Protonigradienttia

Protonigradienttia is the electrochemical gradient of protons (H+) across a biological membrane. It arises when proton pumps move H+ out of the cytosol or mitochondrial matrix, creating a higher proton concentration on one side of the membrane and a corresponding electrical potential due to charge separation. The resultant gradient stores energy that can be converted into work when protons flow back through proton-conducting proteins.

Components: The gradient comprises a chemical component, the difference in proton concentration (pH difference), and an electrical component, the membrane potential (Δψ). Together they form the proton motive force (PMF).

Biological roles: The PMF drives ATP synthesis in mitochondria and chloroplasts via the FoF1-ATP synthase. It also powers transport of metabolites and, in many bacteria, the rotation of flagella. In photosynthetic and respiratory membranes, light or electron transport reactions pump protons to establish the gradient.

Measurement and significance: The magnitude of PMF varies with organism and conditions; in mitochondria it is typically around 150–200 millivolts, with a ΔpH of roughly 0.5–1.0 pH unit. Perturbation of the gradient can impair energy production and increase reactive oxygen species. Uncoupling agents collapse the gradient, releasing energy as heat.

See also: chemiosmotic theory, proton motive force, ATP synthase, electron transport chain.