Apentix/AP Biology/Unit 2: Cell Structure and Function/Question
AP Biologyhardmcq1 pt

Potassium leak channels admit K+ while rejecting Na+, although Na+ has the smaller ionic radius. Which account of this selectivity is accurate?

A.The pore is narrower than Na+, physically barring the smaller ion from entry.
B.Filter carbonyl oxygens are spaced to stabilize dehydrated K+, an energetic fit that Na+ cannot match.
C.Na+ lacks positive charge, so the negatively polarized filter cannot attract it.
D.ATP reshapes the filter after every ion passage, retuning the channel for potassium.

Explanation

Core Concept

The oxygens mimic K+'s hydration shell geometry, stabilizing a stripped potassium ion at low energy. Na+ is too small to contact those oxygens simultaneously, so shedding its own water shell costs more than the filter repays. Selectivity therefore reflects energetic fit rather than pore diameter alone.

Correct Answer

BFilter carbonyl oxygens are spaced to stabilize dehydrated K+, an energetic fit that Na+ cannot match.

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