What happens during the process of facilitated diffusion?

This is where diffusion happens down the concentration gradient i.e. from high to low concentrations (just like simple diffusion) but facilitated diffusion requires a special and specific protein within the cell membrane. The substate binds to the receptor and the receptor itself changes conformation so that the substrate is now on the other side of the membrane. This is done for molecules that are too large to pass straight through the phospholipid bilayer (cell membrane) by simple diffusion e.g. glucose.