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) of secondary carriers (Forrest et al., 2010).
Transport through lipids and aquaporins is entirely driven by the difference in osmotic pressure on the two sides of the membrane. Water transport in cotransporters and uniporters is different: It can be cotransported, energized by coupling to the substrate flux. In the K+/Cl- and the Na+/K+/2Cl- cotransporters (2.A.30), water is cotransported, while water transport in glucose uniporters (2.A.1.1) and Na+-coupled transporters of nutrients (2.A.21) and neurotransmitters (2.A.22) takes place by both osmosis and cotransport (Zeuthen, 2010).
Many transport proteins arose by internal multiplication of repeat units (Saier, 2003). This is particularly true for secondary carriers, a conclusion that has been confirmed (Hennerdal et al., 2010). Topologies of these membrane proteins represent a balance between long and short range lipid-protein interactions (Vitrac et al., 2011).
A system of secondary carrier classification in wide use, specifically for humans and other mammalian species, is the solute carrier (SLC) system. The interconversion of the TC and SLC systems by family is as follows: