Lipoproteins are particles in the bloodstream that transport fatty substances called lipids between different organs, glands, and tissues. The interior of the lipoprotein contains triglycerides (glycerol esterified with three fatty acids) and cholesterol esterified with fatty acids. The covering membrane of a lipoprotein contains chemicals more easily soluble in blood than those in the interior, such as free cholesterol, phospholipids (e.g., lecithin), and apoproteins. Since the different lipoproteins contain different amounts of triglycerides and cholesterol, they may be separated by centrifuging them into particles with different densities, including low-density (LDL), high-density (HDL), and very low-density (VLDL) particles. Apoproteins (such as apo A in HDL particles and apo B in VLDL and LDL particles) function to direct the lipoproteins to their destinations or to act as coenzymes to activate certain other enzymes that process the lipoproteins.