Figure 3. Isopentenyl diphosphate is synthesized in the cytosol via the mevalonic acid (MVA) pathway and in the chloroplast through the methylerythritol phosphate (MEP)pathway. Geranyl diphosphate (C10) is produced by a head-to-tail condensation of isopentenyl diphosphate (C5) and its isomer dimethylallyl diphosphate (C5). Addition of another isoprene unit produces farnesyl diphosphate (C15), a precursor of ABA in some fungi.Geranylgeranyl diphosphate (C20) is produced by another condensation reaction. Head-tohead condensation of two geranylgeranyl diphosphate molecules produces phytoene (C40) in
the first committed step in carotenoid biosynthesis.