Membrane proteins are proteins situated at the lipid bilayer or membrane of the cell. In animal cells, these proteins make up around 50% of the mass of the plasma membrane. Different types of membrane proteins are present at the membrane and each has different structure. They also carry out different functions. These functions are vital to the cell. Membrane protein acts as transporters, anchors, receptors and lastly enzymes. Moreover, different types of membrane proteins associate with the lipid bilayer in different ways: transmembrane, monolayer-associated, lipid-linked and protein-attached. Integral membrane proteins, also called intrinsic proteins, are permanently embedded within the plasma membrane. Structurally, the integral proteins contain residues with hydrophobic side chains that penetrate the fatty acyl regions of the phospholipid bilayer, thus anchoring the protein to the membrane. The only way to remove the integral proteins from the membrane are with synthetic detergents, nonpolar solvents and denaturing agents that disrupt the hydrophobic interactions of the bilayer.