Which type of protein facilitates the transport of molecules down the concentration gradient?

Channel proteins are specifically designed to facilitate the transport of molecules across a cell membrane down their concentration gradient. These proteins create a hydrophilic pathway that allows ions and small polar molecules to move through the membrane more easily than they would through the lipid bilayer. The movement occurs without the expenditure of energy, allowing substances to flow from an area of higher concentration to an area of lower concentration, which is known as passive transport.

Intrinsic proteins, which might have been assumed as the correct answer, refer to proteins that are embedded within the cell membrane but do not specifically denote those that facilitate transport or describe their mechanism of action. On the other hand, extrinsic proteins are typically located on the surface of the membrane and play roles such as signaling or maintaining the structure of the membrane rather than transporting molecules. Structural proteins are involved in maintaining cell shape and integrity, further reinforcing why channel proteins are the correct choice for the transport function described in the question.