Terpenes
Also known as: isoprenoids
Terpenes are a class of lipids built from repeating five-carbon isoprene units. They range from the small volatile molecules that give plants their scents to large structures such as squalene, the precursor of cholesterol and the steroid hormones.
Every terpene is assembled from the same building block: isoprene, a five-carbon branched unit with the formula C₅H₈. Because units are joined predominantly head-to-tail, terpene carbon counts come in multiples of five, and the isoprene rule — mentally chopping a structure into five-carbon pieces — is the standard way to classify one. Terpenes are hydrophobic and are classified as lipids on that basis, even though they lack the fatty acid chains found in triacylglycerols and phospholipids.
Classification follows the number of isoprene units: monoterpenes contain two units and ten carbons, sesquiterpenes three and fifteen, diterpenes four and twenty, triterpenes six and thirty, and tetraterpenes eight and forty. Familiar examples map onto this scheme: limonene and menthol are monoterpenes, squalene is a triterpene, and the carotenoids that color carrots and tomatoes are tetraterpenes. A terpenoid is a terpene that has been chemically modified, usually by the addition of oxygen-containing functional groups or the loss of methyl groups.
Biologically, terpenes matter because of what they become. Isoprene units are activated in the cell as isopentenyl pyrophosphate, produced from acetyl-CoA through the mevalonate pathway. Condensation of these units yields squalene, which cyclizes to form the four-ring sterol skeleton of cholesterol, and cholesterol in turn is the precursor of all steroid hormones and of bile acids. The fat-soluble vitamins A, E, and K are likewise terpenoid derivatives; retinol comes from cleavage of the tetraterpene beta-carotene. In plants, smaller volatile terpenes serve as defensive compounds and pollinator attractants, which is why they dominate essential oils.
On the MCAT, terpenes appear in both the biological and physical sciences sections. The biochemistry material covers them alongside the other lipid classes in the context of the endocrine system, connecting the terpene backbone to cholesterol and steroid hormone synthesis, while the chemical and physical foundations material treats them within lipid structure and reactivity. Test questions most often ask you to count isoprene units to classify a drawn structure, distinguish a terpene from a terpenoid, or trace the path from isoprene through squalene to cholesterol.
Key takeaways
- Terpenes are hydrophobic lipids constructed from repeating five-carbon isoprene units, usually joined head-to-tail.
- They are classified by unit count: monoterpenes have two units, sesquiterpenes three, diterpenes four, triterpenes six, and tetraterpenes eight.
- A terpenoid is a terpene modified by added functional groups, typically oxygen-containing ones.
- Squalene, a triterpene, cyclizes to form cholesterol, the precursor of all steroid hormones and bile acids.
- The fat-soluble vitamins A, E, and K are terpenoid derivatives.