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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.
A atom is the smallest neutral particle that can be identified as still that element. However, a positive ion is smaller than the neutral atom; it still has the same number of protons so it's still the element. Removal of successive electrons makes the ions still smaller.