Facilitated diffusion differs from simple diffusion in that...

Facilitated diffusion is a specialized form of passive transport that allows substances to cross membranes with the assistance of specific proteins. This process is essential for molecules that cannot easily pass through the lipid bilayer of the cell membrane due to their size, polarity, or charge. In facilitated diffusion, transport proteins, such as channel proteins or carrier proteins, help move these molecules across the membrane while still following the concentration gradient—meaning substances move from an area of higher concentration to an area of lower concentration without the need for energy input.

The involvement of these proteins is what differentiates facilitated diffusion from simple diffusion. In simple diffusion, substances move directly through the lipid bilayer without any assistance, as long as they are small and nonpolar. Therefore, the role of proteins as facilitators in this process is the key characteristic that defines facilitated diffusion.