  Ion channels are proteins with holes down their middle that control the flow
of ions and electric current across otherwise impermeable biological membranes.
The flow of sodium, potassium, calcium (divalent), and chloride ions have been
central issues in biology for more than a century. The flow of current is
responsible for the signals of the nervous system that propagate over long
distances (meters). The concentration of divalent calcium ions is a 'universal'
signal that controls many different systems inside cells. The concentration of
divalent calcium and other messenger ions has a role in life rather like the
role of the voltage in different wires of a computer. Ion channels also help
much larger solutes (e.g., organic acid and bases; perhaps polypeptides) to
cross membranes but much less is known about these systems. Ion channels can
select and control the movement of different types of ions because the holes in
channel proteins are a few times larger than the (crystal radii of the) ions
themselves. Biology uses ion channels as selective valves to control flow and
thus concentration of crucial chemical signals. For example, the concentration
of divalent calcium ions determines whether muscles contract or not. Ion
channels have a role in biology similar to the role of transistors in computers
and technology. Ion Channels Control Concentrations Important To Life The Way
Computers Control Voltages Important To Computers.
