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Georgia Lofts

Georgia Lofts


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About Me:Biomedical Science Graduate

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GCSE Revision: Decomposition

GCSE Revision: Decomposition


Please note: Text in bold is what the AQA GCSE biology specification requires an understanding of.

Students should be able to explain how temperature, water and availability of oxygen affect the rate of decay of biological material.


Decomposition- the state or process of rotting; decay. Matter is broken down. The rate at which this occurs is dependent upon the number of decomposing microorganisms, water and oxygen availability and temperature.


We have previously learnt how bacteria and fungi operate in the decomposition process so we can now move on to factors which affect the rate of decay. Well firstly, we know that microorganisms are involved, therefore the more we have, the faster the decay process.


Water availability is important. Decomposers, like all organisms need water to survive. Areas lacking water will have less decomposition due to lack of microorganisms being available. Many decomposers secrete enzymes onto the dead matter, they then absorb dissolved molecules. These reactions require water in order to take place. Therefore, more water means more decomposition.


Temperature too is an important factor for decomposition rates. Decomposers are less active in colder temperatures, slowing down the rate of decomposition. A temperature increase makes the decomposers more active. When temperatures are too high (at extreme temperatures) this will kill decomposers and therefore decomposition will stop.


Oxygen is also needed for survival. Little oxygen means little or no decomposition. Decomposers respire oxygen to enable them to grow and multiply. Hence, why when you see sealed and vacuum-packed food, or when we cling film food before putting it in the fridge, we are trying to prevent microbes for growing through limiting the conditions they need to survive. When oxygen availability increases so does the rate of decomposition. However, some decomposers can survive without oxygen. We use these in biogas generators.


Fun fact: the Egyptians mummified dead kings and queens removing all water and so decomposers could not break down tissue.


Students 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.


Rates of decay are the speed at which dead matter is broken down. We can measure this by measuring changes in pH (e.g. in milk), change in temperature (e.g. in grass) or change in mass (e.g. with decaying fruit).


Skills for this point in the specification have already been covered previously. Top tips are to do practice questions, as practice makes perfect!


 Anaerobic decay produces methane gas. Biogas generators can be used to produce methane gas as a fuel.


The process of anaerobic decay or decomposition can be used to provide us with fuel. Methane and carbon dioxide produced together is called biogas. Biogas is a source of renewable energy. Biogas generators are large vessels that animal waste is deposited to be anaerobically digested. This process is useful because not only is it an energy source but it reduces the volume of domestic waste added to landfill sites. However, it does have its disadvantages as methane and carbon dioxide are greenhouse gases which contribute to global warming.



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