The movement of molecules from an area of lower concentration to one of higher concentration against the concentration gradient requires energy and is called active transport.

Here's why:

* Concentration gradient: This refers to the difference in concentration of a substance between two areas. Molecules naturally move from areas of high concentration to areas of low concentration, following the concentration gradient (this is called passive transport).

* Active transport: To move molecules against this natural flow requires energy. This energy is usually supplied by ATP (adenosine triphosphate), the primary energy currency of cells.

Examples of active transport:

* The sodium-potassium pump in nerve cells moves sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients.

* The uptake of glucose by intestinal cells against its concentration gradient.

Let me know if you have any other questions!