Ion Channels as Devices

Abstract

Ion channels are proteins with a hole down their middle that control an enormous range of biological function. Channels are devices in the engineering sense of the word and engineering analysis helps understand their function. In particular, the current through channels is driven by the power supply of concentration gradient and electrical potential maintained by across membranes by cell metabolism. The current is controlled by the physics of ion permeation in a narrow charged tube. The wall of the tube contains a few fixed charges; the tube is less than 1 nm in diameter. The density of charge (mobile or fixed) in the tube is enormous, ∼10 molar. (Liquid water is ∼55 molar.) Movement of ions through this tube can be well described as the movement of charged spheres according to the Poisson-Drift-Diffusion equations of computational electronics. Selfconsistent computation of the electric field is a necessity. The chemical specificity of channels seems to arise from the crowding of charge in their narrow tunnel. A purely physical description of the energetics of crowded spheres is enough to explain the complex patterns of selectivity found in several types of channels.