What term describes a membrane that allows some substances to pass while blocking others?

• A. Hypertonic
• B. Freely permeable
• C. Impermeable
• D. Selectively permeable

Answer: (D)

Membranes are selectively permeable: they let certain substances cross while blocking others. The phospholipid bilayer forms a barrier to many polar and charged molecules, but proteins embedded in the membrane act as channels, carriers, and pumps that regulate entry and exit. Because movement depends on factors like size, charge, polarity, and the presence of specific transport proteins, some substances pass while others do not. This is why the term that describes a membrane with this selective passage is “selectively permeable.”

If a membrane were freely permeable, nothing would be blocked; if it were impermeable, nothing would pass. Hypertonic describes solute concentration in solutions, not the membrane’s permeability.