Oxidative phosphorylation is a highly efficient process that generates large amounts of adenosine triphosphate (ATP), the basic unit of energy that drives many cellular processes. Oxidative phosphorylation involves two processes— the electron transport chain and chemiosmosis.
Electron Transport Chain
The electron transport chain involves a series of protein complexes on the inner mitochondrial membrane that undergo a series of redox reactions. At the end of this chain, the electrons reduce molecular oxygen to produce water.
The shuttling of electrons between complexes is coupled with proton transfer from the mitochondrial matrix to the intermembrane space against their concentration gradient. Eventually, the high concentration of protons in the intermembrane space drives ATP synthase, a protein complex embedded within the inner membrane, to produce ATP in a process called chemiosmosis. It was biochemist Peter Mitchell who discovered the chemiosmotic mechanism required in respiring cells for ATP synthesis. Similarly, plants also use chemiosmosis to convert energy from sunlight into chemical energy in the form of ATP.
ATP synthase is a multi-subunit complex. It consists of a stator—the channel through which protons 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 binding of the incoming protons to the F0 rotor makes it spin. The spinning rotor then turns the internal stalk called γ-subunit, which passes through the center of the F1 subunits. The rotation of the γ-subunit facilitates changes in the conformation of F1 sub-units such that they can catalyze the synthesis of ATP from ADP and inorganic phosphate.
The process of aerobic respiration can produce a total of 30 or 32 ATPs per molecule of glucose consumed. 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 glucose molecule, producing a net total of two ATP. Finally, 32 to 34 ATP are produced in the electron transport chain through oxidative phosphorylation, depending on whether NADH or FADH2 is used as the electron carrier.