Aerobic & Anaerobic Respiration (AQA GCSE Combined Science: Trilogy: Biology): Revision Note

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 enables essential processes to occur within cells

• Organisms need energy for:

  • chemical reactions to build larger molecules from smaller molecules, e.g. protein synthesis

  • muscle contraction for movement

  • keeping warm to maintain the optimum temperature for enzyme activity

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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:

glucose + oxygen → carbon dioxide + water

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

• Aerobic respiration occurs in the mitochondria

Anaerobic respiration in animals

• Respiration in cells can take place anaerobically (without oxygen); this involves the incomplete breakdown of glucose into lactic acid

• Anaerobic respiration in animals occurs when the body can’t supply enough oxygen for aerobic respiration, such as during vigorous exercise

• Anaerobic respiration in animal cells is represented by the equation:

glucose → lactic acid

• As the oxidation of glucose is incomplete in anaerobic respiration, much less energy is transferred than in aerobic respiration

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

glucose → ethanol + carbon dioxide

• Anaerobic respiration in yeast cells is called fermentation

• Fermentation is economically important in the manufacture of:

  • bread: the production of carbon dioxide makes dough rise

  • alcoholic drinks: ethanol is a type of alcohol

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: