Here's why:

* Moving Against the Gradient: Active transport moves molecules across a membrane *against* their concentration gradient. This means moving molecules from an area of low concentration to an area of high concentration, which requires energy.

* ATP Provides Energy: ATP is the primary energy currency of cells. When ATP is broken down into ADP (adenosine diphosphate) and a phosphate group, it releases energy that can be used to power cellular processes, including active transport.

* Specific Proteins: Active transport is carried out by specific proteins embedded within the cell membrane. These proteins use the energy from ATP to move molecules across the membrane.

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