Apentix/AP Biology/Unit 2: Cell Structure and Function/Question
AP Biologymediummcq1 pt

Oxygen and carbon dioxide cross the plasma membrane rapidly, while sodium ions cross only through specific membrane proteins. Which property of the lipid bilayer explains this difference?

A.Glycoprotein chains on the outer face bind gases and repel metal cations
B.A hydrophobic interior dissolves small nonpolar molecules but excludes hydrated ions
C.Phosphate head groups hydrolyze ATP to pump sodium outward
D.Cholesterol converts the membrane into a sieve with uniform pores

Explanation

Core Concept

The fatty acid tails form a nonpolar core that small nonpolar gases penetrate readily, so O2 and CO2 diffuse straight through down their gradients. Hydrated Na+ carries charge and a shell of water that the nonpolar core will not accommodate, forcing ions through channel or carrier proteins instead. This partitioning of polar from nonpolar traffic is the molecular basis of selective permeability.

Correct Answer

BA hydrophobic interior dissolves small nonpolar molecules but excludes hydrated ions

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