These include:

2.A Porters (uniporters, symporters, antiporters). Transport systems are included in this subclass if they utilize a carrier-mediated process to catalyze uniport (a single species is transported either by facilitated diffusion or in a membrane potential-dependent process if the solute is charged), antiport (two or more species are transported in opposite directions in a tightly coupled process, not coupled to a direct form of energy other than chemiosmotic energy) and/or symport (two or more species are transported together in the same direction in a tightly coupled process, not coupled to a direct form of energy other than chemiosmotic energy).

2.B Non-ribosomally synthesized porters. These substances, like non-ribosomally synthesized channels, may be depsipeptides or non-peptide-like substances. They complex a cation in their hydrophilic interior and facilitate translocation of the complex across the membrane, exposing their hydrophobic exterior, by moving from one side of the bilayer to the other. If the free porter can cross the membrane in the uncomplexed form, the transport process can be electrophoretic, but if only the complex crosses the membrane, transport is electroneutral.

2.C 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 ppproton 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.