Terpenes are a key component in chemotaxonomical classification of cannabis strains as terpene composition is a phenotypic trait. Majority of terpenes found in cannabis are hydrocarbons, which are a direct product of terpene synthase (TPS) enzymes. The molecular make up of terpenes in a cannabis plant involves the linking and elongation of chains in hydrocarbons and isoprene units, formed by isopentenyl pyrophosphate and dimethylallyl pyrophosphate. Terpenoids are basically terpenes with the addition of oxygen, among other structural additions. There are numerous types of unique functional terpenes in green plants and are formed via many differing pathways; methylerythritol phosphate (MEP), cytosolic mevalonate (MEV), or deoxyxylulose phosphate pathway (DOXP) to name a few. In addition, mevalonic acids (MVA) involvement in biosynthesis of complex terpenoids, such as steroids, was demonstrated in 1983. Once produced, specifically within the disk cells, terpenes are stored within the trichomes of the plant. There are several types of terpenes in cannabis composed of varying numbers of isoprene units. They contribute to the signature aroma and insecticidal properties via their emission as volatile organic compounds. Different cannabis strains synthesize different terpenes through their biochemical pathways, and diversity of the terpenes is dependent upon the diversity of the TPS enzymes present in the cannabis plants TPS gene pool. Though, causes of variations in the TPS enzymes are still unknown.
Monoterpenes myrcene and sesquiterpenes β-caryophyllene (binds to the human CB2 cannabinoids receptor) and α-humulene are the most common terpene compounds, and are present in most varieties of cannabis strains. The lack of exact standards makes it sometimes difficult for scientists to classify new terpenes. Terpene profiles are subject to change under different environmental conditions, which may lead to variation in TPS gene expression, ultimately leading to a variation in the synthesized terpenes. Terpenes have unique, distinct aromas, which is why each strain smells different. Cannabis plants, like many others, biochemically synthesize terpenes with intense aromas as a method of chemical defense in attempts to repel predators, and invite pollinators. Because terpenes and terpenoids are biologically active molecules, it is possible variations in terpenes may elicit different biological and psychoactive responses in humans. This is why people claim to have different psychological effects to different strains.