Which of the following cannot pass through the plasma membrane unaided by transport proteins?

• A. Ions
• B. Oxygen
• C. Water
• D. Fatty Acids

Answer: (A)

The key idea is selective permeability of the lipid bilayer. The membrane’s hydrophobic interior strongly resists charged or highly polar substances, so ions cannot cross by simple diffusion and must use transport proteins like channels or pumps to move across. In contrast, small nonpolar or lipid-soluble molecules cross more easily on their own: oxygen is nonpolar and diffuses readily; fatty acids are lipid-soluble and can pass through the bilayer without help; water, while polar, is small enough to diffuse and also moves efficiently through aquaporin channels. So the substance that cannot pass unaided is the charged ion.