8.12: ATP Yield
Cellular respiration produces 30-32 ATP molecules per glucose molecule. Although most of the ATP results from oxidative phosphorylation and the electron transport chain (ETC), 4 ATP are gained beforehand (2 from glycolysis and 2 from the citric acid cycle).
The ETC is embedded in the inner mitochondrial membrane and comprises four main protein complexes and an ATP synthase. NADH and FADH2 pass electrons to these complexes, which in turn pump protons into the intermembrane space. This distribution of protons generates a concentration gradient across the membrane. The gradient drives the production of ATP when protons flow back into the mitochondrial matrix via the ATP synthase.
For every 2 input electrons that NADH passes into complex I, complexes I and III each pump 4 protons and complex IV pumps 2 protons, totaling 10 protons. Complex II is not involved in the electron chain initiated by NADH. FADH2, however, passes 2 electrons to complex II, so a total of 6 protons are pumped per FADH2; 4 protons via complex III and 2 via complex IV.
Four protons are needed to synthesize 1 ATP. Since 10 protons are pumped for every NADH, 1 NADH yields 2.5 (10/4) ATP. Six protons are pumped for every FADH2, so 1 FADH2 yields 1.5 (6/4) ATP.
Cellular respiration produces a maximum of 10 NADH and 2 FADH2 per glucose molecule. Since a single NADH produces 2.5 ATP and a single FADH2 produces 1.5 ATP, it follows that 25 ATP + 3 ATP are produced by oxidative phosphorylation. Four ATP are produced before oxidative phosphorylation, which yields a maximum of 32 ATP per glucose molecule.
Importantly, glycolysis occurs in the cytosol and the ETC is located in the mitochondria (in eukaryotes). The mitochondrial membrane is not permeable to NADH, hence the electrons of the 2 NADH that are produced by glycolysis need to be shuttled into the mitochondria. Once inside the mitochondrion, the electrons may be passed to NAD+ or FAD. Given the different ATP yield depending on the electron carrier, the total yield of cellular respiration is 30 to 32 ATP per glucose molecule.