Which process requires energy to move substances from a region of low concentration to a region of higher concentration across a membrane?

• A. Active Transport
• B. Passive Transport
• C. Facilitated Diffusion
• D. Osmosis

Answer: (A)

Energy input is required when a substance must be moved uphill, from regions of lower concentration to higher concentration across a membrane. This describes active transport, where proteins such as pumps use energy—often from ATP—to change shape and shuttle particles against the gradient. Because the movement goes against the natural flow, energy is needed to drive it.

In contrast, passive transport does not require cellular energy. Substances move down their concentration gradient, either by simple diffusion or via transport proteins in facilitated diffusion, which assist but still rely on the gradient rather than energy input. Osmosis specifically refers to the diffusion of water across a membrane, also without energy. So the scenario described is best explained by active transport.