Here's why:

* Active transport moves substances against their concentration gradient. This means moving them from an area of low concentration to an area of high concentration. Think of trying to push water uphill – it requires effort!

* Passive transport, on the other hand, doesn't require energy. It relies on the natural movement of substances down their concentration gradient (from high to low), like a ball rolling downhill.

Examples of active transport:

* Sodium-potassium pump: Pumps sodium ions out of the cell and potassium ions into the cell, maintaining the cell's electrical potential and allowing nerve impulses to travel.

* Glucose uptake: Cells use active transport to take in glucose from the bloodstream, even when the concentration of glucose inside the cell is higher.

Key takeaway: Active transport is essential for cells to maintain their internal environment and carry out vital functions, but it requires the cell to expend energy.