In the context of transmembrane transport, which statement best distinguishes active transport from facilitated diffusion?

• A. Active transport is not saturable, whereas facilitated diffusion is saturable because it uses membrane proteins.
• B. Active transport requires an external energy source (e.g., ATP hydrolysis or coupling to an ion gradient) to move solutes against their electrochemical gradient, whereas facilitated diffusion moves solutes down their electrochemical gradient without direct energy input.
• C. Active transport can only move small nonpolar molecules across the membrane, whereas facilitated diffusion is specific to ions.
• D. Active transport always occurs through open channels, whereas facilitated diffusion always occurs through carrier proteins that never change conformation.

Answer: (B)