Many of these chemicals are polar (neutral or charged), which explains why the type of barrier chosen by evolution to keep them inside the cell is based on a lipid structure, whose permeability to such molecules is extremely low. In order for a polar compound to cross the membrane, it must –rst partition from the aqueous environment (cytoplasm or exoplasm) into the membrane itself. is process is highly unlikely, due to the de-solvation of the polar compound. e partitioning can be estimated in two steps, as follows.