Uniport carrier systems

There are, however, various types of active transport systems, involving protein carriers and known as uniports, symports, and antiports as indicated in Figure 3.7. Thus, symports and antiports involve the transport of two different molecules in either the same or a different direction. Uniports are carrier proteins, which actively or passively (see section "Facilitated Diffusion") transport one molecule through the membrane. Active transport requires a source of energy, usually ATP, which is hydrolyzed by the carrier protein, or the cotransport of ions such as Na+ or H+ down their electrochemical gradients. The transport proteins usually seem to traverse the lipid bilayer and appear to function like membrane-bound enzymes. Thus, the protein carrier has a specific binding site for the solute or solutes to be transferred. For example, with the Na+/K+ ATPase antiport, the solute (Na+) binds to the carrier on one side of... 

Transport of many compounds including drugs across cell membranes is mediated by membrane proteins called carrier proteins or channel proteins. Some of these proteins transport only one substrate molecule at a time across the membrane (uniport systems), while others act as cotransport systems (Figure 9.4). Depending on the direction of the second substrate, the proteins are also called symporters or antiporters, for example, Na /glucose cotransporter, H " /peptide cotransporter, or Na /K antiporter (—Na /K -ATPase). 

Mitchell [7] visualized three different systems for facilitated secondary transport. Uniport only one solute is translocated by the carrier protein. Symport two or more different solutes are translocated in the same direction by the carrier protein. Antiport two or more different solutes are translocated by one carrier in opposite directions. 

A carrier which transports a single molecule in one direction is called a uniport system. Alternatively, a carrier may carry two molecules simultaneously in the same direction, i.e. a symport system. TTiirdly, a carrier may exchange one molecule for another and therefore transport them in opposite directions, i.e an antiport system (Figure 9.4). 

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