Rockefeller University logoPotassium Ion Channels: How They Work

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potassium ion animation

The structure of the potassium ion channel protein reveals three potassium ions lined up inside the channel. Two of the ions are in a narrow segment bound by protein atoms. The third ion sits in a wide, water-filled chamber. This ion is held in place by water and dielectric charges from the alpha helices. When another potassium ion approaches the channel from either inside or outside the cell, it sends a jolt down the line of ions--much as one billiard ball imparts a force through two others--and an ion "pops out" the opposite side.

 

sodium ion animation
The potassium ion channel is very selective and does not accept sodium ions. It is believed that sodium ions are rebuffed because their much smaller size causes them to bind tighter to surrounding water molecules. When a potassium ion enters the narrow segment of the channel, the water molecules are readily stripped off and the ion is held in place by slightly charged oxygen atoms. The sodium ions, still attached to the water molecules, cannot enter the channel and so move on. Some larger ions, such as cesium, can also fit in the channel.
 

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