Glycolysis, as we have just described it, is an anaerobic process. None of its nine steps involve the use of oxygen. However, immediately upon finishing glycolysis, the cell must continue respiration in either an aerobic or anaerobic direction; this choice is made based on the circumstances of the particular cell. A cell that can perform aerobic respiration and which finds itself in the presence of oxygen will continue on to the aerobic citric acid cycle in the mitochondria. If a cell able to perform aerobic respiration is in a situation where there is no oxygen (such as muscles under extreme exertion), it will move into a type of anaerobic respiration called homolactic fermentation. Some cells such as yeast are unable to carry out aerobic respiration and will automatically move into a type of anaerobic respiration called alcoholic fermentation.
More specifically, the differences in aerobic and anaerobic respiration rest on the different very roles played by the NADH molecule produced in step 5 of glycolysis. In both aerobic and anaerobic respiration, the NADH molecule is part of the enzyme complex and must be restored to its NAD, oxidized state. If there are aerobic conditions, meaning oxygen is available, the NADH molecule can be transported to the mitochondria where it can be immediately converted back to NAD and plays a role in the electron transport chain. However, under anaerobic, oxygen-deficient conditions, NADH gets converted back to NAD through anaerobic mechanisms, whether homolactic or alcoholic fermentation.
Instead of being immediately reoxidized after glycolysis step 5 as it would in aerobic respiration, the NADH molecule remains in its reduced form until pyruvate has been formed at the end of glycolysis. The pyruvate product of glycolysis gets further acted upon under anaerobic conditions by the enzyme lactate dehydrogenase (LDH).
In this reaction, the hydrogen from the NADH molecule is transferred to the pyruvate molecule. This results in its carbon-oxygen double bond being reduced to a carbon-oxygen single bond with the addition of a hydrogen atom. The result is the molecule lactate. From the lactate product, lactic acid can be formed, which causes the muscle fatigue that accompanies strenuous workouts where oxygen becomes deficient.
There is another way that the NADH molecule can be re-oxidized. Anaerobic conditions in yeast convert pyruvate to carbon dioxide and ethanol. This occurs with the help of the enzyme pyruvate decarboxylase which removes a carbon dioxide molecule from the pyruvate to yield an acetaldehyde. The acetaldehyde is then reduced by the enzyme alcohol dehydrogenase which transfers the hydrogen from NADH to the acetaldehyde to yield NAD and ethanol. This enzyme is not found in humans.
As you can see, both of these anaerobic conditions leads to glycolytic products other than pyruvate. These different products are necessary because the NADH molecule must be reoxidized so that it can function in the next round of glycolysis of newly introduced glucose. If oxygen is not present to help oxidize it, other reactions, such as those of homolactic and alcoholic fermentation, must occur.