AQA GCSE Biology

Revision Notes

7.2.4 Decomposition

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 table

Factors affecting rate of decay table, downloadable IGCSE & GCSE Biology revision notes

Investigating Decay

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

Worked example rate of change in pH of milk_1, downloadable IGCSE & GCSE Biology revision notes

Worked example rate of change in pH of milk_2, downloadable IGCSE & GCSE Biology revision notes

Worked example rate of change in pH of milk_3, downloadable IGCSE & GCSE Biology revision notes

Uses of Decomposition


  • Gardeners and farmers try to provide optimum conditions (warmth, moisture and an oxygen supply) for rapid decay of waste biological material (eg. 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 (eg. 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:
methane + oxygen → carbon dioxide + water
  • Biogas generators are large containers in which animal or plant waste is allowed to decay anaerobically

The process of biogas generation, downloadable IGCSE & GCSE Biology revision notes

The process of biogas generation

Exam Tip

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.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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