File:BIOL 111 Figure 9.16.png

Oxidative Phosphorylation

Chemiosmosis couples the electron transport chain to ATP synthesis.

1. NADH and FADH₂ shuttle high-energy electrons extracted from food during glycolysis and the citric acid cycle to an electron transport chain built into the inner mitochondrial membrane. The gold arrows trace the transport of electrons, which finally pass to oxygen at the "downhill" end of the chain, forming water. Most of the electron carriers of the chain are grouped into four complexes (See Figure 9.13). Two mobile carriers, Ubiquinone (Q) and Cytochrome c (Cyt c), move rapidly, ferrying electrons between the large complexes. As complexes I, III, and IV accept and then donate electrons, they pump protons from the mitochondrial matrix into the intermembrane space. (In prokaryotes, protons are pumped outside the plasma membrane.) Note that FADH₂ deposits its electrons via complex II and so results in fewer protons being pumped into the intermembrane space than occurs with NADH, Chemical energy originally harvested from food is transformed into a proton-motive force, a gradient of H⁺ across the membrane.
2. During chemiosmosis, the protons flow back down their concentration gradient via ATP synthase, which is built into the membrane nearby. The ATP synthase harnesses the proton-motive force to phosphorylate ADP, forming ATP. Together, electron transport and chemiosmosis make up oxidative phosphorylation.