AQA GCSE Biology

Revision Notes

4.2.1 Aerobic & Anaerobic Respiration

Respiration: Principles

  • Cellular respiration is an exothermic reaction which is continuously occurring in living cells
  • The chemical process of cellular respiration releases energy
  • The energy transferred supplies all the energy needed for living processes to occur within cells and organisms as a whole
  • Organisms need energy for:
    • Chemical reactions to build larger molecules from smaller molecules
    • Muscle contraction to allow movement
    • Keeping warm (to maintain a constant temperature suitable for enzyme activity)

Uses of energy in the human body, IGCSE & GCSE Chemistry revision notes

Uses of the energy released from respiration

Aerobic Respiration

  • Respiration in cells can take place aerobically (using oxygen) to transfer energy; glucose is reacted with oxygen in this process
  • The equations that summarise the chemical reactions of respiration that release energy from glucose are:


Word equation for aerobic respiration, IGCSE & GCSE Chemistry revision notes

Word equation for aerobic respiration


Balanced equation for aerobic respiration, IGCSE & GCSE Chemistry revision notes

Balanced symbol equation for aerobic respiration


  • Aerobic respiration uses oxygen and most of the reaction takes place in the mitochondria (these are shown above the arrow in the equations)

Anaerobic Respiration in Animals

  • Respiration in cells can take place anaerobically (without oxygen), to transfer energy; it simply involves the incomplete breakdown of glucose into lactic acid
  • This occurs when the body can’t supply enough oxygen for aerobic respiration, such as during vigorous exercise
  • Anaerobic respiration is represented by the equation:


Word equation for anaerobic respiration in animals, IGCSE & GCSE Chemistry revision notes

Word equation for anaerobic respiration in animals – some bacterial cells respire in this way too


  • As the oxidation of glucose is incomplete in anaerobic respiration much less energy is transferred than in aerobic respiration
  • Anaerobic respiration takes place without the need of oxygen

Anaerobic Respiration in Plants & Yeast

  • Plants and yeast can respire without oxygen as well, breaking down glucose in the absence of oxygen to produce ethanol and carbon dioxide
  • Anaerobic respiration in yeast cells is called fermentation
  • Fermentation is economically important in the manufacture of bread (where the production of carbon dioxide makes dough rise) and alcoholic drinks (as ethanol is a type of alcohol)


Word equation for anaerobic respiration in yeast, IGCSE & GCSE Chemistry revision notes

The process outlined above is the same in plants

Comparing Anaerobic & Aerobic Respiration

  • You need to be able to compare the processes of aerobic and anaerobic respiration with regard to the need for oxygen, the differing products and the relative amounts of energy transferred:


Anaerobic Respiration table, IGCSE & GCSE Biology revision notes

Exam Tip

Remember that cellular respiration is not breathing; it is a chemical process of transferring energy from glucose in all living cells.

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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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