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

• A. Facilitated diffusion occurs only in prokaryotic cells, whereas simple diffusion occurs only in eukaryotic cells.
• B. Facilitated diffusion requires specific membrane transport proteins (channels or carriers) to move substances down their concentration gradient, whereas simple diffusion occurs directly through the lipid bilayer without proteins.
• C. Facilitated diffusion moves substances against their concentration gradient using ATP, whereas simple diffusion moves substances down their concentration gradient without ATP.
• D. Facilitated diffusion can only transport water molecules, whereas simple diffusion can only transport ions.

Answer: (B)