Ionic Hopping Defended

In the whirlwind of cloning, mutagenesis, and, suddenly, structure that the ion channel field has been riding for the past 15 yr, it is easy to forget that we still don't have a satisfactory view of that most basic task carried out by these proteins: ion permeation. The diffusion of ions through the aqueous pores of ion channels (a process much simpler than gating) is being treated in two different ways by two increasingly polarized schools of thought. For want of terms that are both precise and concise, I refer to these as the “chemical- kinetic” and “continuum” descriptions of channel- mediated electrodiffusion—both of which treat ions as stumbling through one-dimensional random walks along the pore. In chemical-kinetic descriptions, ions hop along a small number of binding sites (Hille, 1991); in continuum theories, they diffuse in continuous space along the pore under the influence of local electrochemical...