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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.
The smallest atom is a hydrogen atom (37 picometers) whereas some larger atoms more than 5 times larger (lanthanum is 187 picometers). If the atom is an ion, it will also change it's radius significantly (positive ions are smaller and negative ions are larger than the neutral atom).