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.