Subtle shades of permeability

Na+ and K+ ions differ in diameter by only a few tenths of an A, but that is enough for a channel to select one ion over the other by factors that can exceed 1000. Physiologists have been wondering how this can take place for several decades, and now that we are starting to see crystal structures, some answers are emerging. Before the crystal structure, we knew that ion permeation depended on the energetics of ions interacting with water versus the channel protein. We suspected, and in some cases knew, what some of the contributions were. They included dehydration of the permeating ion, image forces due to the different polarizabilities of the water and membrane, interactions with charged …