Movement of specific molecules across cell membranes through protein channels.

• A. Diffusion
• B. Osmosis
• C. Active Transport
• D. Facilitated diffusion

Answer: (D)

Facilitated diffusion is the movement of specific molecules across the cell membrane through protein channels down their concentration gradient, and it does not require energy. The protein channels provide a hydrated, selective route for polar or charged molecules that can’t easily cross the lipid bilayer on their own. This fits the idea of molecules moving specifically through channels and relies on the gradient rather than any cellular energy. For example, ions passing through ion channels or glucose moving via glucose transporters (GLUT) are classic cases. In contrast, diffusion would move through the membrane without channels (not specifically via proteins), osmosis is about water movement, and active transport requires energy to move substances against their gradient.