Simple diffusion

Only comparatively small uncharged or hydrophobic molecules (H2O, O2, and CO2, other gases, urea and ethanol)   cross   the lipid   bilayer   through simple   diffusion.   Membrane proteins are not included, so there is no specificity.  The molecule in aqueous solution on single side of the membrane dissolves into the lipid bilayer crosses it and then dissolves into the aqueous solution on the other side. The rate  of  diffusion  is  straight  proportional  to  the  concentration  gradient  of  the molecule across the membrane and the procedure is not saturable.

Facilitated diffusion

Not like simple diffusion the facilitated or carrier-mediated diffusion of a molecule across a biological membrane is dependent on specific integral membrane

                                        figure: Kinetics of (a) simple and (b) facilitated diffusion