∞ generated and posted on 2016.08.24 ∞
Movement of a substance across a lipid bilayer with the help of proteins and solely down a substance's concentration gradient.
Facilitated diffusion is equivalent to passive diffusion except that the substance is unable to move across the membrane, down their concentration gradient, except given the facilitation of transport proteins.
No transport proteins, of the proper type, then no movement across the membrane.
The following video is nice though longish overview of movement across lipid bilayers except as via active transport: