Which process requires energy to move substances across a membrane against their concentration gradient?

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

Answer: (C)

Moving substances across a membrane against their concentration gradient requires energy because you’re pushing them in the direction they wouldn’t naturally go. This is what active transport does: it uses energy, often from ATP, to power carrier proteins (pumps) that move ions or molecules uphill. A classic example is the sodium-potassium pump, which uses energy to move Na+ out of the cell and K+ in, maintaining essential ionic gradients for many cellular processes.

In contrast, diffusion and osmosis are driven by the natural tendency of substances to move from higher to lower concentrations and do not require direct energy input. Passive transport covers these energy-free movements, including facilitated diffusion, which still follows the gradient even though membrane proteins help speed the process.

So, the energy-dependent movement against a gradient is characteristic of active transport.