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Key role of fermentation under circumstances where electron transport chains are not available and otherwise a byproduct of donation to electron transport chains of electrons obtained from organic compounds during cellular respiration.
NAD+ is absolutely required for glycolysis, which in turn is crucial for the chemoheterotrophic generation of ATP from organic compounds. NAD+ is also required for pyruvate oxidation as well as the Krebs cycle. NAD+, however, is used up by all of these processes.
For cells to use these mechanisms as part of their means of generating ATP, they must have a ready supply of NAD+. That supply is NADH, which is oxidized to regenerate NAD+ either via fermentation pathways or instead by the passing of electrons from NADH to the electron transport chain.
Absent electron transport chain functioning, such as under anaerobic conditions, fermentation pathways represent the only means by which NAD+ may be regenerated. Fermentation therefore is employed to keep glycolysis going—the source of essentially all of the ATP generated by fermenting organisms.
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