The water-filled pore of an ion channel is important in transporting ions across the cell membrane. But how does this water-filled pore work?
Well, we know that ions attract water because water is bipolar: it has an unequal distribution of charges. The oxygen atoms tend to attract electrons, so it has a slightly negative charge, while the hydrogen atom tends to give up electrons, so it has a slightly positive charge.
So, with O atoms being negative and H atoms being positive, O atoms attract cations, and H atoms attract anions. In a similar fashion, ions attract water: in fact, they become surrounded by water, as spheres/waters of hydration.
Now, because ions in solution are surrounded by these spheres of hydration, the ease with which an ion flows through a solution depends more on the size of the ion coupled with the sphere of water surrounding that ion.
This seems counterintuitive to me at first glance, but, smaller ions tend to have lower mobility through a membrane. Why is this?
Well, the smaller the ion, the more localized the charge, and so the stronger its electric field. Therefore, the smaller the ion, the stronger it attracts water.
So, let’s compare Na and K ions. K ions are larger than Na ions, so the supposedly “intuitive” thought would be, well, the larger ion would move slower because it’s heavier, or something like that. Instead, as Na, the smaller ion compared to K, moves through solution, its stronger electrostatic attraction to water causes its sphere of hydration to be larger. This slows down Na compared to K, which would have a smaller sphere because it’s a larger ion.
Therefore, nature has constructed, via the ideas presented in this article, an ion channel that is selective for K ions over Na. Now, of course, ion transporter proteins are still present and important, and may be even more so than these water-filled pores.
Fascinating, isn’t it!