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 highly oxidizing and, therefore, is an excellent acceptor. In anaerobes, other less-oxidizing substances such as sulfate (SO42-), nitrate (NO3-), or sulfur (S) 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.
Contents |
Cellular respiration (both aerobic and anaerobic) utilizes highly reduced species such as NADH and FADH2 (produced during glycolysis and the citric acid cycle in animal cells) to establish a proton electrochemical gradient across a membrane, resulting in a hydrogen potential or pH 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 a proton gradient or involve a membrane. Fermentation is simply the process of glycolysis to produce ATP, coupled with the regeneration of the electron acceptor NAD+ from the NADH generated in glycolysis. In lactic acid bacteria, pyruvate is reduced to lactic acid to regenerate NAD+, in yeasts pyruvate is reduced to ethanol, with the extra carbon from both processes being released in CO2.
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 precipitiate heavy metal ions from solution, leading to the deposition of sulfidic metal ores.
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.
| type | lifestyle | electron acceptor | products | Eo' [V] | example organisms |
| aerobic respiration | obligate and facultative aerobes | oxygen O2 | H2O + CO2 | + 0.82 | eukaryotes |
| [iron] reduction | facultative aerobes, obligate anaerobes | ferric iron Fe(III) | Fe(II) | + 0.75 | Geobacter, Geothermobacter, Geopsychrobacter, Pelobacter carbinolicus, P. acetylenicus, P. venetianus, Desulfuromonadales, Desulfovibrio |
| manganese reduction | facultative or obligate anaerobes | Mn(IV) | Mn(II) | Desulfuromonadales, Desulfovibrio | |
| cobalt reduction | facultative or obligate anaerobes | Co(III) | Co(II) | Geobacter sulfurreducens | |
| uranium reduction | facultative or obligate anaerobes | U(VI) | U(IV) | Geobacter metallireducens, Shewanella putrefaciens, (Desulfovibrio) | |
| nitrate reduction (denitrification) | facultative aerobes | nitrate NO3− | nitrite NO2– | + 0.40 | Paracoccus denitrificans, E. coli |
| fumarate respiration | facultative aerobes | fumarate | succinate | + 0.03 | Escherichia coli |
| sulfate respiration | obligate anaerobes | sulfate SO42− | sulfide HS− | - 0.22 | Desulfobacter latus, Desulfovibrio |
| methanogenesis (carbonate reduction) | methanogens | carbon dioxide CO2 | methane CH4 | - 0.25 | Methanothrix thermophila |
| sulfur respiration (sulfur reduction) | facultative aerobes and obligate anaerobes | sulfur S0 | sulfide HS− | - 0.27 | Desulfuromonadales |
| acetogenesis (carbonate reduction) | acetogens | carbon dioxide CO2 | acetate | - 0.30 | Acetobacterium woodii |
| TCA reduction | facultative or obligate anaerobes | trichloroacetic acid | dichloroacetic acid | Trichlorobacter (Geobacteraceae) |
www.apple.co.uk www.google.co.uk
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||