When oxygen is in short supply, as in muscles during vigorous exercise, the electron transport chain stops because there is no final electron acceptor. Reduced NAD can no longer be reoxidised there, so the link reaction and Krebs cycle also stop.Glycolysis can still make ATP, but only if NAD is regenerated. In mammals this is done by reducing pyruvate to lactate (lactic acid), catalysed by the enzyme lactate dehydrogenase:pyruvate + reduced NAD → lactate + NAD The NAD returns to glycolysis, so glycolysis can continue producing a net 2 ATP per glucose. This is anaerobic respiration (lactate fermentation). No carbon dioxide is produced, because pyruvate is not decarboxylated.Anaerobic respiration has advantages and costs:
• it produces ATP quickly and allows short bursts of intense activity when oxygen cannot be delivered fast enough;
• it produces far less ATP per glucose, because most of the energy is still held in lactate;
• lactate builds up and must be dealt with once oxygen is available again.Some cells rely on it permanently. Mammalian red blood cells have no mitochondria, so all their ATP comes from glycolysis followed by lactate production.Worked comparison: if aerobic respiration yields 32 ATP per glucose and anaerobic respiration yields 2, aerobic respiration produces 32 ÷ 2 = 16 times as much ATP from the same glucose.
• it produces ATP quickly and allows short bursts of intense activity when oxygen cannot be delivered fast enough;
• it produces far less ATP per glucose, because most of the energy is still held in lactate;
• lactate builds up and must be dealt with once oxygen is available again.Some cells rely on it permanently. Mammalian red blood cells have no mitochondria, so all their ATP comes from glycolysis followed by lactate production.Worked comparison: if aerobic respiration yields 32 ATP per glucose and anaerobic respiration yields 2, aerobic respiration produces 32 ÷ 2 = 16 times as much ATP from the same glucose.