Chapter 14: Electron Transport and ATP Synthesis

The process occurs within the inner mitochondrial membrane, where a series of protein complexes work together to extract energy from reducing equivalents such as NADH and FADH2, ultimately generating most of the cell's ATP. The chemiosmotic theory explains how the energy released from electron transfer is captured by pumping protons across the inner membrane, establishing both a concentration gradient and an electrical potential difference known as the protonmotive force. This gradient stores energy that drives ATP synthesis. Electrons flow through four sequential complexes of increasing reduction potential: Complex I oxidizes NADH and pumps four protons; Complex II accepts electrons from succinate but does not pump protons; Complex III processes ubiquinol through the Q cycle mechanism and pumps four protons; and Complex IV catalyzes oxygen reduction to water while pumping two protons. These complexes are linked by mobile electron carriers including ubiquinone and cytochrome c. Complex V, ATP synthase, functions as a molecular motor where protons flowing back into the matrix drive conformational changes in catalytic subunits through Boyer's binding change mechanism, generating ATP with an efficiency of approximately three protons per ATP molecule synthesized. The chapter addresses the energetic cost of transporting ATP out and substrates into the mitochondrion, establishing that four total protons are required per ATP available to the cell. Calculation of P/O ratios reveals that NADH-linked oxidation yields approximately 2.5 ATP per oxygen atom reduced, while succinate-linked oxidation yields 1.5 ATP due to bypassing Complex I. The chapter also discusses how cytosolic NADH from glycolysis must transfer its electrons to the matrix through shuttle systems, including the malate-aspartate shuttle and the glycerol phosphate shuttle. Finally, the chapter addresses reactive oxygen species generated during aerobic metabolism and the protective enzyme systems including superoxide dismutase and catalase that prevent oxidative damage to cells.