What is "chemiosmosis"

n. (context chemistry English) Any chemical reaction that occurs through an intervening semipermeable membrane, especially through a membrane in a biological cell

Chemiosmosis is the movement of ions across a semipermeable membrane, down their electrochemical gradient. More specifically, it relates to the generation of adenosine triphosphate (ATP) by the movement of hydrogen ions across a membrane during cellular respiration or photosynthesis.

Hydrogen ions, or protons, will diffuse from an area of high proton concentration to an area of lower proton concentration, and an electrochemical concentration gradient of protons across a membrane can be harnessed to make ATP. This process is related to osmosis, the diffusion of water across a membrane, which is why it is called "chemiosmosis".

ATP synthase is the enzyme that makes ATP by chemiosmosis. It allows protons to pass through the membrane and uses the kinetic energy to phosphorylate adenosine diphosphate (ADP), making ATP. The generation of ATP by chemiosmosis occurs in mitochondria, chloroplasts as well as in most bacteria and archaea.