In the context of principles of transmembrane transport, which statement best describes how facilitated diffusion differs from simple diffusion across a cell membrane?

• A. Facilitated diffusion can transport any small nonpolar molecule without a protein, whereas simple diffusion requires a carrier protein for transport.
• B. Facilitated diffusion occurs only for water (osmosis), whereas simple diffusion occurs only for ions.
• C. Facilitated diffusion moves solutes against their concentration gradient by using ATP, whereas simple diffusion moves solutes down their concentration gradient without energy input.
• D. Facilitated diffusion requires specific membrane proteins (channels or carriers) and is saturable, whereas simple diffusion occurs directly through the lipid bilayer and is not saturable.

Answer: (D)