Which transport process requires energy input?

• A. Passive diffusion
• B. Facilitated diffusion
• C. Active transport
• D. Simple diffusion

Answer: (C)

Moving substances against their concentration or electrochemical gradient requires energy. This happens in active transport, where a pump uses energy from ATP or from a pre-existing ion gradient to drive the uphill movement of solutes. In primary active transport, ATP hydrolysis powers the pump itself (for example, Na+/K+ ATPase). In secondary active transport, the energy stored in one ion’s gradient powers the transport of another solute, even though the transporter isn’t directly using ATP. The other processes move down their gradient without direct energy input: diffusion occurs passively, and facilitated diffusion uses a protein to assist passage but still follows the gradient without consuming energy. So active transport is the one that requires energy input.