Which statement best defines active transport across a biological membrane?

• A. Movement of a solute down its concentration gradient through a membrane channel without direct energy input.
• B. Nonselective movement of small molecules directly through the lipid bilayer at a rate proportional to membrane surface area.
• C. Movement of a solute across a membrane against its electrochemical gradient, requiring an energy source (e.g., ATP hydrolysis or coupling to another gradient).
• D. Net movement of water across a semipermeable membrane driven solely by differences in solute concentration.

Answer: (C)