Anaerobic respiration occurs in muscle cells in the absence of oxygen.
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When you exercise and your muscles are not getting the oxygen they need, they resort to undergoing lactic acid fermentation . This reaction generates 2NAD+ molecules for every 2 pyruvate molecules with the help of the enzyme lactate dehydrogenase .
This reaction is important because the 2NAD+ that are regenerated allows glycolysis to continue so that it may produce 2 net ATP molecules which can be used to provide the energy for the muscle cells.
Yeast cells undergo fermentation as well but the process produces a different end product, ethanol, via a 2 step process.
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As you can see, pyruvate will be decarboxylated to become acetaldehyde. Decarboxylation just means a molecule will lose a CO2 group. The enzyme that helps with the process is called pyruvate decarboyxlase.
Acetaldehyde is then reduced to ethanol with the help of alcohol dehydrogenase.
Now a little bit of organic chemistry will help you. We know that acetaldehyde is reduced to ethanol because the carbonyl group disappears and an alcohol appears. Reduction cannot occur without something else being oxidized. What was oxidized? The NADH molecule from glycolysis becomes oxidized to become NAD+. Again, the NAD+ molecules regenerated will be used back up again for glycolysis, continuing the cycle.
