ABSTRACT
The great majority of physiologically important solutes, both ions and nonelectrolytes, use specific protein vehicles to cross the plasma membrane. The protein which alternately exposes its substrate binding site to the external and to the internal medium is usually called the carrier, although other terms, such as permease, translocator, and porter, are found in the literature. There is a plethora of postulated models which attempt to physically visualize the concept of carrier-mediated transport. The chapter considers the important groups of substances transported by a carrier mechanism. The following membrane proteins, apparently involved in monosaccharide transport, have been isolated and at least partially purified: the inducible galactose-binding protein, the constitutive glucose-binding lipoprotein, and two glucose-binding proteins and a mannose-binding protein. It is clear that there is a good deal of direct and indirect interaction among different classes of compounds during transport events.
