Facilitated diffusion is the passive transport of molecules across membranes with the help of proteins.

• A. It requires energy from the cell
• B. It moves molecules from low to high concentration
• C. It moves solutes against a gradient
• D. It uses membrane proteins to move substances down their concentration gradient

Answer: (D)

Facilitated diffusion is a form of passive transport that uses membrane proteins to help substances cross the lipid bilayer. Since it’s passive, it does not require energy from the cell. The movement occurs down the substance’s concentration gradient, not against it, which distinguishes it from active transport. Channel and carrier proteins provide a passageway for specific solutes, such as glucose or ions, enabling their rapid diffusion across the membrane without energy input. Therefore, describing facilitated diffusion as using membrane proteins to move substances down their concentration gradient best captures how this process works.