Figure 1. Structure of (a) chlorophyll a, (b) chlorophyll b, (c) bacteriochlorophyll a and (d) bacteriochlorophyll b. The major esterifying alcohol (= R) is phytol in chlorophyll a and b, geranylgeraniol in bacteriochlorophyll a, and phytadienol in bacteriochlorophyll b.

Figure 2. Potential feedback control mechanisms in the metabolic pathway of tetrapyrroles. The synthesis of 5-aminolaevulinate (ALA) from glutamate is the rate-limiting step of tetrapyrrole biosynthesis. Feedback regulation on the enzymes of the ALA pathway, glutamyl-tRNA reductase or glutamate 1-semialdehyde aminotransferase, can start out from three possible sites: at haem formation, at the reduction of protochlorophyllide, and at the level of Mg-protoporphyrin IX and Mg-protoporphyrin IX monomethylester (Mg-Proto MME) formation.

Figure 3. The possible hazardous reactions of excited porphyrins after formation of their long-lived excited triplet state. Free radicals can be generated by electron or hydrogen transfer (type I reaction) or energy is transferred to oxygen giving rise to the formation of toxic singlet oxygen (type II reaction). Additionally, the quench mechanism of excited porphyrins and singlet oxygen by carotenoids (Car) are illustrated (green arrows). Zeaxanthin is assumed to dissipate efficiently excitation energy.