisms called decomposers
Decomposers include microorganisms (bacteria and fungi) and detritus feeders
Decomposition is a crucial process as it ensures that materials such as carbon and mineral ions are recycled and returned to the environment
Remember, there is a finite (fixed) number of elements on Earth, new atoms cannot be created out of nothing!
The rate of decay is the speed that decomposers break down biological material and is affected by three key factors:
Factors affecting rate of decay
Temperature
At warmer temperatures, enzymes involved in decomposition can work at a faster rate, increasing the rate of decay
If the temperature is too high, these enzymes will denature and the rate of decay will decrease
At low temperatures, the enzymes involved in decomposition work slowly, decreasing the rate of decay — this is why we keep food in a fridge
Water
Decomposers require water to survive (water being essential for certain biological processes)
Many decomposers also function by secreting enzymes onto decaying biological matter and absorbing the products of this chemical digestion — without water these reactions cannot occur
As water availability decreases, so does the rate of decay
Availability of oxygen
Oxygen is needed by many decomposers for aerobic respiration — without oxygen, they cannot survive
For these decomposers, the rate of decay decreases as oxygen availability decreases
However, some microorganisms can respire anaerobically (they don’t require oxygen to survive), resulting in anaerobic decay (such as in biogas generators)
Investigating decay
Worked Example
A student is investigating rate changes in the decay of milk. They incubated milk at three different temperatures and recorded the pH every 24 hours. Their results table is shown below.
0 hours | 24 hours | 48 hours | 72 hours | |
|---|---|---|---|---|
10°C | 6.5 | 6.3 | 6.2 | 5.9 |
20°C | 6.5 | 6.0 | 5.4 | 4.7 |
30°C | 6.5 | 5.0 | 4.7 | 4.7 |
Calculate the rate of change in pH of the milk in each 24 hour period for each of the three temperatures.
Draw a graph of the results.
Answer:
To calculate the rate of change we first need to work out how much the pH value changed over each 24 hour period. This is calculated by finding the difference between the current pH value and the previous pH value.
(Note: no change at 0 hours as no time has passed so the pH has not changed)
Change in pH:
Temperature | 24 hours | 48 hours | 72 hours |
|---|---|---|---|
10°C | 6.5 − 6.3 = 0.2 | 6.3 − 6.2 = 0.1 | 6.2 − 5.9 = 0.3 |
20°C | 6.5 − 6.0 = 0.5 | 6.0 − 5.4 = 0.6 | 5.4 − 4.7 = 0.7 |
30°C | 6.5 − 5.0 = 1.5 | 5.0 − 4.7 = 0.3 | 4.7 − 4.7 = 0 |
You can now calculate the rate of change for each 24 hour period by dividing each change in pH by the time taken for this change to occur:
Rate of change = change in value ÷ change in time
Rate of change in pH:
Temperature | 24 hours | 48 hours | 72 hours |
|---|---|---|---|
10°C | 0.2 ÷ 24 = 0.0083 | 0.1 ÷ 24 = 0.0042 | 0.3 ÷ 24 = 0.013 |
20°C | 0.5 ÷ 24 = 0.021 | 0.6 ÷ 24 = 0.025 | 0.7 ÷ 24 = 0.029 |
30°C | 1.5 ÷ 24 = 0.063 | 0.3 ÷ 24 = 0.013 | 0 ÷ 24 = 0 |
Examiner Tips and Tricks
Graph tips:
Ensure your graph takes up at least half the space you are given
Ensure axes scales go up in suitable multiples
Ensure axes have labels and units
Ensure data points are plotted accurately
Independent variable on x-axis
Dependent variable on y-axis
Examiner Tips and Tricks
When studying rates of decay, you should be able to:
calculate rate changes in the decay of biological material
translate information between numerical and graphical form
plot and draw appropriate graphs selecting appropriate scales for the axes
Uses of decomposition
Compost
Gardeners and farmers try to provide optimum conditions (warmth, moisture and an oxygen supply) for rapid decay of waste biological material (e.g. waste plant matter)
The compost produced is used as a natural fertiliser for growing garden plants or crops
Once the compost is spread onto the soil, it is broken down further by decomposing microorganisms (bacteria and fungi) and detritivores (e.g. earthworms and woodlice)
This ensures the recycling of minerals (such as magnesium and nitrates) that can then be absorbed by plants to be used for growth (magnesium is used to make chlorophyll, nitrates to make amino acids)
Biogas generators
Some decomposing microorganisms can break down biological material without oxygen
This is called anaerobic decay
Anaerobic decay produces methane gas (as well as carbon dioxide) – together these products are given the term ‘biogas’
The methane produced can be burned as a fuel:
Biogas generators are large containers in which animal or plant waste is allowed to decay anaerobically
Diagram of a biogas generator
Examiner Tips and Tricks
It is important that the decomposing microorganisms are kept in anaerobic conditions (no oxygen) in order to ensure anaerobic decay occurs.
Don’t forget, however, that water (moisture) is still required for the microorganisms to survive. In addition, the biogas generator should be kept at a constant, optimum temperature to allow the decomposing microorganisms to continue respiring and decomposing the biological material.
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