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 speci?city.  The molecule in aqueous solution on one side of the membrane dissolves into the lipid bilayer crosses it and then dissolves into the aqueous solution on the opposite side. The rate  of  diffusion  is  straight  proportional  to  the  concentration  gradient  of  the molecule across the membrane and the procedure is not saturable