Chemiosmosis

Chemiosmosis is the movement of ions across a semipermeable membrane, down their electrochemical gradient. An example of this would be 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 free energy difference to phosphorylate adenosine diphosphate (ADP), making ATP. The generation of ATP by chemiosmosis occurs in and chloroplasts, as well as in most bacteria and oarchaea, an electron transport chain pumps H+ ions in the thylakoid spaces through thylakoid membranes. The energy from the electron movement through electron transport chains cross through ATP synthase which allows the proton to pass through them and use this free energy difference to photophosphorylate ADP making ATP.

Peter D. Mitchell proposed the chemiosmotic hypothesis in 1961. The theory suggests essentially that most adenosine triphosphate (ATP) synthesis in respiring cells comes from the electrochemical gradient across the inner membranes of by using the energy of NADH and FADH₂ formed from the breaking down of energy-rich molecules such as glucose.

...
Wikipedia