Chemiosmosis

Chemiosmosis is the process by which ions diffuse across a mitochondrial permeable membrane. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration.

In chemiosmosis, ions (electrons) of hydrogen diffuse from an area of high electron concentration to an area of lower electron concentration. An electrochemical concentration gradient of electrons across a membrane could be harnessed to make ATP. This process is similar to osmosis, which is the diffusion of water across a membrane, hence the name chemiosmosis.

ATP synthase is the enzyme that makes ATP by chemiosmosis. It allows electrons to pass through the membrane using the kinetic energy to phosphorylate ADP making ATP. The generation of ATP by chemiosmosis occurs in chloroplasts and mitochondria as well as in some bacteria.

Chemiosmotic phosphorylation is the third pathway that produces ATP from inorganic phosphate and an ADP molecule. This process is part of oxidative phosphorylation. The complete breakdown of glucose in the presence of oxygen is called cellular respiration. The last steps of this process occur in mitochondria. The reduced molecules NADH and FADH2 are generated by the Krebs cycle and glycolysis. These molecules pass electrons to an electron transport chain, which uses the energy released to create a proton gradient across the inner mitochondrial membrane. ATP synthase then uses the energy stored in this gradient to make ATP. This process is called oxidative phosphorylation because oxygen is the final electron acceptor and the energy released by reducing oxygen to water is used to phosphorylate ADP and generate ATP.

Electron Transport Chain (Video)