How are terpenes primarily formed?

Terpenes are primarily formed by the joining of isoprene units. Isoprene, which has the chemical formula C5H8, is a building block for terpenes and is characterized by its simple structure featuring a conjugated double bond system. The fundamental process of terpene biosynthesis occurs through the head-to-tail condensation of these isoprene units, leading to a vast array of terpene structures.

This process takes place in the biosynthetic pathways involving enzymes that catalyze the formation of various terpenes. The way in which isoprene units are assembled allows for the diversity of terpenes found in nature, ranging from monoterpenes (which contain two isoprene units) to larger molecules like geraniol or even squalene, which have multiple isoprene units linked together.

Other options discuss different biochemical processes: the decomposition of sugars does not relate to terpene formation, fermentation of alcohol involves converting sugars into alcohol through microorganisms, and hydrophobic reactions often refer to interactions between nonpolar molecules rather than the addition of isoprene units. These distinctions clarify why the option about isoprene units is the correct choice.