The absorption mechanism that requires a carrier protein but does not use energy is known as which?

• A. Passive diffusion
• B. Active transport
• C. Endocytosis
• D. Facilitated diffusion

Answer: (D)

Facilitated diffusion is the process described: substances that can’t easily cross the lipid bilayer use a carrier protein to help them move across the membrane, and this happens without using cellular energy. The carrier protein changes shape to shuttle the solute across as it flows down its concentration gradient, so no ATP or other energy source is needed. This is what distinguishes it from passive diffusion, which doesn’t involve a carrier protein at all, and from active transport or endocytosis, which require energy. A common example is glucose moving into cells through specific glucose transporters.