Oxidative Phosphorylation: How Cells Make Most of Their ATP

Oxidative phosphorylation is the metabolic pathway that produces the majority of a cell's energy currency, ATP (Adenosine Triphosphate): The Universal Energy Currency of Living Cells. It is the last stage of cellular respiration, downstream of glycolysis and the citric acid cycle, and it harvests the chemical energy stored in electron carriers such as NADH and FADH2. The pathway has two tightly linked halves. The first is The Electron Transport Chain: The Mitochondrial Assembly Line That Makes Most of Your ATP, a series of protein complexes embedded in the inner membrane of the Mitochondria: The Powerhouse Organelles with Their Own DNA (in eukaryotes) or the plasma membrane (in prokaryotes). As electrons hop from one complex to the next toward oxygen, energy is released in controlled steps rather than all at once. The complexes use that energy to pump protons (H+) from the mitochondrial matrix into the intermembrane space, building up an electrochemical gradient. That gradient is the proton-motive force, which has two parts: a difference in pH (chemical) and a difference in electrical charge across the membrane. It stores potential energy much like water held behind a dam. The second half of the pathway, chemiosmosis, releases it: protons flow back into the matrix through ATP synthase, a rotary molecular motor whose spinning drives the phosphorylation of ADP into ATP. Roughly three to four protons pass through for each ATP made. Oxygen is essential because it is the terminal electron acceptor. The final complex, Cytochrome c Oxidase: The Final Enzyme in Cellular Respiration, hands electrons to molecular oxygen, which combines with protons to form water. Without oxygen to pull electrons through, the whole chain backs up and stalls. Oxidative phosphorylation is far more efficient than fermentation: complete oxidation of a single glucose molecule yields roughly 30 to 36 ATP, compared with just 2 from glycolysis alone.