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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+.
Anaerobic respiration plays a major role in the global nitrogen, sulfur, and carbon cycles through the reduction of the oxyanions of nitrogen, sulfur, and carbon to more-reduced compounds. Dissimilatory denitrification is the main route by which biologically fixed nitrogen is returned to the atmosphere as molecular nitrogen gas. Hydrogen sulfide, a product of sulfate respiration, is a potent neurotoxin and responsible for the characteristic 'rotten egg' smell of brackish swamps. Along with volcanic hydrogen sulfide, biogenic sulfide has the capacity to precipitate heavy metal ions from solution, leading to the deposition of sulfidic metal ores. Many terrestrial environments become temporarily flooded, and the resulting decrease in oxygen availability results in transient anoxia. Sequential changes in redox conditions and associated adapted microorganisms will follow a flooding event (such as initially aerobic conditions becoming nitrate-reducing followed by iron-reducing, sulfate reducing and eventually methanogenic). Redox gradients such as these may occur in either time (called sequential reduction) or space (the redox regime becomes increasingly negative with distance from an oxygen source). Environmental redox cycling often has strong effects on natural biogeochemical cycling as well as biodegradation of anthropogenic organic pollutants.
Dissimiltory denitrification is widely used in the removal of nitrate and nitrite from municipal wastewater. An excess of nitrate can lead to eutrophication of waterways into which treated water is released. Elevated nitrite levels in drinking water can lead to problems due to its toxicity. Denitrification converts both compounds into harmless nitrogen gas.
Methanogenesis is a form of carbonate respiration that is exploited to produce methane gas by anaerobic digestion. Biogenic methane is used as a sustainable alternative to fossil fuels. On the negative side, uncontrolled methanogenesis in landfill sites releases large volumes of methane into the atmosphere, where it acts as a powerful greenhouse gas.
Specific types of anaerobic respiration are also used to convert toxic chemicals into less-harmful molecules. For example, toxic arsenate or selenate can be reduced to less toxic compounds by various bacteria.
Examples of respiration
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