How do ion channels achieve selectivity?
Hypothesis One: selectivity simply depends on the size of the ion. A channel for Na+ (which has a smaller crystal radius than K+) would have a smaller diameter pore than a K+ channel and therefore exclude would K+ on the basis of size (sez Prof Yoda): Partially correct: Non-specific cation channels (eg channel associated with ACh receptor) tend to be larger than specific ones. Two problems with this explanation: 1)how you could have a channel that was selective for K+ and not for Na+, and 2) ignores the effect of the waters of hydration. It turns out that smaller ions are able to attract water molecules more strongly than larger ones so that they’re surounded by a bigger shell of water molecules and so the HYDRATED RADIUS for Na+ is actually larger than the hydrated radius for K+. (People figured this out on the basis of measuring the speed of diffusion of ions in solution: lithium diffused slower than sodium, and sodium diffused slower than potassium, which is opposite to what you’d e
