Figure 1: Monoterpene biosynthesis. A. The terpenoid precursors isopentenyl pyrophosphate (IPP, 10) and dimethylallyl pyrophosphate (DMAPP, 11) are synthesized in plants via two different pathways, the mevalonate and the methylerythritol phosphate (MEP) pathway. DMAPP (11) and IPP (10) are further condensed to geranyl pyrophosphate (GPP, 12) which can be converted into monoterpenes such as thymol (15). The isoprene unit derived from IPP (10) is highlighted in red, the isoprene unit derived from DMAPP (11) in green. B. Biosynthesis of thymol (15) by aromatization of γ-terpinene (13) to p-cymene (14) followed by hydroxylation of p-cymene (14).