Facilitated diffusion relies on what to allow molecules to move down their concentration gradient?

Facilitated diffusion is a passive transport mechanism that allows molecules to move across a cell membrane down their concentration gradient without the input of energy. This process relies on specific transport proteins embedded in the cell membrane, which can include channels and carrier proteins.

In facilitated diffusion, channels serve as pathways that allow specific molecules, such as ions or small polar molecules, to bypass the hydrophobic lipid bilayer of the membrane, facilitating their movement. These channels are typically selective for particular molecules, enabling them to flow freely when they move from an area of higher concentration to one of lower concentration.

While carrier proteins also facilitate the movement of substances down their concentration gradient, option B specifically points to the presence of a channel that serves this purpose. Therefore, the focus on channels accurately highlights a key component integral to the process of facilitated diffusion.

The role of ATP or active transporters is to mediate active transport, which moves substances against their concentration gradients, and is not relevant in facilitated diffusion since it does not require energy input.