Carrier-mediated transport systems (facilitated diffusion and active transport) have a transport maximum (T_m: the maximum rate at which a carrier can transport a solute). This is because the carriers exhibit specificity and saturation. Specificity is the ability to bind with only one ligand or a few closely related ligands. A carrier will only be able to transport a specific solute. As solute concentration increases, the rate of transport increases as well, until the point of saturation. When all carriers are transporting solute, no additional solute can be transported. There are no more carriers available to increase the speed of transport. This is the transport maximum.

Some proteins can transport two or more solutes. If a second, competing solute is present, transport of the first solute is slower.