Integral membrane proteins Membrane proteins are classified as either peripheral (extrinsic) or integral (intrinsic) depending on how tightly they are associated with the membrane. Integral membrane proteins are tightly bound to the membrane through interactions with the hydrophobic core of the bilayer (see Figure 4) and can be extracted from it only by using agents that disrupt the membrane structure, such as organic solvents (e.g. chloroform) or detergents (see below). Most integral membrane proteins have one or more regions of the polypeptide chain that span the lipid bilayer and interact noncovalently with the hydrophobic fatty acid chains. However, some are anchored in the membrane by a covalently attached fatty acid or hydrocarbon chain (see below). Like lipids, integral membrane proteins are amphipathic, having both hydrophobic and hydrophilic regions, and are asymmetrically distributed across the bilayer (Section E1).