Which type of membrane transport occurs when a channel protein or carrier protein assists in the movement of molecules down their concentration gradient without the use of cellular energy?

Step 3: Compare facilitated diffusion with other types of transport mentioned in the problem. Simple diffusion involves molecules moving directly through the lipid bilayer without assistance, while active transport requires energy (ATP) to move molecules against their concentration gradient. Endocytosis involves the engulfing of substances into the cell, which is an active process.

Step 4: Identify the key characteristics of facilitated diffusion in the problem. The movement of molecules down their concentration gradient and the involvement of channel or carrier proteins are specific to facilitated diffusion.