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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.
In the cell, channel proteins are responsible for bringing in ions and other small molecules. Each channel protein can only bring in a specific molecule. For example, a calcium channel can only be used to transport calcium in and out of the cell.