Which transport mechanism requires energy to move substances against their concentration gradient?

• A. Diffusion
• B. Facilitated diffusion
• C. Active transport
• D. Osmosis

Answer: (C)

Moving substances against their concentration gradient requires energy because you’re pushing material from a lower concentration to a higher one, which is not the natural flow of particles. The mechanism that does this is active transport, which uses energy to drive the movement through membrane proteins known as pumps. The energy source is usually ATP, though some systems use the energy stored in existing electrochemical gradients created by other pumps.

A classic example is the sodium–potassium pump, which uses ATP to swap Na+ for K+, creating essential ion gradients for many cellular processes. Energy can also be used in a secondary way: a primary pump creates a gradient, and that gradient is then exploited to move other substances uphill, as in how the sodium gradient drives glucose uptake in certain intestinal cells.

In contrast, diffusion and osmosis move substances down their gradients and don’t require energy, and facilitated diffusion helps molecules cross membranes down the gradient via carrier or channel proteins without input of energy.