Here's why:

* Diffusion: Movement of molecules from an area of high concentration to an area of low concentration. This is driven by the concentration gradient and doesn't require additional energy.

* Facilitated: The movement is aided by a protein, in this case, a membrane protein. These proteins act as channels or carriers, allowing specific molecules (like ions) to cross the cell membrane.

Key characteristics of facilitated diffusion:

* Passive: Doesn't require the cell to expend energy.

* Specific: The protein only allows certain molecules to pass through.

* Saturable: The rate of transport can be limited by the number of available protein channels or carriers.

In contrast to active transport:

* Active transport: Requires energy to move molecules against their concentration gradient (from low to high concentration). This is typically done by using ATP as an energy source.

Let me know if you have any other questions!