Here's why:
* Facilitated diffusion relies on concentration gradients: It moves substances from an area of high concentration to an area of low concentration, just like simple diffusion. The difference is that facilitated diffusion uses transport proteins to help the substances cross the cell membrane.
* Transport proteins act as facilitators: They bind to the substance and help it move across the membrane, but they don't actively pump the substance against its concentration gradient. This means no ATP is needed.
Think of it like a doorway: The transport protein acts like a doorway that makes it easier for the substance to pass through the membrane. It doesn't require the cell to use energy to open or close the door.
In contrast, active transport requires ATP because it moves substances against their concentration gradients. This means the cell needs to use energy to pump the substance from an area of low concentration to an area of high concentration.
