Which term best describes the property of a cell membrane that allows some substances to cross more easily than others?

• A. Hypertonic solution
• B. Ion channel
• C. Fluid mosaic model
• D. Selectively permeable membrane

Answer: (D)

The main idea here is selective permeability. A membrane that allows some substances to cross more easily than others is selectively permeable. The lipid bilayer itself is hydrophobic, so small nonpolar molecules (like O2 and CO2) diffuse through readily, while most polar molecules and ions need help from transport proteins. That help comes in the form of channels and carriers that are specific for certain substances, which is how the membrane control is achieved. The other terms point to different ideas: an ion channel is a protein that provides a passage for ions but doesn’t describe the membrane’s overall property, the fluid mosaic model describes structure, and a hypertonic solution refers to solute concentration outside the cell, not how the membrane behaves.