Facilitated diffusion differs from simple diffusion by requiring

• A. A membrane protein (channel or carrier)
• B. No protein involvement
• C. Endocytosis
• D. ATP for energy

Answer: (A)

Facilitated diffusion relies on membrane proteins to help polar or charged solutes cross the lipid bilayer down their concentration gradient. The essential point is that the solute can’t easily slip through the membrane on its own, so a channel or carrier provides a passage or a binding site and then changes shape to shuttle the solute across. This process is still passive—no energy input is required—unlike active transport, which uses ATP. Endocytosis isn’t part of this mechanism either, as it involves vesicle formation and energy-dependent uptake. An everyday example is glucose entering cells via specific transporter proteins; without these membrane proteins, glucose wouldn’t diffuse efficiently through the membrane. Simple diffusion, by contrast, doesn’t need any protein and can cross the membrane for small nonpolar molecules like O2 or CO2.