protonmotive

The protonmotive force, commonly called the proton motive force (PMF), is the electrochemical gradient across a biological membrane formed by protons (H+) that have been pumped across the membrane. It consists of two components: a chemical gradient (ΔpH) and an electrical gradient (ΔΨ). The PMF can be expressed as pmf = ΔΨ − (2.303 RT/F) ΔpH, and is typically measured in millivolts.

Formation occurs in mitochondria, chloroplasts, and many bacteria, where electron transport chains pump protons from the interior (matrix or stroma) to the opposite side of the membrane. This activity creates a higher proton concentration on one side and a more negative interior. The resulting gradient stores energy that can be tapped to perform work as protons re-enter through membrane protein complexes.

Functionally, the primary use of the PMF is ATP synthesis via the F-type ATP synthase. Protons flowing down the gradient drive rotation and the phosphorylation of ADP to ATP. The PMF also powers other processes, including active transport of nutrients through cotransporters and, in many bacteria, the rotation of the flagellar motor.

Significance and history: The chemiosmotic theory, proposed by Peter Mitchell, explains how electron transport chains couple to ATP formation through the PMF and is central to understanding respiration, photosynthesis, and energy transformation in cells. The magnitude of the PMF can vary among organisms and conditions and can be altered by ionophores, uncouplers, or inhibitors that disrupt the gradient.