<P> Cellular respiration (both aerobic and anaerobic) utilizes highly reduced chemical compounds 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 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). 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 . </P> <P> 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+ . The two processes thus generate ATP in very different ways, and the terms should not be treated as synonyms . </P> <P> Anaerobic 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 . The biogeochemical cycling of these compounds, which depends upon anaerobic respiration, significantly impacts the carbon cycle and global warming . Anaerobic respiration occurs in many environments, including freshwater and marine sediments, soil, subsurface aquifers, deep subsurface environments, and biofilms . Even environments, such as soil, that contain oxygen also have micro-environments that lack oxygen due to the slow diffusion characteristics of oxygen gas . </P> <P> An example of the ecological importance of anaerobic respiration is the use of nitrate as a terminal electron acceptor, or dissimilatory denitrification, which is the main route by which fixed nitrogen is returned to the atmosphere as molecular nitrogen gas . Another example is methanogenesis, a form of carbonate respiration, that is used 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 . Sulfate respiration produces hydrogen sulfide, which is responsible for the characteristic' rotten egg' smell of coast wetlands and has the capacity to precipitate heavy metal ions from solution, leading to the deposition of sulfidic metal ores . </P>

Why does anaerobic respiration take place when o2 is not available
find me the text answering this question