Isoprenoids, a major class of nonsaponifiable lipids that occur in plants, animals, and bacteria, are characterized by chains consisting of modular groups of five carbon atoms in which the typical pattern has four of the carbon atoms in a linear chain and a single carbon attached at the carbon one position removed from the end of the chain. Isoprenoids, which play widely varying roles in the physiological processes of plants and animals, also have a number of commercial uses. The isoprenoids are also known as terpenes. The smallest terpene molecules—those containing 10 carbon atoms—are called monoterpenes. The larger molecules, which increase by five-carbon units at a time, are called sesquiterpenes, diterpenes, triterpenes, and tetraterpenes, respectively. The monoterpenes are mostly volatile, which accounts for their fragrances. While terpenes of higher molecular weight are less volatile, sesquiterpenes contribute to the flavors of some foods. The addition of water molecules to a pyrophosphate group connected to a monoterpene produces geraniol, a component of geranium oil and an important contributor to rose perfumes. Related molecules are menthol from peppermint oil; citral from lemongrass oil; and limonene from lemon and orange oils. Other monoterpenes include pinene, from turpentine, and camphor. While most of the larger terpenes are cyclic—i.e., they contain ring structures—the diterpene called phytol, an important precursor of the plant pigment chlorophyll, is acyclic. A related diterpene that contains one ring of six carbon atoms is vitamin A. Squalene, although acyclic, is a precursor of the four-ring steroids that play an essential role in controlling many biological activities. Found widely in nature, notably in fish-liver oils, squalene is among the most important triterpenes. Five-ring triterpenes, which are derived from squalene, like steroids, and found in many plants, are often linked chemically to sugars to form compounds called saponins. The tetraterpenes include, among many others, the plant pigments lycopene, which accounts for the red of tomatoes, and ß-carotene, which accounts for the yellow in carrots. The digestive process splits ß-carotene, a symmetrical molecule, to produce vitamin A. Among the best-known polyterpenes, which are polymers having numerous isoprene units, is natural rubber, in which 4,000 to 5,000 five-carbon units are linked together. Gutta-percha, a tough plastic substance used in insulation and in dentistry, is another polymeric isoprene.