<P> Facilitated diffusion is the passage of molecules or ions across a biological membrane through specific transport proteins and requires no energy input . Facilitated diffusion is used especially in the case of large polar molecules and charged ions; once such ions are dissolved in water they cannot diffuse freely across cell membranes due to the hydrophobic nature of the fatty acid tails of the phospholipids that make up the bilayers . The type of carrier proteins used in facilitated diffusion is slightly different from those used in active transport . They are still transmembrane carrier proteins, but these are gated transmembrane channels, meaning they do not internally translocate, nor require ATP to function . The substrate is taken in one side of the gated carrier, and without using ATP the substrate is released into the cell . They may be used as potential biomarkers </P> <P> Reverse transport, or transporter reversal, is a phenomenon in which the substrates of a membrane transport protein are moved in the opposite direction to that of their typical movement by the transporter . Transporter reversal typically occurs when a membrane transport protein is phosphorylated by a particular protein kinase, which is an enzyme that adds a phosphate group to proteins . </P> <P> (Grouped by Transporter Classification database categories) </P> <Ul> <Li> α - helical protein channels such as voltage - gated ion channel (VIC), ligand - gated ion channels (LGICs) </Li> <Li> β - barrel porins such as aquaporin </Li> <Li> channel - forming toxins, including colicins, diphtheria toxin, and others </Li> <Li> Nonribosomally synthesized channels such as gramicidin </Li> <Li> Holins; which function in export of enzymes that digest bacterial cell walls in an early step of cell lysis . </Li> </Ul>

What type of membrane protein is a carrier protein