Among the compounds found in the essential oils of plants are a group of substances called terpenes, all of which have in common the fact that their carbon skeletons can be divided into two or more carbon units that are identical with the five-carbon skeleton of isoprene. Carbon 1 of an isoprene unit is called the head, and carbon 4 is called the tail. A terpene is a compound in which the tail of one isoprene unit becomes bonded to the head of another isoprene unit.

CH3     | CH2=C-CH=CH2	C 1 |23 4 C-C-C-C
2-Methyl-1,3-butadiene	Isoprene unit

Terpenes are among the most widely distributed compounds in the biological world, and a study of their structure provides a glimpse into the wondrous diversity that nature can generate from a simple carbon skeleton. Terpenes also illustrate an important principle of the molecular logic of living systems: In building large molecules, small subunits are bonded together by a series of enzyme-catalyzed reactions and then chemically modified by additional enzyme-catalyzed reactions. Chemists use the same principles in the laboratory, but their methods cannot match the precision and selectivity of the enzyme-catalyzed reactions of cellular systems.

Probably the terpenes most familiar to you - at least by odor - are components of the so-called essential oils extracted from various parts of plants. Essential oils contain the relatively low-molecular-weight substances that are largely responsible for characteristic plant fragrances.