Thus double bonds interfere with transition to the crystalline state, and lower the phase transition temperature. Cholesterol inserts into bilayers with its polar hydroxyl interacting with polar phospholipid head groups. Cholesterol inhibits the transition from liquid crystal to crystalline state , because its rigid ring system interferes with close packing of fatty acid tails of phospholipids. However the rigid cholesterol makes the membrane somewhat less fluid. Membrane Proteins Membrane proteins may be classified as peripheral , integral , or having a lipid anchor .
Peripheral proteins are on the membrane surface.
They are water-soluble, with mostly hydrophilic surfaces. Many peripheral proteins adhere to exposed domains of integral proteins, by ionic and H-bond interactions. Some cytosolic proteins include domains that recognize and bind to polar head-groups of particular membrane lipids. PH domains mediate reversible binding of many signal proteins to the cytosolic surface of the plasma membrane. The kinases that catalyze transfer of phosphate from ATP to the inositol moiety of phosphatidylinositol, creating ligands recognized by PH domains, are themselves regulated by signal pathways.