1.3. Lipid Rafts :
Some micro domains of cholesterol and spingolipids from discrete membrane are known as lipid rafts. These clusters move laterally within the plasma membrane and get associate with specific membrane proteins. Two types of lipid rafts are (1) planar (Non coveolar) (2) Caveolae. Caveolae are flask shaped invagination of plasma membrane that contain caveolin protein. Caveolin is an intrinsic cholesterol binding membrane protein. Although the function of lipid rafts remain to be fully understood, they play an important role in processes such as cell movement (Cholesterol trafficking) and the uptake of extracellular molecules by endocytosis as well as in cell signaling. The lipid composition of a bilayer also influences its thickness which in turn may play a role in localizing proteins to a particular membrane. It is found that spingomyelin associates into a more gel-like & thicker bilayer than phospholipids do. Similarly, cholesterol and other molecules that decrease membrane fluidity increase membrane thickness. Because Spingomyelin tails are already optimally stabilized, the extra addition of cholesterol has no effect on the thickness of a spingomyelin bilayer. Lipid rafts are the centers of organizing for the assembly of signaling molecules, influencing membrane fluidity and membrane protein trafficking and regulating neurotransmission and receptor trafficking lipid rafts are more ordered and tightly packed than it's surrounding stsucture of biomembrane. Lipid rafts are present in plasma membranes and other parts of cells like Lysosomes and Golgi complex.