In the context of principles of transmembrane transport, which statement correctly describes the difference between simple diffusion and facilitated diffusion across a cell membrane?

• A. Simple diffusion requires ATP hydrolysis, whereas facilitated diffusion does not require ATP.
• B. Simple diffusion is saturable at high solute concentrations, whereas facilitated diffusion is never saturable.
• C. Simple diffusion moves solutes against their concentration gradient, whereas facilitated diffusion moves solutes down their concentration gradient.
• D. Simple diffusion occurs directly through the lipid bilayer without a transport protein, whereas facilitated diffusion requires a membrane transport protein (channel or carrier) but still moves solute down its concentration gradient.

Answer: (D)