Answer the following question given this paragraph: 

The dual character is critical for the formation of the lipid bilayer. The hydrophilic portions of the molecule are in contact with the aqueous environment of the cell, while the hydrophobic portion of the lipids is in the interior of the bilayer and provides a barrier to the passive diffusion of most molecules.


Q: The dual character is critical for the formation of the what?


A:
lipid bilayer