Simple diffusion occurs when solutes move directly across which structure?

• A. Across the phospholipid bilayer
• B. Through a channel protein
• C. Across a carrier protein
• D. Via vesicles

Answer: (A)

Simple diffusion is passive transport that moves small, nonpolar molecules directly through the lipid bilayer down their concentration gradient, without any help from proteins. The phospholipid bilayer provides the hydrophobic interior that these molecules can dissolve in and traverse on their own. Channel proteins, carrier proteins, and vesicles all involve other forms of transport (facilitated diffusion or vesicular transport), not simple diffusion. So the structure solutes cross in this process is the phospholipid bilayer itself.