Fermentation is respiration with no external electron acceptor. It is really important to grasp this concept. Simply put, the challenge of fermentation is to regenerate NAD+, an essential cofactor for glycolysis, so that glycolysis can occur again. Remember glyceraldehyde 3-phosphate is oxidized by NAD+ to form 1,3 biphosphoglycerate (BPG) in glycolysis. The challenge of fermentation is to convert NADH back into NAD+.
In yeast fermentation, after decarboxylization of pyruvate to form acetaldehyde, reduction by NADH occurs forming ethanol, which regenerates NAD+. In lactate fermentation, pyruvate itself is reduced by NADH. The regeneration of NAD+ is necessary to sustain glycolysis.
These two fermentations contrast with the oxidative decarboxylization of pyruvate to form acetyl CoA in aerobic metabolism, which actually consumes NAD+. In aerobic metabolism, there is no need to use the by-products of glycolysis (acetaldehyde or pyruvate) to oxidize NADH, when NADH can donate its electron to the electron transport system for eventual reduction of O2.
In yeast fermentation, after decarboxylization of pyruvate to form acetaldehyde, reduction by NADH occurs forming ethanol, which regenerates NAD+. In lactate fermentation, pyruvate itself is reduced by NADH. The regeneration of NAD+ is necessary to sustain glycolysis.
These two fermentations contrast with the oxidative decarboxylization of pyruvate to form acetyl CoA in aerobic metabolism, which actually consumes NAD+. In aerobic metabolism, there is no need to use the by-products of glycolysis (acetaldehyde or pyruvate) to oxidize NADH, when NADH can donate its electron to the electron transport system for eventual reduction of O2.
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