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)

Verified step by step guidance

1. Understand that facilitated diffusion is a type of passive transport, meaning it does not require energy input like ATP.
2. Recognize that facilitated diffusion involves the movement of molecules down their concentration gradient, from an area of higher concentration to an area of lower concentration.
3. Identify that facilitated diffusion requires specific membrane proteins, such as channel proteins or carrier proteins, to help molecules cross the lipid bilayer because these molecules cannot easily diffuse directly through the membrane.
4. Distinguish facilitated diffusion from simple diffusion, which involves small nonpolar molecules moving directly through the lipid bilayer without protein assistance.
5. Note that facilitated diffusion differs from active transport, which moves molecules against their concentration gradient and requires energy, and from osmosis, which specifically refers to water movement.