Terpenoids

Terpenoids are a class of organic compounds, which are widespread in nature. The mixtures of substances that contain the fragrant compounds of plants are called “essential oils,’’ and the majority of terpenoids occur in the essential oils of many plants and flowers, and some in gums and resins. A few terpenoids have been isolated from animal sources also. They contain the “essence” or the odour of the plant. Practically, you are familiar with distinctly pleasant odours (aromas) produced by many plants. The fragrances of pine and cedar trees, flowers (e.g. rose, jasmine, etc.) peppermint, turpentine, cinnamon, cloves, and camphor (moth balls) are just a few of the examples one can name. All these aromas, of course, are the result of the volatile compounds produced by the plants. Many of them are valuable, because of their use as flavourings, perfumes, spices, incense, medicines, and insect repellents.

Classification of terpenoids

Number of isoprene units	Number of carbon atom	Class
1	5	Hemiterpenoids  (C5H8)
2	10	Monoterpenoids  (C10H16)
3	15	Sesquiterpenoids (C15H24)
4	20	Diterpenoids  (C20H32)
5	25	Sesterterpenoids  (C25H40)
6	30	Tritepenoids  (C30H48)
8	40	Tetraterpenoid  (C40H64) (carotenoids)
>8	>40	Polyterpenoids  (C5H8)n

Biological importance and industrial uses of terpenoids

As insect attractants

The most important biological function of terpenods is to attract beneficial insects and other animals to the plants, which are essential for pollination of flowers and seed dispersal. Some plants like rose, sandalwood, geranium, jasmine and iris, etc. emits soft floral scents due to the presence of volatile mono and some sesquiterpenoids present in the plants.

 For example, monoterpenoids, α –Pinene, myrcene, geraniol, citral and can act as aggregation, trail, alarm, and sex pheromones for bees, weevils, silkworm and other insects. These insects, therefore, can pollinate the flowers.

As insect repellents

The strong aromatic adours of some terpenoids repel harmful insects and pests and protect the plants from predators. For example, grass such as citronella and lemon grass, due to their strong aromatic odours, are never eaten by herbivorous animals like cattle. The sesquiterpenoid, trans-β-farnesene, is a potent aphid repellent present in hops and sweet potatoes.

As defensive secretion

Camphor and 1,8-cineol are emitted b various plants like eucalyptus and sagebrush. They inhibit germination and development of seeds of other competing species. Cubitene is a diterpenoids  present in the defensive secretion of species of African termite. Similarly α –Pinene, myrcene,  geraniol  and citral  occur  as  defensive secretions in ants and beetles, for  protection.

As insect pheromones

Another biological function of terpenoids is to act as insect sex pheromones. For example the sesquiterpenoids, germacrene  diepoxide act as a sex pheromone of the cockroach.

As plant growth substances

The most biologically significant class of diterpenoids is the Gibberellins, which are plant hormones. They are widely distributed in higher plants.

As juvenile hormones

Another group of biologically active sesquiterpenoids, are the juvenile hormones, which control the metamorphosis (maintains the larval character) of immature insects.

As receptors in visual process

Vitamin A1 (retinol) is a monocyclic diterpenoid. It occurs free and as esters in fats, in fish livers, and in blood. It influences growth in animals and also increases resistances to disease. Vitamin A1  deficiency in the human diet cause night blindness and prolonged deficiency lead to xerophthalmia.

As photoreceptors in plants

Carotenoids are physiologically very important for photosynthetic plants. They occur in association with chlorophyll. They act as scavengers to remove the potentially harmful oxygen containing radicals and excited states formed during the early stages of photoreceptors.