<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>

Which type of membrane transport requires a carrier molecule