Facilitated diffusion occurs across a membrane via which mechanism?

• A. Across a channel or carrier protein
• B. Across the phospholipid bilayer directly
• C. Requires ATP to operate
• D. Endocytosis

Answer: (A)

Facilitated diffusion uses membrane proteins to move substances across the membrane down their concentration gradient without using cellular energy. Channel proteins create pores that allow ions or small polar molecules to pass through by diffusion, while carrier proteins bind the solute and change shape to shuttle it across. Because the process is downhill, no ATP is required. This is distinct from moving straight through the lipid bilayer, which polar or charged molecules can’t do efficiently, and from endocytosis, which is vesicle-based and energy-dependent. So the mechanism is movement across the membrane via a channel or carrier protein.