The other class of photosynthetic pigments is the carotenoids. Most land plants contain a variety of carotenoids including beta-carotene, lutein, neoxanthin and violaxanthin. Their basic structure is composed of a repeating, branched five-carbon unit. Molecules formed from these five-carbon units are often called isoprenoids. Notice that the carotenoid structures also contain systems of conjugated double bonds that are responsible for the absorption of light.
Most carotenoids absorb photons in the blue region of the light spectrum (400 to 500 nm) and appear to be yellow; an exception is the primary red pigment in tomatoes, a carotenoid named lycopene.
The synthesis of carotenoids utilizes the isoprenoid pathway (Figure: Isoprenoid Pathway). This pathway is also the basis for the production of such diverse molecules as those involved in aroma molecules, vitamins (e.g. vitamin A), steroids and rubber. The biochemistry involves the addition of successive five-carbon units (or multiples of five carbons) and then rearrangements, cyclizations and additions of functional groups. The pathway is also important in that the geranylgeranyl pyrophosphate intermediate is utilized to make a phytyl group in the biosynthesis of chlorophyll. Carotenoid biosynthesis takes place inside the envelope membranes surrounding the chloroplast, not in the interior membranes containing the chlorophyll.