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||The outer membrane allows free diffusion
of molecules smaller than 600 Da due to the presence of
non-specific transporters like porins [primary sources]. See Arora et al., 2000 PMID 10636850 p.1594 left column bottom paragraph:"The outer membrane of Gram-negative bacteria serves as a molecular sieve, resisting the entry of noxious compounds, while at the same time allowing the uptake of essential nutrients. Molecules up to about 600 Da in size are taken up by a set of more or less substrate-specific porins and transporters. Outer membrane proteins were among the first integral membrane proteins for which crystal structures have been solved by x-ray diffraction."