The biogeochemical cycling of these compounds, which depends upon anaerobic respiration, significantly impacts the carbon cycle and global warming. Ecological importance Īnaerobic respiration is a critical component of the global nitrogen, iron, sulfur, and carbon cycles through the reduction of the oxyanions of nitrogen, sulfur, and carbon to more-reduced compounds. There are two important anaerobic microbial methane formation pathways, through carbon dioxide / bicarbonate ( HCO −ģ) reduction (respiration) or acetate fermentation. In yeast, acetaldehyde is reduced to ethanol to regenerate NAD +. 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. These oxidized compounds are often formed during the fermentation pathway itself, but may also be external. The electron acceptor NAD + is regenerated from NADH formed in oxidative steps of the fermentation pathway by the reduction of oxidized compounds. The resulting current drives ATP synthesis from ADP and inorganic phosphate.įermentation, in contrast, does not use an electrochemical gradient but instead uses only substrate-level phosphorylation to produce ATP. A proton motive force drives protons down the gradient (across the membrane) through the proton channel of ATP synthase.
OXYGEN SERVES AS THE TERMINAL ELECTRON ACCEPTOR IN SERIES
The reduced chemical compounds 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 chemical substance (in anaerobic respiration). This results in an electrical potential or ion concentration difference across the membrane.
Cellular respiration (both aerobic and anaerobic) uses highly reduced chemical compounds such as NADH and FADH 2 (for example produced during glycolysis and the citric acid cycle) to establish an electrochemical gradient (often a proton gradient) across a membrane. Therefore, anaerobic respiration is less efficient than aerobic.Īnaerobic cellular respiration and fermentation generate ATP in very different ways, and the terms should not be treated as synonyms. Less energy per oxidized molecule is released. These terminal electron acceptors have smaller reduction potentials than O 2. Anaerobes instead use less-oxidizing substances such as nitrate ( NO −Ĥ), or elemental sulfur (S). Molecular oxygen is an excellent electron acceptor. In aerobic organisms undergoing respiration, electrons are shuttled to an electron transport chain, and the final electron acceptor is oxygen. Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain. Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O 2).
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