Living Transistors: a Physicist's View of Ion Channels

Ion channels are proteins with a hole down the middle embedded in cell membranes. Membranes form insulating structures and the channels through them allow and control the movement of charged particles, spherical ions, mostly Na, K, Ca, and Cl. Membranes contain hundreds or thousands of types of channels, most of which are closed at any time. Channels control an enormous range of biological channel by opening and closing in response to specific stimuli by mechanisms that are not yet understood in physical language. Open channels conduct current of charged particles following laws of electrodiffusion rather like the laws of electrodiffusion of quasiparticles in semiconductors. Open channels select between similar ions using a combination of electrostatic and ‘crowded charge’ (Lennard-Jones) forces. The specific location of atoms and the exact atomic structure of the channel protein seems much less important than certain properties of the structure, namely the volume accessible to ions and the effective density of fixed and polarization charge. There is no sign of other chemical effects like delocalization of electron orbitals between ions and the channel protein. Channels play a role in biology as important as transistors in computers, and they use rather similar physics to perform part of that role. Version 2: 3 Feb 2008 arXiv:q-bio/0506016v2 [q-bio.BM]