Question

How does each molecule of NADH generate 2.5 ATPs after electron transport and oxidative phosphorylation?

Why does FADH2 only generate 1.5?

Answer 1

NADH produces 2.5 ATP during the ETC (Electron Transport Chain) with oxidative phosphorylation because NADH gives up its electron to Complex I, which is at a higher energy level than the other Complexes. When Complex I transfers the electron to Complex III, energy is given off to pump protons across the membrane, creating a gradient. The electron moves again to Complex IV and again pumps more electrons across the membrane. Because NADH started with Complex I, it had more chances to pumps more protons across the gradient, which powers the ATP synthase and gives us 3 ATP per molecule of NADH.

FADH2 produces 1.5 ATP during the ETC because it gives up its electron to Complex II, bypassing Complex I. By bypassing Complex I, we missed a chance to pump protons across the membrane, so less protons have been pumped by the time we get to Complex IV. Protons still have been pumped, enough to fuel 1.5 ATP created by ATP synthase.