Here's why:

* Up its concentration gradient: This means the substance is moving from an area of lower concentration to an area of higher concentration. This is energetically unfavorable and requires energy input.

* Protein carriers: These are specialized proteins embedded in the membrane that bind to the substance and facilitate its movement across the membrane.

* Expenditure of chemical energy: This energy is typically provided by ATP (adenosine triphosphate), which is the primary energy currency of cells.

Let me know if you would like more details about active transport mechanisms!