In the context of principles of transmembrane transport, which statement best distinguishes passive transport from active transport across a biological membrane?

• A. Passive transport requires ATP hydrolysis, whereas active transport never requires cellular energy.
• B. Passive transport moves substances down their electrochemical gradient without direct energy input, whereas active transport moves substances against their electrochemical gradient using energy (e.g., ATP hydrolysis or coupling to another gradient).
• C. Passive transport can move solutes against their electrochemical gradient if the concentration difference is small, whereas active transport cannot.
• D. Passive transport always requires a membrane protein pump, whereas active transport occurs only by simple diffusion through the lipid bilayer.

Answer: (B)