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Many, perhaps most, porins form non-specific channels across the outer membrane. Any molecule small enough to pass through the channel can move through the porin by passive diffusion. A recent reference is: Kojima S, Nikaido H.Permeation rates of penicillins indicate that Escherichia coli porins function principally as nonspecific channels. Proc Natl Acad Sci U S A. 2013 Jul 9;110(28):E2629-34. doi: 10.1073/pnas.1310333110. Epub 2013 Jun 24. http://www.ncbi.nlm.nih.gov/pubmed/23798411Siegele (talk) 04:31, 7 February 2014 (UTC)