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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.
Figure 1: Two ion channels in the lipid bilayer of the cell membrane. These channels are made from four subunits. The one on the right is cut away to allow a view of the pore inside. Ion channels are ion-permeable pores in the lipid membranes of all cells.