What term describes a semipermeable membrane that allows some substances to pass while others cannot?

• A. Selectively permeable
• B. Permeable
• C. Nonselective
• D. Impermeable

Answer: (A)

A selectively permeable membrane is one that lets some substances cross while blocking others. This property comes from the membrane’s structure: the lipid bilayer forms a mostly hydrophobic barrier that few polar or charged molecules can cross on their own, while transport proteins create specific pathways for certain ions and larger polar molecules. As a result, small nonpolar molecules like O2 and CO2 diffuse easily, water passes through via aquaporins, and ions or larger polar solutes require channels or carriers. An impermeable membrane would block nearly everything, while a fully permeable one would not regulate entry at all. So, the best term for a membrane that allows some substances to pass and not others is selectively permeable.