A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle. Membrane proteins can be classified into two groups, based on their attachment to the membrane.
Integral membrane protein
Integral membrane proteins are firmly attached to the membrane. In most cases, the protein spans the entire membrane (a transmembrane protein) but in all cases, the protein is anchored in the membrane's hydrophobic region, by a covalently attached lipid or glycolipid. A detergent (such as SDS or Triton X-100) is necessary to disrupt the lipid bilayer and dissociate the protein.
Peripheral membrane protein
Peripheral membrane proteins are attached to integral proteins, usually by electrostatic bonds. Peripheral proteins easily dissociate following treatment with a polar reagent, such as a solution with an elevated pH or high salt concentrations. In either case, the lipid bilayer is not disrupted.