Anaerobic respiration refers to the oxidation of molecules in the absence of oxygen to produce energy. These processes require another electron acceptor to replace oxygen. Anaerobic respiration is often used interchangeably with fermentation, especially when the glycolytic pathway exists in the cell. However, certain anaerobic prokaryotes generate all of their ATP using an electron transport system and ATP synthase.

When glycolysis is used

Oxygen is not necessary for glycolysis to occur. In some organisms, such as C. tetani (causes tetanus) or C. perfringens (causes gangrene), called obligate anaerobes, the presence of oxygen is lethal. In organisms which use glycolysis, the absence of oxygen prevents pyruvate from being metabolised to CO₂ and water via the citric acid cycle and the electron transport chain (which relies on O₂) does not function. Fermentation does not yield more energy than that already obtained from glycolysis (2 ATPs) but serves to regenerate NAD⁺ so glycolysis can continue. Various end products can also be created, such as lactate or ethanol.

Fermentation in animals

(see Lactic acid formation )

Fermentation in other organisms

In some plant cells and yeasts, fermentation produces CO₂ and ethanol. The conversion of pyruvate to acetaldehyde generates CO₂ and the conversion of acetaldehyde to ethanol regenerates NAD⁺.

The end product of fermentation in C. perfringens is a gas which causes the condition gas gangrene.

When ATP synthase is used

Anaerobic respiration is also defined as a membrane bound biological process coupling the oxidation of electron donating substrates (e.g. sugars and other organic compounds, but also inorganic molecules like hydrogen, sulfide/sulfur, ammonia, metals or metal ions) to the reduction of suitable alternative electron acceptors other than molecular oxygen. During these redox processes, protons are translocated over the membrane from "inside" to "outside", establishing a concentration gradient over the membrane which temporarily stores the energy released in the chemical reactions. This energy is then converted into ATP by the same enzyme used during aerobic respiration, ATP synthase. Possible electron acceptors for anaerobic respiration are nitrate, nitrite, nitrous oxide, oxidised amines and nitro-compounds, fumarate, oxidised metal ions, sulfate, sulfur, sulfoxo-compounds, halogenated organic compounds, selenate, arsenate or carbon dioxide (in acetogenesis and methanogenesis). All these types of anaerobic respiration are restricted to prokaryotic organisms.