Na+-coupled versus H+-coupled energy transduction in bacteria

Protons and sodium ions are the major coupling ions in membrane associated bioenergetic processes in bacteria. Most bacteria maintain electrochemical gradients of both proton and Na + across the cytoplasmic membrane. The gradients exert a force on the cations to move from the extracellular medium across the membrane into the cell. From a thermodynamic point of view the protonmotive force and the sodium ion motive force are equivalent, and composed of both a chemical and an electrical component. The ion motive force (imf) expressed in electrical field units is described by