In the process of anaerobic respiration, carbohydrate can be metabolised by a process that utilises oxidative phosphorylation via an electron transport chain, but instead of oxygen serving as the terminal electron acceptor a (usually) inorganic molecule such as nitrate or sulphate is used. These processes are referred to, respectively, as dissimilatory nitrate or sulphate reduction. Obligate anaerobes carry out this process, as they are unable to utilise oxygen; in addition, other organisms may turn to this form of res-piration if oxygen is unavailable (facultative anaerobes). Other examples of inorganic electron acceptors for anaerobic respiration include Fe3+ , CO2 and Mn4+ . In certain circumstances, an organic molecule such as fumarate may be used instead.
Anaerobic respiration is not as productive as its aerobic counterpart in terms of ATP production, because electron acceptors such as nitrate or sulphate have less positive redox potentials than oxygen. Anaerobic respiration tends to occur in oxygen-depleted environments such as waterlogged soils.
It must be stressed that anaerobic respiration is not the same as fermentation. The latter process does not involve the components of the electron transport chain (i.e. there is no oxidative phosphorylation), and much smaller amounts of energy are generated.