Which part of the phospholipid bilayer primarily restricts passage of polar molecules?

• A. Cholesterol content determines permeability.
• B. Membrane proteins act as channels.
• C. Hydrophilic heads face inward.
• D. Hydrophobic core of the bilayer.

Answer: (D)

The main idea is that the nonpolar interior of the lipid bilayer acts as the primary barrier to polar molecules. The hydrophobic core formed by the fatty acid tails resists interactions with water and charged or highly polar substances, making it energetically unfavorable for polar molecules to pass through without help. While small nonpolar molecules can diffuse through easily, polar molecules and ions typically need channels or transporters in the membrane to cross. Cholesterol can adjust membrane fluidity and packing, subtly influencing permeability but not serving as the main barrier. The hydrophilic heads face the water on either side and don’t limit passage in the same way; they help interface with the aqueous environment. So, the hydrophobic core is the part that primarily restricts passage of polar molecules.