Adding up the products of each stage shows how much ATP aerobic respiration can make from one glucose molecule.Products per glucose:
• Glycolysis: net 2 ATP, 2 reduced NAD
• Link reaction (×2): 2 reduced NAD
• Krebs cycle (×2): 2 ATP, 6 reduced NAD, 2 reduced FAD
• Total: 4 ATP made by substrate-level phosphorylation, 10 reduced NAD and 2 reduced FADMost ATP is made by oxidative phosphorylation. Reduced FAD enters the electron transport chain further along than reduced NAD, so it yields less ATP.Worked example (theoretical maximum), using 3 ATP per reduced NAD and 2 ATP per reduced FAD:
• 10 reduced NAD × 3 = 30 ATP
• 2 reduced FAD × 2 = 4 ATP
• substrate-level phosphorylation = 4 ATP
• total = 38 ATPIn real cells the yield is lower, about 30 to 32 ATP per glucose, because:
• some protons leak back across the inner membrane without passing through ATP synthase;
• energy is used to transport pyruvate, ADP and phosphate into the mitochondrion and ATP out;
• the reduced NAD from glycolysis is made in the cytoplasm, and passing its hydrogen into the mitochondrion can cost energy.Anaerobic respiration yields only the 2 ATP from glycolysis, so aerobic respiration releases around 15 times more usable energy from each glucose. Even so, only about a third of the energy in glucose ends up in ATP; the rest is released as heat.
• Glycolysis: net 2 ATP, 2 reduced NAD
• Link reaction (×2): 2 reduced NAD
• Krebs cycle (×2): 2 ATP, 6 reduced NAD, 2 reduced FAD
• Total: 4 ATP made by substrate-level phosphorylation, 10 reduced NAD and 2 reduced FADMost ATP is made by oxidative phosphorylation. Reduced FAD enters the electron transport chain further along than reduced NAD, so it yields less ATP.Worked example (theoretical maximum), using 3 ATP per reduced NAD and 2 ATP per reduced FAD:
• 10 reduced NAD × 3 = 30 ATP
• 2 reduced FAD × 2 = 4 ATP
• substrate-level phosphorylation = 4 ATP
• total = 38 ATPIn real cells the yield is lower, about 30 to 32 ATP per glucose, because:
• some protons leak back across the inner membrane without passing through ATP synthase;
• energy is used to transport pyruvate, ADP and phosphate into the mitochondrion and ATP out;
• the reduced NAD from glycolysis is made in the cytoplasm, and passing its hydrogen into the mitochondrion can cost energy.Anaerobic respiration yields only the 2 ATP from glycolysis, so aerobic respiration releases around 15 times more usable energy from each glucose. Even so, only about a third of the energy in glucose ends up in ATP; the rest is released as heat.