concept

Proton-motive force

The total force driving protons across a membrane, in millivolts, combining two parts: the electrical potential already there and the difference in proton concentration (that is, the pH difference). At warm temperatures each unit of pH difference contributes roughly 60–70 mV. Modern cells run on about 150–250 mV; an alkaline vent with a four-unit pH difference can exceed that — which is why energy was never the bottleneck in the origin of life.