When respiration is not possible, due to either a lack of an external electron acceptor or an impairment of the respiratory chain, fermentation is the catabolic pathway that is used for the production of energy from the partial oxidation of glucose or other carbon sources (Figure 1). The oxidation of the substrate, which occurs mainly through the Embden-Meyerhoff and Parnas (EMP) or Entner-Doudoroff (ED) pathways, results in the production of pyruvate, adenosine triphosphate (ATP), and nicotinamide adenine dinucleotide phosphate (NAD(P)H). In the absence of external electron acceptors, the pyruvate or the other organic compounds that derive from the initial substrate reaction undergo reduction, with the regeneration of NAD+(P). This step is essential for the fermentation process to progress and it leads to the production of waste products (ethanol and organic acids) that are excreted from the cell. ATP is the main product of fermentation, and it is generated by phosphorylation at the substrate level. An exception to this general rule is seen with the fermentation of car-boxylic acids. The catabolism of these substrates generates a gradient of H+ or Na+ ions across the plasma membrane, and the production of ATP involves the activity of the H+ or Na+ membrane ATPases.
While the complete oxidation of 1 mol of glucose to CO2 through oxidative phosphorylation (respiration) generates up to 38 mol of ATP, fermentation produces only a few moles of ATP (1-3) per mole of glucose. Thus, the recovery of energy from fermentation is rather low compared to that yielded by respiration. Moreover, it varies depending on the initial substrate and the fermentation process itself.
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