12.3) Anaerobic respiration

12.3) Anaerobic respiration


Anaerobic respiration: is the term for the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen.

  • The word anaerobic means ‘in the absence of oxygen’.
  • Anaerobic respiration happens in muscles during hard exercise
  • glucose → lactic acid
  • C6H12O6 → 2C3H6O3
  • Anaerobic respiration also happens in plant cells and some microorganisms. Anaerobic respiration in yeast is used during brewing and bread-making
  • glucose → ethanol + carbon dioxide
  • C6H12O6 → 2C2H5OH + 2CO2

Anaerobic respiration is much less efficient than aerobic respiration because it releases much less energy per glucose molecule broken down (respired).


  • There is a buildup of lactic acid in the muscles during vigorous exercise.
  • The lactic acid needs to be oxidised to carbon dioxide and water later.
  • This causes an oxygen debt, that needs to be ‘repaid’ after the exercise stops.
  • Lactic acid is removed in the bloodstream.
  • The blood needs to move more quickly during and after exercise to maintain this lactic acid removal process, so the heart rate is rapid.
  • On reaching the liver, some of the lactic acid is oxidised to CO2 and H2O, using up oxygen in the process.
  • After exercise has stopped, a high level of oxygen consumption may persist until the excess of lactic acid has been oxidised.
  • This is characterised by deeper breathing.