In the context of principles of transmembrane transport, which statement best describes a key difference between simple diffusion and facilitated diffusion across a plasma membrane?

• A. Facilitated diffusion moves solutes from low to high concentration, whereas simple diffusion moves solutes from high to low concentration.
• B. Simple diffusion requires ATP hydrolysis to move solutes down their concentration gradient, whereas facilitated diffusion is always ATP-independent.
• C. Simple diffusion uses ion channels to move charged solutes rapidly, whereas facilitated diffusion occurs only for small nonpolar molecules such as $O_2$ and ${\mathrm{CO}}_2$.
• D. Facilitated diffusion requires a membrane transport protein 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.

Answer: (D)