In the context of principles of transmembrane transport, what best defines secondary active transport?

• A. Transport of a solute against its concentration gradient by coupling it to the downhill movement of another ion or solute whose gradient was previously established by primary active transport (e.g., symport or antiport).
• B. Net movement of molecules down their concentration gradient through a membrane channel or carrier without any energy input.
• C. Direct transport of a solute against its concentration gradient using energy from ATP hydrolysis by the transporter itself (e.g., P-type ATPases).
• D. Vesicle-mediated movement of macromolecules across the plasma membrane via endocytosis or exocytosis.

Answer: (A)