Which absorption mechanism requires energy?

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

Answer: (C)

The key idea here is energy use to move substances. Active transport is the mechanism that requires energy because it moves items against their concentration gradient—from low to high concentration—using a transporter protein and energy, usually from ATP or from another electrochemical gradient. This energy-driven process lets cells uptake nutrients even when they're scarce outside the cell.

In contrast, passive diffusion lets substances move down their gradient without energy input, driven purely by concentration differences. Facilitated diffusion also moves things down their gradient but uses a protein channel or carrier—still without energy. Osmosis is specifically the diffusion of water across a membrane, following water potential, and also does not require energy.