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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.
For reference, the radius of the first minimum for a diffraction pattern formed on the retina is about 4.6 micrometers with 550-nanometer light and a pupil diameter of 2 millimeters. Thus, the arrangement of sensory elements in the retina will determine the limiting resolution of the eye.