Respiration

The above reactions of cellular respiration are often referred to as aerobic respiration because the final series of reactions, the electron transfer chain, require oxygen as an electron acceptor. When oxygen is absent or in short supply, cells may rely upon glycolysis alone for their supply of ATP. Glycolysis presumably originated in primitive cells early in Earth's history when very little oxygen was present in the atmosphere.

In an anaerobic environment, pyruvate is typically broken down into lactate or into acetaldehyde and then ethanol, instead of being degraded to acetyl CoA and then introduced to the citric acid cycle. The NADH made during glycolysis (see above) is required for synthesis of ethanol or lactate. Obviously, exclusive reliance upon glycolysis for the manufacture of ATP is very inefficient, since only two molecules of ATP are made from each glucose molecule, whereas aerobic respiration makes 36 molecules of ATP from each glucose molecule.

Needless to say, synthesis of ethanol is essential in the making of wine and beer. In this case, the sugars present in the must (sweet juice of the crushed grapes) or wort (sweet liquid from the malted barley) are broken down to pyruvate and from there into ethanol. Interestingly, when humans drink ethanol, our livers metabolize it in the reverse direction, into acetaldehyde and other carbohydrates. Accumulation of acetaldehyde has been implicated in causing hangovers as well as in fetal alcohol syndrome, a suite of developmental abnormalities in an infant caused by exposure to alcohol as a fetus.