Anaerobic Respiration

This lesson covers: 

  1. Differences between anaerobic and aerobic respiration
  2. Alcohol fermentation and lactate fermentation

Differences between anaerobic and aerobic respiration

Anaerobic respiration acts as an emergency measure for ATP production when oxygen levels are low, as it does not require oxygen.


Differences between aerobic and anaerobic respiration:

  • Some steps of aerobic respiration occur within mitochondria, while anaerobic respiration only occurs in the cytoplasm.
  • Only aerobic respiration includes the link reaction, the Krebs cycle, and oxidative phosphorylation.
  • Aerobic respiration yields approximately 34 ATP per glucose molecule, while anaerobic respiration yields only 2 ATP per glucose molecule.

Alcohol fermentation and lactate fermentation

In anaerobic conditions, which are those without oxygen, cells can produce a small yield of ATP through glycolysis. This can only continue if the reduced NAD that is produced can be oxidised again.


The link reaction and Krebs cycle cannot continue if all the FAD and NAD are reduced, because they cannot accept any more protons and electrons. Oxidative phosphorylation cannot occur without oxygen as the final electron acceptor.

There are two major pathways through which cells can undergo anaerobic respiration: alcohol fermentation and lactic acid fermentation.


Alcohol fermentation:

  • Occurs in yeasts and some plants.
  • Pyruvate loses a molecule of CO2 and accepts a hydrogen from reduced NAD.
  • This produces ethanol, and regenerates NAD.
  • In yeast, it can be used to produce ethanol for wine and beer production.


Lactate fermentation:

  • Occurs in some animals and bacteria.
  • Pyruvate accepts a hydrogen from reduced NAD.
  • This produces lactate, and regenerates NAD.
  • Lactate is removed by the blood and taken to the liver to be converted to glycogen, or oxidised to regenerate pyruvate when oxygen is available.


If too much anaerobic respiration occurs in muscle tissue, the reduced quantity of ATP produced is insufficient to maintain vital processes for extended time periods. This means lactate accumulates, causing cramp and muscle fatigue, and it also reduces the pH affecting enzymes.