Terpenoids

The class is organized by how many isoprene units a molecule contains, and whether its carbon chain closes into rings or stays open:

Monoterpenes

two units, ten carbons

Sesquiterpenes

three units, fifteen carbons

Diterpenes

four units, twenty carbons

Triterpenes

six units, thirty carbons

Tetraterpenes (carotenoids)

eight units, forty carbons

As a rule, the smaller and lighter the molecule, the more volatile it is — which is why the monoterpenes dominate essential oils while the triterpenes sit in resins and barks.

Terpenoids are close to universal, but certain families are famous for concentrating them, usually as defensive or aromatic chemistry.

Beyond these, the saponin-rich roots of ginseng and liquorice and the carotenoid pigments of fruits and leaves show how far the class reaches.

A compound is classed as a terpenoid when its carbon skeleton can be traced back to repeating isoprene units, typically with oxygen-bearing groups added. Strictly, the pure hydrocarbons are the terpenes and the broader oxygenated majority are the terpenoids, though the two words are often used interchangeably.

Structure, not botanical origin or biological activity, is the sorting criterion — which is why a fragrant monoterpene and a foaming saponin sit in the same class.

Terpenoids give herbal medicine much of its character. The aromatic monoterpenes and sesquiterpenes are the carminative, antimicrobial and anti-inflammatory actives of the essential-oil herbs, while the triterpenes and saponins supply adaptogenic, expectorant and vein-toning activity.

The sterols and carotenoids are more nutritional in character, acting as membrane components, antioxidants and, in the case of the provitamin-A carotenes, vitamin precursors.