Facilitated diffusion is a form of passive transport used to transport substances into or out of the cell. Passive transport means that it does not require any additional ATP or energy from the cell in order for the process to occur. Facilitated diffusion is a way to move large substances into the cell down their concentration gradient from an area of high concentration to an area of low concentration through protein channels. Protein channels are structures that are embedded in the cell membrane that act like little tunnels for materials to move through. In the diagram below you can see how a protein channel might work to transport substances through the cell membrane and into the cell.