How the potassium channel on cell membranes allows potassium diffusion, but DOES NOT allow sodium diffusion
The correct answer and explanation is:
Correct Answer:
Potassium channels on cell membranes allow potassium (K⁺) diffusion but do not allow sodium (Na⁺) diffusion because of ion selectivity based on size, charge, and hydration shell of the ions.
300-Word Explanation:
Cell membranes contain specialized ion channels that regulate the movement of specific ions in and out of cells. Potassium channels are designed to be selectively permeable, meaning they allow only potassium ions (K⁺) to pass through while excluding sodium ions (Na⁺), even though both are positively charged.
This selectivity arises from the structure of the potassium channel, particularly the selectivity filter located in the narrowest part of the channel. The filter is lined with carbonyl oxygen atoms from the amino acids of the channel protein. These oxygen atoms are precisely spaced to mimic the water molecules that normally surround a potassium ion in solution. When K⁺ enters the filter, it loses its hydration shell and is stabilized by these oxygen atoms, allowing it to pass through.
Sodium ions, though smaller than potassium, cannot be stabilized as effectively by the selectivity filter. The spacing of the carbonyl oxygens does not match the size of a dehydrated Na⁺ ion. As a result, Na⁺ does not interact favorably with the channel and is excluded. In other words, the channel is too large to stabilize Na⁺ properly, and so it does not allow it to pass through.
This high selectivity is crucial for many physiological processes, such as nerve impulse conduction, muscle contraction, and maintaining the resting membrane potential. The movement of potassium through these channels helps establish the electrochemical gradient, which is essential for signal transmission in nerve and muscle cells.
In summary, potassium channels are specialized to permit only potassium ions by using precise structural features that reject ions like sodium, ensuring proper cell function.