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Anaerobic respiration
Anaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen. In order for the electron transport chain to function, an exogenous final electron acceptor must be present to allow electrons to pass through the system. In aerobic organisms, this final electron acceptor is oxygen. Molecular oxygen is a highly oxidizing agent and, therefore, is an excellent acceptor. In anaerobes, other less-oxidizing substances such as sulfate (SO42−), nitrate (NO3−), sulphur (S), or fumarate are used. These terminal electron acceptors have smaller reduction potentials than O2, meaning that less energy is released per oxidized molecule. Anaerobic respiration is, therefore, in general energetically less efficient than aerobic respiration.
Anaerobic respiration is used mainly by prokaryotes that live in environments devoid of oxygen. Many anaerobic organisms are obligate anaerobes meaning that they can respire only using anaerobic compounds and will die in the presence of oxygen.
== Anaerobic respiration as compared with fermentation ==
Cellular respiration (both aerobic and anaerobic) utilizes highly reduced species such as NADH and FADH2 (for example produced during glycolysis and the citric acid cycle) to establish an electrochemical gradient (often a proton gradient) across a membrane, resulting in an electrical potential or ion concentration difference across the membrane. The reduced species are oxidized by a series of respiratory integral membrane proteins with sequentially increasing reduction potentials with the final electron acceptor being oxygen (in aerobic respiration) or another species (in anaerobic respiration). The membrane in question is the inner mitochondrial membrane in eukaryotes and the cell membrane in prokaryotes. A proton motive force or pmf drives protons down the gradient (across the membrane) through the proton channel of ATP synthase. The resulting current drives ATP synthesis from ADP and inorganic phosphate.
Fermentation, in contrast, does not utilize an electrochemical gradient. Fermentation instead only uses substrate-level phosphorylation to produce ATP. The electron acceptor NAD+ is regenerated from NADH formed in oxidative steps of the fermentation pathway by the reduction of oxidized compounds. These oxidized compounds are often formed during the fermentation pathway itself, but may also be external. For example, in homofermentative lactic acid bacteria, NADH formed during the oxidation of glyceraldehyde-3-phosphate is oxidized back to NAD+ by the reduction of pyruvate to lactic acid at a later stage in the pathway. In yeast, acetaldehyde is reduced to ethanol to regenerate NAD+.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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