Lipids are naturally occuring molecules found in organisms that are used primarily for energy storage, cell membranes, and for signaling molecules. Typical lipids are found in fats, waxes, fat-soluble vitamins, such as A, D, E, and K, mono- , di-, and triglycerides, and phospholipids. Lipids may occur in saturated form or undaturated form, saturated refers to the number of hydrogen atoms bonded to each carbon atom in the chain. Unsaturated means there is minimal hydrogen bonding in the carbon chain. The primary categories of lipids are fatty acids, glycerolipids, glycerophospholipids, sphingolipids, and sterols. Some of these are hydrophobic ("water hating", will not mix with water) while others are hydrophilic ("water loving", will mix with water). Of these, the more common are the fatty acids, the glyceolipids (used in adipose tissue, fat cells), glycerolipids (used in bilayer cellular membranes), and sterols, such as cholesterol.