Facilitated diffusion and active transport both rely on membrane-spanning proteins. What distinguishes the work these proteins do, and why does this distinction dictate the cell's energetic cost?

D. Active transport moves solutes up a gradient whereas facilitated diffusion runs down one

Proteins cannot bypass thermodynamics: moving cargo against an established gradient demands metabolic coupling via ATP hydrolysis. Channels and facilitated carriers merely provide a path down an existing electrochemical gradient, so the cell pays no direct metabolic surcharge.