Which statement best describes active transport?

• A. With the concentration gradient via a pump
• B. Against the concentration gradient via a pump that requires ATP
• C. Through a channel with no energy
• D. By diffusion

Answer: (B)

Active transport moves substances against their concentration gradient by using energy, typically supplied by ATP through a pump protein that changes shape to move the cargo. This energy input drives an uphill movement, which is what sets active transport apart from diffusion. A classic example is the Na+/K+ ATPase, which hydrolyzes ATP to power ion movement against their gradients and establish essential concentration differences across the membrane. In contrast, diffusion or movement through a channel happens passively, requiring no energy and proceeding down the gradient.