Anaerobic Respiration

This lesson covers: 

  1. Differences between anaerobic and aerobic respiration
  2. Categories of organisms based on their oxygen needs
  3. Alcohol fermentation and lactic acid 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.

Categories of organisms based on their oxygen needs

Organisms can be categorised by their oxygen requirements for metabolic processes:

  • Obligate anaerobes - These organisms cannot survive in the presence of oxygen.
  • Facultative anaerobes - These organisms switch between aerobic and anaerobic respiration, depending on oxygen availability.
  • Obligate aerobes - These organisms depend on oxygen to synthesise ATP.


Eukaryotic cells, including those in humans, can perform either type of respiration depending on the situation.

Alcohol fermentation and lactic acid 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 is converted into ethanal.
  • Ethanal accepts a hydrogen from reduced NAD.
  • This produces ethanol, and regenerates NAD.
  • Ethanol cannot be further metabolised so must be removed as a waste product.


Lactic acid fermentation:

  • Occurs in some animals and bacteria.
  • Pyruvate accepts a hydrogen from reduced NAD, catalysed by lactate dehydrogenase.
  • This forms lactic acid, and regenerates NAD.
  • Lactic acid can be oxidised back to pyruvate to generate more ATP, or it can be stored as glycogen.


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 lactic acid accumulates, causing cramp and muscle fatigue, and it also reduces the pH affecting enzymes.