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Arsenic metabolism in living organisms is dependent on the ability of different arsenic species to traverse biological membranes.Simple diffusion provides an altative influx and efflux route to mediated transport mechanisms that can increase the amount of arsenic available for metabolism in cells.Using octanol-water and liposome-water partition coefficients,the ability of arsenous acid,arsenate,methylarsonate,dimethylarsinate,thio-methylarsonate,thiodimethylarsinic acid,arsenotriglutathione and monomethylarsonic diglutathione to diffuse through the lipid bilayer of cell membranes was investigated.Molecular modelling of arsenic species was used to explain the results.All arsenic spedes with the exception of arsenate,methylarsonate and thio-methylarsonate were able to diffuse through the lipid bilayer of liposomes,with liposome-water partition coefficients between 0.04 and 0.13.Trivalent arsenic species and thio-pentavalent arsenic species showed higher partition coefficients,suggesting that they can easily traverse cell membranes by passive simple diffusion.Given the higher toxicity of these species compared to oxo-pentavalent arsenic species,this study provides evidence supporting the risk associated with human exposure to trivalent and thio-arsenic species.