Here's why:
* Concentration gradient: The driving force behind facilitated diffusion is the difference in concentration of the molecule across a membrane. Molecules naturally move from areas where they are more concentrated to areas where they are less concentrated, like a ball rolling downhill.
* Membrane proteins: Facilitated diffusion uses specific membrane proteins called transport proteins. These proteins act like channels or carriers, helping the molecules move across the membrane. They do not expend energy themselves, but rather facilitate the movement down the concentration gradient.
* No energy expenditure: Because the movement is driven by the concentration gradient, the cell doesn't need to provide energy to make it happen. This is unlike active transport, which requires energy to move molecules against their concentration gradient.
Analogy: Imagine a crowded room with people trying to get out. They can all push and shove to get out (like simple diffusion), or they can use the door (like facilitated diffusion). The door doesn't require any extra energy to open, but it makes moving people out much faster and easier.
In summary: Facilitated diffusion is a passive process that uses membrane proteins to help molecules move down their concentration gradient. It doesn't require energy from the cell because it relies on the natural tendency of molecules to move from areas of high concentration to areas of low concentration.
