Isopentenyl diphosphate (IPP) is the biological C₅ precursor of isoprenoids. By labeling experiments using [1-¹³C]glucose, higher plants were shown to possess two distinct biosynthetic routes for IPP biosynthesis: while the cytoplasmic sterols were formed via the acetate/mevalonate pathway, the chloroplast-bound isoprenoids (β-carotene, lutein, prenyl chains of chlorophylls and plastoquinone-9) were synthesized via a novel IPP biosynthesis pathway (glyceraldehyde phosphate/pyruvate pathway) which was first found in eubacteria and a green alga. The dichotomy in isoprenoid biosynthesis in higher plants allows a reasonable interpretation of previous odd and inconclusive results concerning the biosynthesis of chloroplast isoprenoids, which so far had mainly been interpreted in the frame of models using compartmentation of the mevalonate pathway.