In which type of transport can ions not diffuse directly across the membrane?

• A. Simple diffusion
• B. Facilitated diffusion
• C. Diffusion through channels
• D. Osmosis

Answer: (A)

Ions cannot diffuse directly across the membrane because of their charge and the hydrophobic nature of the lipid bilayer. Simple diffusion refers to the process whereby small nonpolar molecules and uncharged polar molecules can pass directly through the membrane without the need for any specific transport mechanisms. Since ions are charged, they cannot pass freely through the lipid bilayer and require assistance for transport.

In contrast, facilitated diffusion, diffusion through channels, or osmosis involve specific mechanisms or channels that help move molecules or ions across the membrane. Facilitated diffusion allows ions and polar molecules to cross the membrane with the help of carrier proteins. Diffusion through channels specifically refers to the movement of ions and molecules through protein channels that span the membrane. Osmosis refers to the movement of water through a semipermeable membrane, which is also facilitated by specialized channels known as aquaporins.

Thus, the correct answer highlights the inability of ions to simply diffuse across the membrane due to their charged nature.