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# Selectivity Filter of the Potassium Ion Channel

BIOCHM-I6YMNV

Potassium ion channels are the most widely distributed ion channel; they are found in virtually all types of cells and organisms. Potassium ions are able to diffuse from the cytoplasm into cells through these selective channels.

Each channel is made up of a series of proteins that form a path through the membrane bilayer, so that the charged species do not have to interact with the lipid environment of the membrane. The opening of the potassium ion channel is fairly small, approximately 3.5 Angstroms wide, and is called the selectivity filter.

Potassium ions have an ionic radius of 1.52 Angstroms. Even though sodium ions are smaller, with an ionic radius of 1.16 Angstroms, these channels are at least 10,000 times more permeable to potassium ions than to sodium ions.

Choose the MOST appropriate explanation for how the selectivity filter of the potassium ion channel works.

A

Potassium ions are just large enough to get 'stuck' in the selectivity filter and are unable to diffuse back out of the opening of the ion channel.

B

The concentration of sodium inside the cell makes the diffusion of sodium through the channel thermodynamically unfavorable.

C

The energy cost of desolvating the potassium ion is recovered because it interacts with the side chains making up the selectivity filter.