How Cells Use Energy

Oxidative Phosphorylation

During oxidative phosphorylation, the oxidation of electron carriers is coupled to the phosphorylation of ADP to produce most of the ATP made by the cell.
After high-energy electrons are created during the citric acid cycle, the conversion of pyruvate to acetyl-CoA, and glycolysis, they are shuttled to an electron transport chain, where ATP can be generated for the cell. The electron transport chain is a series of electron transporters that move electrons from NADH and FADH2 to oxygen molecules; protons are pumped from the mitochondrial matrix to the intermembrane space, and oxygen is reduced to form water. In other words, protein complexes embedded in the inner mitochondrial membrane transfer electrons from electron donors to electron acceptors. Oxidative phosphorylation is the production of ATP through chemiosmosis in the mitochondria through an electron transport chain. (Chemiosmosis is the movement of ions across a semipermeable membrane down their electrochemical gradient, which results in a charge that can be used as a source of energy.) It involves two distinct processes: electron transport with the help of the electron transport chain and phosphorylation of ADP to ATP. This entire process requires oxygen and takes place in the mitochondria of the cell. Oxidative phosphorylation is the main source of cellular energy in the form of ATP.

Steps of Oxidative Phosphorylation

During oxidative phosphorylation, electrons are transferred to molecules near the start of the electron transport chain by NADH and FADH2 at mitochondrial protein complexes I and II. Protein complexes III and IV pump protons from the matrix to the intermembrane space, creating a proton gradient. Near the end of the transport chain, electrons are transferred to oxygen (O) molecules, which split to form water (H2O). ATP synthase pumps protons back into the matrix and generates ATP.
The first step that occurs is the oxidation of NADH and FADH2, which is part of the electron transport chain. At the inner mitochondrial membrane, NADH and FADH2 are oxidized to NAD+ and FAD and release high-energy electrons. These electrons are transferred to proteins in the electron transport chain. As the electrons move along the electron transport chain, they fall to successively lower energy states. The energy released by electrons is used to move protons (H+ ions) from within the mitochondrial matrix to the intermembrane space. Meanwhile, electrons are shuttled through four different protein complexes, with the help of electron carriers.

In Protein complex I, NADH dehydrogenase works to remove protons (H+ ions) from NADH in an oxidation reaction, while in Protein complex II, succinate dehydrogenase removes electrons from FADH2 via a similar oxidation reaction. The energy from these molecules is used to pump protons (H+) away from the mitochondrial matrix, creating a gradient and serving as temporary storage of the energy. Protein complex III also acts as a proton pump. Additional hydrogen ions are pumped away from the mitochondrial matrix, and cytochrome b-c transfers electrons onto Protein complex IV, where cytochrome c oxidase combines the electrons with protons and the final electron acceptor in the mitochondria, oxygen. Oxygen receives electrons that are in the last protein complex and then combines with protons inside the mitochondrial cell to make water.

The proteins of the electron transport chain use the energy released by the liberated electrons to drive the hydrogen pumps that move protons across the inner membrane from the mitochondrial matrix to the intermembrane space. The resulting proton gradient serves as a source of energy for ATP synthase because, when NADH and FADH2 are oxidized, the intermembrane space becomes positively charged due to the protons that were pumped there. The mitochondrial matrix, therefore, becomes negatively charged because there is a loss of protons and a high concentration of electrons in the space. This difference in charge creates a potential-energy difference that drives the diffusion of protons back inside the mitochondrial matrix through an enzyme complex called ATP synthase. ATP synthase generates ATP by phosphorylating adenosine diphosphate (ADP). When this happens, ATP synthase takes energy produced from the proton shuttling process and uses it to convert ADP to ATP.