Decomposition (AQA GCSE Biology): Revision Note
Exam code: 8461
Rate of decay
Decay is a crucial process as it ensures that materials such as carbon and mineral ions are recycled and returned to the environment
The rate of decay of biological material can be affected by:
temperature
water availability
oxygen availability
Factors affecting the rate of decay
Temperature
At higher 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
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
Investigating decay
Worked Example
A student investigated 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.
Temperature / °C | pH at 0 hours | pH at 24 hours | pH at 48 hours | pH at 72 hours |
|---|---|---|---|---|
10 | 6.5 | 6.3 | 6.2 | 5.9 |
20 | 6.5 | 6.0 | 5.4 | 4.7 |
30 | 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:
Step 1: calculate the pH change 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 / °C | pH at 24 hours | pH at 48 hours | pH at 72 hours |
|---|---|---|---|
10 | 6.5 − 6.3 = 0.2 | 6.3 − 6.2 = 0.1 | 6.2 − 5.9 = 0.3 |
20 | 6.5 − 6.0 = 0.5 | 6.0 − 5.4 = 0.6 | 5.4 − 4.7 = 0.7 |
30 | 6.5 − 5.0 = 1.5 | 5.0 − 4.7 = 0.3 | 4.7 − 4.7 = 0 |
Step 2: calculate the rate of change for each 24 hour period
This can be done 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 / °C | pH at 24 hours | pH at 48 hours | pH at 72 hours |
|---|---|---|---|
10 | 0.2 ÷ 24 = 0.0083 | 0.1 ÷ 24 = 0.0042 | 0.3 ÷ 24 = 0.013 |
20 | 0.5 ÷ 24 = 0.021 | 0.6 ÷ 24 = 0.025 | 0.7 ÷ 24 = 0.029 |
30 | 1.5 ÷ 24 = 0.063 | 0.3 ÷ 24 = 0.013 | 0 ÷ 24 = 0 |
Ensure that your graph has the following features:
Dependent variable on the y axis and independent variable on the x axis
Labelled axes with units and an even scale
Graph takes up at least half of the available space
Accurately plotted data points
A line that matches the requirement of the question
If the question does not specify a line type, then join the points if appropriate
This should not be done for a scatter graph
If the question asks for a line of best fit, do not join the points
If the data form a straight line then the line of best fit should be straight
If the data points form a curve then the line of best fit should be curved
A line of best fit should have a roughly even number of points above and below it, and should not extend beyond the points
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
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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