A potassium channel is an ion channel that preferentially conducts potassium ions. Its selectivity filter can compensate for the energetic cost of removing water from an ion, while discriminating between ions of different geometry. A crystal structure of a bacterial potassium channel revealed how repeated backbone oxygen atoms line the narrow filter. [ref]
Different potassium channels open under different conditions. Leak channels contribute to resting membrane potential, whereas voltage-dependent channels can support repolarization after an action potential. The direction of current depends on voltage relative to the potassium reversal potential, not simply on the channel's name. Selectivity and gating describe different aspects of its function.
