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

• A. Facilitated diffusion requires specific membrane proteins (channels or carriers) and can become saturated at high solute concentrations, whereas simple diffusion occurs directly through the lipid bilayer and is not saturable in the same way.
• B. Simple diffusion requires a transporter to provide a hydrophilic pathway, whereas facilitated diffusion occurs without any membrane proteins.
• C. Facilitated diffusion moves solutes against their concentration gradient using ATP, whereas simple diffusion moves solutes down their concentration gradient without ATP.
• D. Simple diffusion occurs only for ions, whereas facilitated diffusion occurs only for nonpolar molecules such as O2 and CO2.

Answer: (A)