Here's why:

* Against the concentration gradient: This means molecules are moving from an area of low concentration to an area of high concentration. This is the opposite of what would happen naturally due to diffusion.

* Through protein pumps: Protein pumps are specialized transmembrane proteins that act as "carriers" for molecules. They require energy to move molecules against their concentration gradient.

* Active transport: This refers to any movement of molecules across a membrane that requires energy input.

Key points about active transport:

* Energy requirement: Active transport always requires energy, typically from ATP (adenosine triphosphate).

* Specificity: Protein pumps are specific to the molecules they transport.

* Importance: Active transport is vital for maintaining cell function, allowing cells to concentrate nutrients, eliminate waste, and regulate ion gradients.

Let me know if you have any other questions!