FIGURE 12-5 Anaerobic versus aerobic metabolism of glucose. The ultimate fate of pyruvate formed during glycolysis depends on the presence or absence of oxygen. (a) In the absence of oxygen, pyruvate is only partially degraded and no further ATP is made. However, two electrons are transferred from each NADH molecule produced during glycolysis to an acceptor molecule to regenerate NAD⁺, which is required for continued glycolysis. In yeast (left), acetaldehyde is the electron acceptor and ethanol is the product. This process is called alcoholic fermentation. When oxygen is scarce in muscle cells (right), NADH reduces pyruvate to form lactic acid, regenerating NAD⁺, a process called lactic acid fermentation. (b) In the presence of oxygen, pyruvate is transported into mitochondria, where it is first converted by pyruvate dehydrogenase into one molecule of CO₂ and one of acetic acid, the latter linked to coenzyme A (CoA-SH) to form acetyl CoA, concomitant with reduction of one molecule of NAD⁺ to NADH. Further metabolism of acetyl CoA and NADH generates approximately an additional 28 molecules of ATP per glucose molecule oxidized.