8.14 : ATP Yield

Cellular respiration produces 30 - 32 ATP 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 is comprised of four main protein complexes and an ATP synthase. NADH and FADH₂ pass electrons to these complexes, which 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 two electrons that NADH passes through complex I, a total of 10 protons are pumped: complex I and complex III each pump 4 protons, while complex IV pumps 2 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 protons 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, 25 ATP + 3 ATPs 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. As a result, 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 yields depending on the electron carrier, the total yield of cellular respiration is 30 to 32 ATP per glucose molecule.