The concept of channel permeability
The one-way fluxes of a certain species of ion through a passive channel in a fixed state are assumed to be well-behaved functions of seven variables only, all of which are macroscopically observable: the cis and trans concentrations of that ion, the charge of the ion, the voltage difference between the cis and trans solutions, the thermodynamic temperature, the ion concentration at half maximal conductance of the channel, and the channel permeability. It is then proved that, if these fluxes are independent and Nernstian, the absolute permeability is uniquely and explicitly defined in terms of the channel current near zero voltage; no assumptions about the detailed workings of the permeation process are necessary. It is also proved that, if the one-way fluxes of the species under consideration are merely unaffected by the other species present, then the channel's absolute permeability can still be uniquely and explicitly defined if a weak constraint on the flux ratio is assumed; again, no assumptions on the inner workings of the channel are necessary.