  Ions in water are the liquid of life. Life occurs almost entirely in 'salt
water'. Water itself (without ions) is lethal to animal cells and damaging for
most proteins. Water must contain the right ions in the right amounts if it is
to sustain life. Physical chemistry is the language of electrolyte solutions.
Physical chemistry and biology are intertwined. Physical chemists and
biologists come from different traditions that separated for several decades as
biologists described the molecules of life. Communication is not easy between a
fundamentally descriptive tradition and a fundamentally analytical one.
Biologists have now learned to study well defined systems with physical
techniques, of considerable interest to physical chemists. Physical chemists
are increasingly interested in spatially inhomogeneous systems with structures
on the atomic scale so common in biology. Physical chemists will find it
productive to work on well defined systems built by evolution to be reasonably
robust, with input output relations insensitive to environmental insults. This
article deals with properties of ion channels that in my view can be dealt with
by 'physics as usual', with much the same tools that physical chemists apply to
other systems. Indeed, I introduce and use a tool of physicists-a field theory
(and boundary conditions) based on an energy variational approach developed by
Chun Liu-not too widely used among physical chemists. My goal is to provide the
knowledge base, and identify the assumptions, that biologists use in studying
ion channels, avoiding jargon. Rather simple models of selectivity and
permeation in ion channels work quite well in important cases. Those physical
models and cases are the main focus of this review because they demonstrate the
strong essential link between the traditional treatments of ions in chemical
physics, and the biological function of ion channels.
