Cells rely on energy carriers, most notably ATP, to perform cellular work. ATP fuels so many reactions that a person may use as many as 1025 of these molecules every day. The process for making ATP during cellular respiration (in mitochondria) and photosynthesis (in chloroplasts) is known as chemiosmosis.
In chemiosmosis, proton (H+) diffusion is coupled to ATP synthesis. When protons build up on one side of a membrane, they form an electrochemical gradient across the membrane. The proton concentration gradient and the electric charge difference constitute a source of potential energy called the proton-motive force. This force tends to drive the protons back across the membrane. Protons diffuse across the membrane through a specific proton channel, called ATP synthase, which couples proton flow with the formation of ATP.
The chemiosmotic mechanism was first proposed by Peter Mitchell in 1961 and has since been widely confirmed through experimentation, beginning with an experiment using chloroplasts.
The chemiosmosis hypothesis was a bold departure from the conventional scientific thinking of the time, which did not consider a role for membranes in the process of ATP synthesis.
The chemiosmotic mechanism involves a complex of transmembrane proteins—including a proton channel and the enzyme ATP synthase—that couples proton diffusion to ATP synthesis. The potential energy of the proton gradient, or the proton-motive force, is harnessed by ATP synthase. ATP synthase acts as a channel allowing protons to diffuse back across the membrane (from high proton concentration to low), and it uses the energy of that diffusion to make ATP from ADP and Pi.
Textbook Reference: Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy