Ion Channels: How They Work
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.
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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