Membrane proteins in the fluid mosaic model facilitate transport of which molecules?

• A. Lipophilic molecules move via proteins
• B. Proteins transport large proteins
• C. Proteins provide pathway for small hydrophilic molecules such as ions and glucose
• D. Proteins pump water exclusively

Answer: (C)

In the fluid mosaic model, the lipid bilayer forms a hydrophobic barrier, so polar or charged molecules need a protein pathway to cross. Membrane proteins create channels or carriers that provide selective routes for small hydrophilic solutes. Ions and glucose, for example, cannot diffuse freely through the lipid core, so they move via these proteins either by facilitated diffusion down their gradients or by active transport when energy is used. Water can cross mainly through aquaporins, but not exclusively by pumping, and large proteins don’t cross the membrane through simple diffusion. Thus, the best description is that proteins provide a pathway for small hydrophilic molecules such as ions and glucose.