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

• A. Active transport is always faster than passive transport because it does not require a membrane protein.
• B. Active transport occurs only through channels, whereas passive transport occurs only through pumps (ATPases).
• C. Active transport requires an energy input (e.g., ATP or an ion gradient) to move solutes against their electrochemical gradient, whereas passive transport moves solutes down their electrochemical gradient without direct energy input.
• D. Active transport moves solutes down their electrochemical gradient without energy input, whereas passive transport requires ATP hydrolysis to move solutes against the gradient.

Answer: (C)