Chemiosmosis

Chemiosmosis is the process by which cells make ATP using the energy stored in a gradient of hydrogen ions (protons) across a membrane. The British biochemist Peter Mitchell proposed the idea in 1961 and was awarded the 1978 Nobel Prize in Chemistry for it.

As electrons pass along the electron transport chain, protein complexes pump protons across a membrane, creating a difference in concentration and in electrical charge, called the proton-motive force. The protons flow back through the enzyme ATP synthase, a rotary motor that uses their movement to join ADP and phosphate into ATP. The idea was resisted at first because it did not need a chemical intermediate, but experiments confirmed it.

It takes place across the inner membrane of mitochondria during oxidative phosphorylation, across the thylakoid membrane of the chloroplast in photosynthesis, and across the plasma membrane of bacteria and archaea. The shared mechanism is one reason that endosymbiotic theory traces these organelles to bacterial ancestors.

The proton-motive force also turns bacterial flagella and powers the uptake of nutrients. Uncoupling proteins let protons leak back to produce heat, as in brown fat. Chemiosmosis is a central topic of bioenergetics, and the voltage part of the gradient follows the logic of the Nernst equation. A hypothesis based on similar gradients is proposed for energy sources at the origin of life near a hydrothermal vent.

• ATP Synthase — the enzyme that uses the gradient
• Electron Transport Chain — the system that builds the gradient
• Oxidative Phosphorylation — the process in mitochondria