Integral membrane proteins typically span the lipid bilayer while peripheral proteins are typically bound by salt bridges or hydrogen bonds to the surfaces of integral proteins. Integral membrane proteins contain large sections with amino acids containing nonpolar side chains, which is definitely something to know for the MCAT. Because the transfer of proteins with nonpolar residues from the interior of the membrane to the surrounding water would require the input of free energy, equilibrium favors the sequestration of nonpolar protein segments by the hydrocarbon core of the membrane. In fact, such a free energy increase of +20 kcal/mol or more for a twenty residue segment strongly suggests that the polypeptide segment is a membrane spanning α-helix.