integral protein

Meaning: An integral protein is embedded in the phospholipid bilayer, held there by hydrophobic interactions between its nonpolar side chains and the fatty acid tails. Most are transmembrane, crossing the bilayer as one or more α-helices of roughly 20 nonpolar residues or as β-barrels; a few sit in only one leaflet. Polar and charged domains project into the aqueous space on either side, and in channels polar residues line the pore. Channels, carriers, pumps such as the Na⁺/K⁺ ATPase, and many receptors are integral.

On the MCAT: Expect an extraction experiment. High salt or a pH change releases peripheral proteins, but integral proteins stay put until a detergent dissolves the bilayer. You may also see a sequence or hydropathy plot, where a run of nonpolar residues (Leu, Val, Ile, Phe) suggests a membrane-spanning segment. In the fluid mosaic model, integral proteins can diffuse laterally unless the cytoskeleton anchors them.

Common confusion: Students treat integral and transmembrane as identical. Most integral proteins are transmembrane, but the defining test is how tightly the protein is bound. If only detergent or disruption of the bilayer releases it, it is integral. Peripheral proteins bind by weaker electrostatic or hydrogen-bond interactions to lipid head groups or to integral proteins.