Chemiosmotic

Chemiosmotic theory, often referred to as chemiosmosis, is a biochemical model that explains how cells generate ATP during oxidative phosphorylation and photosynthesis. Proposed by Peter Mitchell in 1961, the theory posits that energy from electron transfer is stored as an electrochemical gradient of protons across a membrane. This gradient, the proton motive force, has two components: a chemical gradient (pH difference) and an electrical gradient (membrane potential).

In mitochondria, electrons are transferred through the respiratory chain located in the inner mitochondrial membrane. Complexes I, III, and IV pump protons from the matrix into the intermembrane space, creating a proton motive force. Protons flow back into the matrix through the F0F1-ATP synthase, driving the phosphorylation of ADP to ATP. Not all complexes pump protons; for example, Complex II does not contribute to the proton gradient.

In chloroplasts, light energy drives electron transport across the thylakoid membrane, pumping protons into the thylakoid lumen. The resulting proton motive force powers ATP synthase to produce ATP needed for carbon fixation in the Calvin cycle. In both systems, the energy stored in the proton gradient is efficiently converted into chemical energy in the form of ATP.

The chemiosmotic model gained strong support from experiments showing that uncouplers and ionophores that dissipate the proton gradient inhibit ATP synthesis. The theory provides a unifying explanation for energy conservation in aerobic respiration, photosynthesis, and various bacterial processes that rely on proton or ion gradients to drive cellular work.