Pyruvate can be broken down by fermentation in the absence of oxygen. (A) When inadequate oxygen is present, for example, in a muscle cell undergoing vigorous contraction, the pyruvate produced by glycolysis is converted to lactate as shown. This reaction restores the NAD⁺ consumed in step 6 of glycolysis, but the whole pathway yields much less energy overall than complete oxidation. (B) In some organisms that can grow anaerobically, such as yeasts, pyruvate is converted via acetaldehyde into carbon dioxide and ethanol. Again, this pathway regenerates NAD⁺ from NADH, as required to enable glycolysis to continue. Both (A) and (B) are examples of fermentations.  Fair Use and Copyright info