In the context of principles of transmembrane transport, why does active transport require an input of energy (e.g., ATP hydrolysis or an ion gradient)?

• A. Because it moves solutes down their electrochemical gradient, which always requires ATP to proceed.
• B. Because it moves solutes against their electrochemical gradient, which is thermodynamically unfavorable without energy input.
• C. Because it eliminates membrane selectivity by forming nonspecific pores that require ATP to remain open.
• D. Because it allows solutes to diffuse directly through the lipid bilayer without any transport protein.

Answer: (B)