Flow of ions through narrow pores, either induced in biological membranes or created in synthetic membrane filters, exhibits, under appropriate conditions: 1) rapid switching of ion current between high and low conducting states; 2) selectivity between different ions; 3) inhibition by protons or divalent cations with an order of efficacy usually H+ >Zn2+>Ca2+ >Mg2+. It seems reasonable to conclude that these common properties arise from a common cause-the nature of the flow of ions close to a charged surface.

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