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Many terpenes are derived commercially from conifer resins, such as those made by this pine.

Terpenes ({{#invoke:IPAc-en|main}}) are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers,<ref>Merriam-Webster Online Dictionary</ref> though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeteria. They are often strong-smelling. They may protect the plants that produce them by deterring herbivores and by attracting predators and parasites of herbivores.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Many terpenes are aromatic hydrocarbons and thus may have had a protective function.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The difference between terpenes and terpenoids is that terpenes are hydrocarbons, whereas terpenoids contain additional functional groups.

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 terpenoids. Some authors will use the term terpene to include all terpenoids. Terpenoids are also known as isoprenoids.

Terpenes and terpenoids are the primary constituents of the essential oils of many types of plants and flowers. Essential oils are used widely as fragrances in perfumery, and in medicine 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 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=news }}</ref>

The aroma and flavor of hops, highly desirable in some beers, comes from terpenes. Of the terpenes in hops myrcene, β-pinene, β-caryophyllene, and α-humulene are found in the largest quantities.<ref>Glenn Tinseth, "Hop Aroma and Flavor", January/February 1993, Brewing Techniques. <http://realbeer.com/hops/aroma.html> Accessed July 21, 2010.</ref>

Structure and biosynthesis

Isoprene

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.

Isopentenyl pyrophosphate
Dimethylallyl pyrophosphate

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.


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