Oxidative phosphorylation is a highly efficient process that generates large amounts of adenosine triphosphate (ATP), the basic unit of energy that drives many processes in living cells. Oxidative phosphorylation involves two processes—electron transport and chemiosmosis. During electron transport, electrons are shuttled between large complexes on the inner mitochondrial membrane and protons (H+) are pumped across the membrane into the intermembrane space, creating an electrochemical gradient. In the next step, protons flow back down their gradient into the mitochondrial matrix via ATP synthase, a protein complex embedded within the inner membrane. This process, called chemiosmosis, uses the energy of the proton gradient to drive the synthesis of ATP from adenosine diphosphate (ADP).
The electron transport chain is a series of complexes that transfer electrons from electron donors to electron acceptors via simultaneous reduction and oxidation reactions, otherwise known as redox reactions. At the end of the chain, electrons reduce molecular oxygen to produce water.
The shuttling of electrons between complexes is coupled with proton transfer, whereby protons (H+ ions) travel from the mitochondrial matrix to the intermembrane space against their concentration gradient. Eventually, the high concentration of protons in the intermembrane space forces protons down their concentration gradient back into the mitochondrial matrix through ATP synthase, thus producing ATP. This process, which uses energy stored in the proton gradient across the membrane to drive cellular work, is called chemiosmosis.
The structure responsible for the movement of protons across the inner mitochondrial membrane is the protein complex ATP synthase. It consists of a stator—the channel in which hydrogen ions enter and leave the complex, a multi-unit rotor (F0) embedded within the membrane, and a knob of catalytic proteins (F1) located in the mitochondrial matrix. The F0 rotor spins as hydrogen ions bind to, and change the shape of, each sub-unit. The spinning rotor then turns an internal rod that changes the conformation of F1 that facilitates its binding to ADP and inorganic phosphate, resulting in the production of ATP.
The process of aerobic respiration can produce a total of 30 or 32 ATP per molecule of glucose consumed (Figure 3). Four ATP are produced during glycolysis, but two are consumed in the process, resulting in a net total of two ATP molecules. One ATP molecule is produced per round of the Krebs cycle, and two cycles occur for every molecule of glucose, producing a net total of two ATP. Finally, 26 or 28 ATP are produced in the electron transport chain through oxidative phosphorylation, depending on whether NADH or FADH2 is used as the electron carrier.
