Chloroplast Pigments
The pigments involved in the process of photosynthesis are known as photosynthetic pigments. Pigments like chlorophylls, carotenoids (carotenes and xanthophylls), anthocyanin, phycobilins (phycocyanin and phycoerythrin) may be found in a plant cell.
a) Chlorophylls: The chlorophylls, the green pigments in chloroplast are of seven types i.e. chlorophyll a, b, c, d, e, bacteriochlorophyll and bacterioviridin. Chlorophyll molecule in made up of a square tetrapyrrolic ring known as head and a phytol alcohol called tail. The magnesium atom is present in the central position of tetrapyrrolic ring. The four pyrrole rings of porphyrin head are linked together by methane (CH=) groups forming a ring system. Each pyrrole ring is made up of four carbon and one nitrogen. The porphyrin head bears many characteristic side groups at many points. Different side groups are indicative of various types of chlorophylls.
Phytol tail is made up to 20 carbon alcohol attached to carbon 7 position of pyrrole ring IV with a propionic acid ester bond. The name chlorophyll was given to the green pigment by Pelletier and Caventou in 1817. In majority of green plants chlorophyll a and b are present.
Chlorophyll a – C35H72O5N4Mg
Chlorophyll b – C55H70O6N4Mg
Chlorophyll a is blue black while chlorophyll b is green black. Both are soluble in organic solvents like alcohol, acetone etc. Chlorophyll a appears red in reflected light and bright green in transmitted light as compared to chlorophyll b which looks brownish red in reflected light and yellow green in transmitted light. Chlorophyll is a green colored pigment because it does not absorb green light. Chlorophyll a possesses –CH3 (methyl group), which is replaced by –CHO (an aldehyde) group in chlorophyll b.
(b) Carotenoid pigments
Carotenoids are the lipids and can be classified into two groups:
(i) Carotenes (ii) Xanthophylls.
Carotenes are orange red in color. The have general formula C40H56. They were isolated from carrot. On the other hand, xanthophylls are yellow in color. Xanthophylls contain oxygen also along with carbon and hydrogen (C40H56O2). All the carotenoid pigments have 40 carbon atoms. Lutein a widely distributed xanthophyll is responsible for yellow color autumn foliage. Fucoxanthin is another important xanthophyll present in Phaecophycease (Brown algae).
Carotenoids absorb light energy and transfer it to Chlorophyll a and thus act as accessory pigments. They protect the chlorophyll molecules from photo-oxidation by picking up nascent oxygen and converting it into harmless molecular stage.
(c) Phycobilins
These pigments are mainly found in blue-green algae (cynobacteria) and red algae. These pigments have open tetrapyrrolic in structure and do not bear magnesium and phytol chains.
Blue-green algae have more quantity of phycocyanin and red algae have more phycoertythrin together form phycobilins. These water-soluble pigments are thought to be associated with small granules attached with lamellae.
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