Here's why:

* Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. This movement is driven by the concentration gradient itself, meaning it requires no additional energy input.

* ATP (adenosine triphosphate) is the primary energy currency of cells. It is used to power many cellular processes that require energy, such as active transport, muscle contraction, and protein synthesis.

Let me know if you'd like to know more about other cellular processes and whether they require ATP!