Ion channels allow atomic control of macroscopic transport

Ion channels are proteins with a hole down their middle that control an enormous range of biological function. Thousands of biologists and clinicians study ion channels every day because of their crucial involvement in disease. Ion channels have a definite structure once open and ions move through them by electrodiffusion. Theories that describe the movement of charged spheres through definite structure are able to account for the experimental behavior of channels in a wide range of conditions with just a few parameters with fixed values. Selectivity is produced by the balance of electrostatic attraction and hard sphere repulsion in at least three types of channels. More 'chemical' forces are not involved. The free energy landscapes of these systems are variables. Preformed binding sites are not involved. Indeed, in some cases selectivity is produced by depletion zones, not binding sites. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.