<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2009) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2009) (Learn how and when to remove this template message) </Td> </Tr> <P> A membrane transport protein (or simply transporter) is a membrane protein involved in the movement of ions, small molecules, or macromolecules, such as another protein, across a biological membrane . Transport proteins are integral transmembrane proteins; that is they exist permanently within and span the membrane across which they transport substances . The proteins may assist in the movement of substances by facilitated diffusion or active transport . The two main types of proteins involved in such transport are broadly categorized as either channels or carriers . The solute carrier s and atypical SLC s are secondary active or facilitative transporters in humans . </P> <P> A carrier is not open simultaneously to both the extracellular and intracellular environments . Either its inner gate is open, or outer gate is open . In contrast, a channel can be open to both environments at the same time, allowing the solutes it transports to diffuse without interruption . Carriers have binding sites, but pores and channels do not . When a channel is opened, millions of ions can pass through the membrane per second, but only 100 to 1000 molecules typically pass through a carrier molecule in the same time . Each carrier protein is designed to recognize only one substance or one group of very similar substances . Research has correlated defects in specific carrier proteins with specific diseases . </P>

A carrier is a transmembrane protein that moves molecules across the membrane through
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