  We consider a simple working hypothesis that all permeation properties of
open ionic channels can be predicted by understanding electrodiffusion in fixed
structures, without invoking conformation changes, or changes in chemical
bonds. We know, of course, that ions can bind to specific protein structures,
and that this binding is not easily described by the traditional electrostatic
equations of physics textbooks, that describe average electric fields, the
so-called `mean field'. The question is which specific properties can be
explained just by mean field electrostatics and which cannot. I believe the
best way to uncover the specific chemical properties of channels is to invoke
them as little as possible, seeking to explain with mean field electrostatics
first. Then, when phenomena appear that cannot be described that way, by the
mean field alone, we turn to chemically specific explanations, seeking the
appropriate tools (of electrochemistry, Langevin, or molecular dynamics, for
example) to understand them. In this spirit, we turn now to the structure of
open ionic channels, apply the laws of electrodiffusion to them, and see how
many of their properties we can predict just that way.
