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The channel has a large diameter of 0.8x1.1nm, and is non-selective for small ions. However, it has an upper exclusion size limit corresponding to molecular weights of about 600 Dalton of permeants. Because most metabolites have molecular weights lower than 600 Da and have been shown to pass through porin channels, porins are called general diffusion pores.
Because ions are charged and water-soluble, they must move through small tunnels or channels (specialized proteins) that span the cell membrane's lipid bilayer. Each channel is specific for only one type of ion. There are specific channels for sodium ions, potassium ions, calcium ions and chloride ions. These channels make the cell membrane selectively permeable to various ions and other substances (like glucose).