In the context of passive transport across a cell membrane, what is the key difference between simple diffusion and facilitated diffusion?

• A. Simple diffusion can only move water across membranes, whereas facilitated diffusion can only move ions.
• B. Facilitated diffusion moves molecules against their concentration gradient, whereas simple diffusion moves molecules down their concentration gradient.
• C. Simple diffusion uses ATP to pump molecules across the membrane, whereas facilitated diffusion does not require ATP.
• D. Facilitated diffusion requires specific membrane proteins (channels or carriers) to move substances down their concentration gradient, whereas simple diffusion occurs directly through the lipid bilayer without transport proteins.

Answer: (D)