 Cholesterol is an essential non-polar lipid. Most cholesterol in the body is found in cell membranes where it maintains the fluidity of the membrane by preventing the fatty acid tails sticking together. Cholesterol is also an important constituent of myelin and is used in the production of bile salts. Very small quantities of cholesterol are used to make steroid hormones. Cholesterol is insoluble in water so it is carried in blood plasma in the form of lipoproteins. High-density lipoprotein HDL, which has a high protein-to-lipid ratio, and low-density lipoprotein LDL. HDLs are often called good because they pick up excess cholesterol and transport it back to the liver where it is made into bile salts and later excreted. LDLs transport cholesterol to cells where it is needed to maintain membrane fluidity. An LDL particle contains over a thousand cholesterol molecules, a monolayer of phospholipids surrounds the cholesterol core and is embedded with proteins called apoproteins. Most mammalian cell membranes have receptors that recognize and bind specifically to the apoprotein in LDL particles. An LDL particle binds to a cluster of specific receptors and is then internalized through receptor-mediated endocytosis. The LDL is now contained in a vesicle inside the cell. Once the vesicle is inside the cell, the receptors are released and returned to the membrane. The vesicle fuses with a lysosome. Enzymes from the lysosome digest the vesicle, releasing the contents, amino acids, fatty acids, and cholesterol. The cell may use the cholesterol immediately. Once cells have enough cholesterol, they stop returning LDL receptors to the membrane. If LDL cannot bind to the cell membrane, it will not be internalized into the cell. The concentration of LDLs in blood increases. If too much LDL accumulates in the blood, plaques atheroma may form on arterial walls leading to atherosclerosis. Because of this, LDL is often called bad.