What does the fluid mosaic model describe about cellular membranes, and which molecules diffuse most readily?

• A. The membrane is a phospholipid bilayer with embedded proteins and cholesterol; small nonpolar molecules diffuse most readily
• B. The membrane is a static barrier that prevents all diffusion
• C. The membrane is a phospholipid bilayer with embedded proteins and cholesterol; large polar molecules diffuse most readily
• D. The membrane is a water-filled gel; ions diffuse fastest

Answer: (A)

Cell membranes are dynamic, fluid mosaics made of a phospholipid bilayer with embedded proteins and cholesterol that can move laterally within the membrane. This fluid structure means the interior of the bilayer is hydrophobic, so molecules that are small and nonpolar diffuse through readily because they don’t have to deal with a charged or polar barrier. Tiny nonpolar molecules like oxygen and carbon dioxide slip through the lipid core with ease, while larger polar molecules and ions face a steep barrier and usually need transport proteins or channels to cross. So describing the membrane as a phospholipid bilayer with embedded proteins and cholesterol, where small nonpolar molecules diffuse most readily, captures how membranes permit some diffusion while restricting others. The other ideas—viewing the membrane as a static barrier, a water-filled gel, or asserting that large polar molecules or ions diffuse fastest—don’t align with how the membrane’s structure and selective permeability actually work.