Which statement best describes active transport?

• A. Passive transport moves substances down their concentration gradient without energy.
• B. Passive transport uses energy to move substances against their gradient.
• C. Active transport moves substances against the gradient and requires energy.
• D. Active transport moves substances down their gradient without energy.

Answer: (C)

Active transport is about moving substances in the direction opposite to their concentration gradient, and it requires energy to do so. This energy input lets the cell move molecules from areas of lower concentration to higher concentration, which is essential for things like maintaining ion gradients across the membrane and Uptaking nutrients that are scarce outside the cell. The energy can come from ATP directly or from the energy stored in existing gradients (secondary active transport), where movement of one substance down its gradient powers another substance against its gradient. In contrast, substances moving with their gradient do so without expending cellular energy (passive transport). That’s why this statement is the best: it correctly combines the idea of opposing the gradient with the need for energy.