Ion gradient-driven energizers. Normally, outer membrane porins (1.B) of Gram-negative bacteria catalyze passive transport of solutes across the membrane, but coupled to eeenergizers,, they may accumulate their substrates in the periplasm against large concentration gradients. These energizers use the proton motive force (pmf) across the cytoplasmic membrane, probably by allowing the electrophoretic transport of protons, and conveying conformational change to the outer membrane receptor/porins. Homologous energizers drive bacterial flagellar motility. The mechanism is poorly understood, but these energizers undoubtedly couple proton H⁺ or sodium Na⁺ fluxes through themselves to the energized process