Which statement best describes facilitated diffusion in the context of passive transport across a cell membrane?

• A. Direct diffusion of small nonpolar molecules through the lipid bilayer without involvement of membrane proteins
• B. Movement of molecules against their concentration gradient using energy from ATP-driven pumps
• C. Net movement of water across a selectively permeable membrane toward the side with lower solute concentration
• D. Movement of molecules down their concentration gradient through specific membrane proteins (channels or carriers) without ATP use

Answer: (D)