A simple theory of ion permeation through a channel is presented, in whichdiffusion occurs according to Fick's law and drift according to Ohm's law, inthe electric field determined by all the charges present. This theory accountsfor permeation in the channels studied to date in a wide range of solutions.Interestingly, the theory works because the shape of the electric field is asensitive function of experimental conditions, e.g., ion concentration. Rateconstants for flux are sensitive functions of ionic concentration because thefixed charge of the channel protein is shielded by the ions in an near it. Suchshielding effects are not included in traditional theories of ionic channels,or other proteins, for that matter.
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