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Potassium ion is necessary for the function of all living cells, and is thus present in all plant and animal tissues. It is found in especially high concentrations in plant cells, and in a mixed diet, it is most highly concentrated in fruits.
There are four major classes of potassium channels:
- Calcium-activated potassium channel - open in response to the presence of calcium ions or other signalling molecules.
- Inwardly rectifying potassium channel - passes current (positive charge) more easily in the inward direction (into the cell).
- Tandem pore domain potassium channel - are constitutively open or possess high basal activation, such as the "resting potassium channels" or "leak channels" that set the negative membrane potential of neurons. When open, they allow potassium ions to cross the membrane at a rate which is nearly as fast as their diffusion through bulk water.
- Voltage-gated potassium channel - are voltage-gated ion channels that open or close in response to changes in the transmembrane voltage.