In the context of principles of transmembrane transport, what does it mean that the cell membrane is selectively permeable?

• A. It allows all molecules to diffuse freely across the lipid bilayer at the same rate.
• B. It permits passage only of molecules moving against their concentration gradient.
• C. It prevents all ions and polar molecules from crossing under any conditions, regardless of transport proteins.
• D. It allows some substances to cross more easily than others based on properties such as size, charge, and polarity, often requiring specific transport proteins.

Answer: (D)