Chemiosmose

Chemiosmosis is the process by which a transmembrane electrochemical gradient, typically of protons, powers the synthesis of ATP. The gradient, known as the proton motive force, combines a difference in proton concentration (pH) with an electrical potential across the membrane. As protons move back across the membrane through the enzyme ATP synthase, the energy released drives the phosphorylation of ADP to ATP.

In mitochondria, the electron transport chain pumps protons from the matrix across the inner mitochondrial membrane into the intermembrane space, generating a gradient. This proton motive force drives ATP production by oxidative phosphorylation as protons re-enter the matrix through ATP synthase.

In chloroplasts, light energy drives electron transport in the thylakoid membranes, pumping protons into the thylakoid lumen. ATP synthase then generates ATP in the stroma, a process called photophosphorylation, which supplies energy for the Calvin cycle.

Chemiosmotic mechanisms also operate in bacteria, where the proton motive force across the plasma membrane powers ATP synthesis, as well as other energy-dependent processes such as nutrient transport and, in some species, rotation of flagella.

The chemiosmotic theory, proposed by Peter Mitchell in 1961, posits that energy from redox reactions is first stored as a proton gradient rather than being transferred directly to ATP. The concept explains how cells convert energy efficiently in respiration, photosynthesis, and other biological processes.