Figure 12.8 Integral and peripheral membrane proteins. Integral membrane proteins (a and b) interact extensively with the hydrocarbon region of the bilayer. Most known integral membrane proteins traverse the lipid bilayer. Some peripheral membrane proteins (c) interact with the polar head groups of the lipids. Other peripheral membrane proteins (d) bind to the surfaces of integral proteins. Some proteins (e) are tightly anchored to the membrane by a covalently attached lipid molecule.