lipoprotein (lĬp´əprō´tēn), any organic compound that is composed of both protein and the various fatty substances classed as lipids, including fatty acids and steroids such as cholesterol. The lipoprotein complex of proteins and steroids is usually provided by a weak, noncovalent interaction; proteins complexed with some other lipids do so by the information of covalent chemical bonds. There are several types of lipoproteins present in human blood, including low-density lipoproteins (LDLs)—molecules with a larger molecular weight and a relatively low percentage of protein—and high-density lipoproteins (HDLs)—molecules with a smaller molecular weight and a relatively high percentage of protein. LDLs are the main transport for cholesterol through the body. HDLs appear to carry excess cholesterol to the liver for processing. Studies have found that high levels of HDLs, which seem to retard or even reverse the formation of cholesterol plaque in the arteries (see arteriosclerosis), reduce the risk of cardiovascular disease. Cell membranes are essentially lipoprotein in nature; the membrane is a continuous sheet of lipid molecules, largely phospholipids, in close association with proteins that either face one side of the membrane or penetrate all the way through the membrane.