Here's why:

* Active transport moves molecules against their concentration gradient, meaning from an area of low concentration to an area of high concentration. This process requires energy because it's going against the natural flow of molecules.

* ATP is the primary energy currency of cells. It stores chemical energy in its phosphate bonds. When these bonds are broken, energy is released, which can be used to power cellular processes, including active transport.

* Proteins are involved in active transport. These proteins act as pumps or carriers, using the energy from ATP to move molecules across the cell membrane.

In summary: Active transport requires energy to move molecules against their concentration gradient, and this energy is supplied by ATP.