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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 the nucleus can't hold the 18 electrons in the Cl-ion as tightly as the 17 electrons in the neutral atom, the negative ion is significantly larger than the atom from which it forms. For the same reason, positive ions should be smaller than the atoms from which they are formed. The 11 protons in the nucleus of an Na+ ion, for example, should be able to hold the 10 electrons on this ion more tightly than the 11 electrons on a neutral sodium atom.