Here's why:

* Active transport requires energy: Active transport moves molecules against their concentration gradient (from low concentration to high concentration). This requires energy, usually supplied by ATP.

* Proteins act as pumps: Membrane proteins embedded in the cell membrane act as pumps. They bind to the molecule being transported, use energy from ATP, and change shape to move the molecule across the membrane.

There are two main types of protein pumps involved in active transport:

* Carrier proteins: These proteins bind to the molecule and change shape to move it across the membrane.

* Channel proteins: These proteins form pores through the membrane, allowing specific molecules to pass through.

In summary, proteins are essential for active transport because they provide the mechanism for moving molecules against their concentration gradient, utilizing energy from ATP.