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'''Terpenes''' ( ) are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifer+s, though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeteria+. They are often strong smelling and thus may have had a protective function.

They are the major components of resin+, and of turpentine+ produced from resin. The name "terpene" is derived from the word "turpentine+". In addition to their roles as end-products in many organisms, terpenes are major biosynthetic building blocks within nearly every living creature. Steroids+, for example, are derivatives of the triterpene squalene+.

When terpenes are modified chemically, such as by oxidation or rearrangement of the carbon skeleton, the resulting compounds are generally referred to as terpenoid+s. Some authors will use the term terpene to include all terpenoids. Terpenoids are also known as isoprenoid+s.

Terpenes and terpenoids are the primary constituents of the essential oil+s of many types of plants and flowers. Essential oils are used widely as natural flavor additives for food, as fragrances in perfumery, and in traditional and alternative medicines such as aromatherapy+. Synthetic variations and derivatives of natural terpenes and terpenoids also greatly expand the variety of aromas used in perfumery and flavors used in food additives. Vitamin A+ is an example of a terpene.

Terpenes are released by trees more actively in warmer weather, acting as a natural form of cloud seeding+. The clouds reflect sunlight, allowing the forest to regulate its temperature.

The aroma and flavor of hops+, highly desirable in some beers, comes from terpenes. Of the terpenes in hops myrcene+, b-pinene+, b-caryophyllene+, and a-humulene+ are found in the largest quantities.

Terpenes are derived biosynthetically from units of isoprene+, which has the molecular formula C5H8. The basic molecular formulae of terpenes are multiples of that, (C5H8)n where n is the number of linked isoprene units. This is called the '''isoprene rule''' or the ''C5 rule''. The isoprene units may be linked together "head to tail" to form linear chains or they may be arranged to form rings. One can consider the isoprene unit as one of nature's common building blocks.

Isoprene itself does not undergo the building process, but rather activated forms, isopentenyl pyrophosphate+ (IPP or also isopentenyl diphosphate) and dimethylallyl pyrophosphate+ (DMAPP or also dimethylallyl diphosphate), are the components in the biosynthetic pathway. IPP is formed from acetyl-CoA+ via the intermediacy of mevalonic acid+ in the HMG-CoA reductase pathway+. An alternative, totally unrelated biosynthesis pathway of IPP is known in some bacterial groups and the plastids of plants, the so-called MEP(2-Methyl-D-erythritol-4-phosphate)-pathway, which is initiated from C5-sugars. In both pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase.
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As chains of isoprene units are built up, the resulting terpenes are classified sequentially by size as hemiterpenes, monoterpenes, sesquiterpenes, diterpenes, sesterterpenes, triterpenes, and tetraterpenes. Essentially, they are all synthesised by Terpene synthase+.

Terpenes may be classified by the number of isoprene units in the molecule; a prefix in the name indicates the number of terpene units needed to assemble the molecule.

* '''Hemiterpenes''' consist of ''a single isoprene'' unit. Isoprene itself is considered the only hemiterpene, but oxygen-containing derivatives such as prenol+ and isovaleric acid+ are hemiterpenoids.
* '''Monoterpenes+''' consist of ''two isoprene'' units and have the molecular formula C10H16. Examples of monoterpenes are: geraniol+, limonene+ and terpineol+.
* '''Sesquiterpenes+''' consist of ''three isoprene'' units and have the molecular formula C15H24. Examples of sesquiterpenes are: humulene+, farnesene+s, farnesol+. (The ''sesqui-'' prefix means one and a half.)
* '''Diterpenes+''' are composed of ''four isoprene'' units and have the molecular formula C20H32. They derive from geranylgeranyl pyrophosphate+. Examples of diterpenes are cafestol+, kahweol+, cembrene+ and taxadiene+ (precursor of taxol+). Diterpenes also form the basis for biologically important compounds such as retinol+, retinal+, and phytol+. They are known to be antimicrobial and antiinflammatory.
* '''Sesterterpenes''', terpenes having 25 carbons and ''five isoprene'' units, are rare relative to the other sizes. (The ''sester-'' prefix means half to three, i.e. two and a half.) An example of a sesterterpene is geranylfarnesol+.
* '''Triterpene+s''' consist of ''six isoprene'' units and have the molecular formula C30H48. The linear triterpene squalene+, the major constituent of shark liver oil+, is derived from the reductive coupling of two molecules of farnesyl pyrophosphate+. Squalene is then processed biosynthetically to generate either lanosterol+ or cycloartenol+, the structural precursors to all the steroid+s.
* '''Sesquarterpenes''' are composed of ''seven isoprene'' units and have the molecular formula C35H56. Sesquarterpenes are typically microbial in their origin. Examples of sesquarterpenes are ferrugicadiol and tetraprenylcurcumene.
* '''Tetraterpenes''' contain ''eight isoprene'' units and have the molecular formula C40H64. Biologically important tetraterpenes include the acyclic lycopene+, the monocyclic gamma-carotene+, and the bicyclic alpha-+ and beta-carotene+s.
* '''Polyterpenes''' consist of long chains of ''many isoprene'' units. Natural rubber+ consists of polyisoprene in which the double bonds are cis+. Some plants produce a polyisoprene with trans double bonds, known as gutta-percha+.
* '''Norisoprenoids''', such as the C13-norisoprenoids 3-oxo-α-ionol+ present in Muscat of Alexandria+ leaves and 7,8-dihydroionone+ derivatives, such as megastigmane-3,9-diol+ and 3-oxo-7,8-dihydro-α-ionol+ found in Shiraz+ leaves (both grapes in the species ''Vitis vinifera+'') or wine+ (responsible for some of the spice notes+ in Chardonnay+), can be produced by fungal peroxydases or glycosidase+s.

Research into terpenes has found that many of them possess qualities that make them ideal active ingredients as part of natural agricultural pesticides.

'''Terpin hydrate''' is a derivative of turpentine+. An expectorant+ and humectant+, it is commonly used in the treatment of acute+ or chronic+ bronchitis+ and related conditions.

Terpenes are used by termites+ of the Nasutitermitinae+ family to attack enemy insects, through the use of a specialized mechanism called a fontanellar gun+.

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Terpenes
* Link: Institute of Chemistry - terpenes
* Link: Structures of alpha pinene and beta pinene
* Terpenes

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 Terpene+ Terpenes ( ) are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeteria.
 Terpineol+ Terpineol is a naturally occurring monoterpene alcohol that has been isolated from a variety of sources such as cajuput oil, pine oil, and petitgrain oil.
Terpene synthase N terminal domain+ In molecular biology, this protein domain belongs to the terpene synthase family (TPS). Its role is to synthesize terpenes which are part of primary metabolism, such as sterols and
Terpene synthase C terminal domain+ In molecular biology, this protein domain belongs to the terpene synthase family (TPS). Its role is to synthesize terpenes which are part of primary metabolism, such as sterols and