Which process uses carrier proteins to move substances against their concentration gradient?

Which process uses carrier proteins to move substances against their concentration gradient?

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

Answer: (C)

Moving substances against their concentration gradient requires energy and a carrier protein that acts as a pump. In active transport, the cell uses energy—often from ATP—to drive a carrier protein to move molecules from low to high concentration, uphill in the gradient. The sodium–potassium pump is a classic example, using ATP to power the protein and shift Na+ and K+ ions against their respective gradients. In contrast, diffusion lets particles spread out from high to low concentration without energy input and without a pump, while osmosis specifically describes the passive diffusion of water across a membrane, also without energy. Some transport involves carrier proteins (facilitated diffusion), but that still moves substances down their gradient and does not require energy, so it doesn’t fit the description of moving against the gradient.

Moving substances against their concentration gradient requires energy and a carrier protein that acts as a pump. In active transport, the cell uses energy—often from ATP—to drive a carrier protein to move molecules from low to high concentration, uphill in the gradient. The sodium–potassium pump is a classic example, using ATP to power the protein and shift Na+ and K+ ions against their respective gradients.

In contrast, diffusion lets particles spread out from high to low concentration without energy input and without a pump, while osmosis specifically describes the passive diffusion of water across a membrane, also without energy. Some transport involves carrier proteins (facilitated diffusion), but that still moves substances down their gradient and does not require energy, so it doesn’t fit the description of moving against the gradient.