Which transport method uses carrier proteins to move substances across the cell membrane when they cannot pass directly?

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

Answer: (A)

Active transport uses carrier proteins to move substances across the cell membrane when they can’t pass directly through the lipid bilayer. It needs energy because the substances are often moved against their concentration or electrochemical gradient. The carrier proteins, or pumps, bind the substance on one side and, powered by ATP hydrolysis (or by stored energy from ion gradients in secondary active transport), change shape to push it to the other side. This mechanism lets cells accumulate nutrients, remove wastes, and maintain ion balances, as seen with the sodium–potassium pump.

Diffusion and osmosis are passive—they don’t require energy or carrier proteins and move substances down their gradients. Filtration is driven by pressure differences rather than carrier-mediated transport.