Which type of molecule can most readily diffuse directly through the lipid bilayer?

• A. Large Polar Molecules Like Glucose
• B. Ions Like Na+
• C. Small Nonpolar Molecules Such As Oxygen And Carbon Dioxide
• D. Charged Proteins

Answer: (C)

The lipid bilayer interior is hydrophobic, so molecules that are nonpolar and small can dissolve in it and pass through without help. Small nonpolar molecules like oxygen and carbon dioxide can diffuse directly across the membrane down their concentration gradient, since they do not carry charge and don’t interact strongly with the polar heads or aqueous surroundings. In contrast, large polar molecules such as glucose are bulky and interact unfavorably with the hydrophobic core, ions like Na+ are charged and hydrated, and charged proteins are large; all of these require specific transport mechanisms or channels. So, the best answer is small nonpolar molecules such as oxygen and carbon dioxide.