Ace Your Biology Skills 2026 — Dive into Marking Period 2 Practice!

Diffusion is a passive process where particles spread from places of higher concentration to lower concentration because of their random motion, and it happens without the cell spending energy. In contrast, active transport moves substances against their concentration gradient—from low to high concentration—by using energy, usually from ATP, through specific pumps or transport proteins. This energy investment lets the cell concentrate essential substances inside even when they’re scarce outside.

So the best description is that diffusion follows the concentration gradient without energy, while active transport uses energy to push substances up the gradient via pumps or transporters. The other statements confuse energy use or where diffusion can occur, so they don’t fit the idea of how these two processes differ.