Which statement about carrier proteins is correct?

• A. Polar molecules; in one direction.
• B. Nonpolar molecules; in both directions.
• C. Polar molecules; in both directions.
• D. Polar molecules; against the gradient only.

Answer: (C)

Carrier proteins enable facilitated diffusion of polar molecules across the membrane. They bind a polar molecule on one side, change shape, and release it on the other side, with the direction determined by the concentration gradient. If there is more molecule on one side, it will move toward the other until equilibrium, so the transport can occur in either direction. Nonpolar molecules don’t need these carriers because they can dissolve in the lipid bilayer. Movement against the gradient isn’t typical for carrier-facilitated diffusion unless energy-driven active transport is involved, which is a different mechanism.