In the context of transmembrane transport, what is the main difference between simple diffusion and facilitated diffusion across a lipid bilayer?

• A. Simple diffusion is saturable at high solute concentrations, whereas facilitated diffusion is not saturable.
• B. Facilitated diffusion moves solutes against their concentration gradient using ATP, whereas simple diffusion moves solutes down their concentration gradient without ATP.
• C. Facilitated diffusion requires specific membrane transport proteins (channels or carriers), whereas simple diffusion occurs directly through the lipid bilayer without proteins.
• D. Simple diffusion occurs only for charged molecules, whereas facilitated diffusion occurs only for nonpolar molecules.

Answer: (C)