Which process requires energy to move substances across the cell membrane?

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

Answer: (D)

The key idea here is that some membrane movements need energy while others don’t. Active transport is the process that requires energy because it moves substances against their concentration gradient, from where there’s less of the substance to where there’s more, using energy from ATP to power protein pumps. This is different from diffusion, where particles spread out naturally from high to low concentration without energy, and from osmosis (water diffusion) and facilitated diffusion, which also move substances down their gradient but rely on proteins to help them cross rather than on energy input. So, moving substances across the membrane with energy input is what defines active transport.