Fig. 1.

Summary of transmembrane transporter classes. Channels are pore-forming proteins which open a gate in a ligand- or electric potential-dependent manner. Transporters are proteins which change their conformation and thereby transport a ligand from one side of the membrane to the other. Human ATP-dependent transporters work against a gradient as exporters (primary active transport). Secondary active transporters harness the electrochemical gradient of one solute to drive the transport of another solute in a symport or antiport mechanism. Uniporters facilitate diffusion of a solute along its gradient, but may exhibit different affinities for the solute depending on the direction of transport.