Membrane lipids exist as closed bilayers in which the hydrophobic regions of the phospholipids are protected from the aqueous environment, while the hydrophilic regions are immersed in water.
Membranes contain two types of proteins: integral proteins and peripheral proteins. Integral proteins are anchored to membrane through a direct interaction with the lipid bilayer. Peripheral proteins are weakly bound to the hydrophilic regions of specific integral proteins. They are located on both surfaces of the membrane.