FLUX, COUPLING, AND SELECTIVITY IN IONIC CHANNELS OF ONE CONFORMATION

Ions crossing biological membranes are described as a concentration of charge flowing through a selective open channel of one conformation a nd analyzed by a combination of Poisson and Nernst-Planck equations an d boundary conditions, called the PNP theory for short. The ion fluxes in this theory interact much as ion fluxes interact in biological cha nnels and mediated transporters, provided the theoretical channel cont ains permanent charge and has selectivity created by (electrochemical) resistance at its ends. Interaction occurs because the flux of differ ent ionic species depends on the same electric field. That electric fi eld is a variable, changing with experimental conditions because the s creening (i.e., shielding) of the permanent charge within the channel changes with experimental conditions. For example, the screening of ch arge and the shape of the electric field depend on the concentration o f all ionic species on both sides of the channel. As experimental inte rventions vary the screening, the electric field varies, and thus the flux of each ionic species varies conjointly, and is, in that sense, c oupled. Interdependence and interaction are the rule, independence is the exception, in this channel.