The main photosynthetic pigments are chlorophyll, carotenes and phycobilins. Chlorophyll is the main pigment of photosynthesis. Other pigments are called antenna pigments. Because they absorb light energy and give chlorophyll-a.
1. Chlorophyll: Chlorophyll is a green photosynthetic molecule. Chloroplasts are abundant in leaf mesophyll tissue. Green plants have five types of chlorophyll. Chlorophyll a, b, c, d and e. Chlorophyll-a is yellowish-green and chlorophyll-b is bluish-green. Some chlorophyll-a (P-700) absorb light at a wavelength of 700 nm, while some chlorophyll-a (P-680) absorb light at a wavelength of 680 nm. These chlorophylls participate in photosynthesis. Photoreactions occur in quantosomes of thylakoids and photoneutral reactions occur in the stroma. Modern scientists believe that light energy absorbed by chlorophyll-a is used in photosynthesis. Light energy absorbed by other pigments is passed on to chlorophyll-a. Some green sulfur bacteria like Rhodopseudomonas, Rhodospirillum, Chlorobium etc. contain bacteriochlorophyll. The chemical constituents of chlorophyll are carbon, hydrogen, oxygen, nitrogen and magnesium.
Chlorophyll-a : C55H72O5N4Mg
Chlorophyll-b : C55H70O6N4Mg
Chlorophyll-c : C35H30O5N4Mg
Chlorophyll-d : C54H70O6N4Mg
Role of Chlorophyll in Photosynthesis: In photosynthesis, chlorophyll converts solar energy into chemical energy. It is activated by absorbing photons or quanta of sunlight and dissociates water molecules to produce H+ and OH-.
2. Carotenoids: Orange carotene and yellow xanthophyll together are called carotenoids. Carotenoids are of two types. Carotene and xanthophyll.
(i) Carotene: Carotene is an unsaturated hydrocarbon compound. Its chemical symbol is C40H56O. It is of four types. Carotene a, b, c and d. It absorbs blue-violet light (449-478nm) the most. Color diversity of carotene is most interesting. Green carotene is found in avocados, red in tomatoes, reddish orange in carrots and yellow in various flowers.
(ii) Xanthophyll: Oxygenated carotene is called xanthophyll. Its chemical symbol is C40H56O2. Among the xanthophylls are lutein in green plants, lycoxanthin in tomatoes and zeaxanthin in Bhutra.
3. Phycobilins: Phycocyanin, phycoerythrin and allo-phycocyanin are collectively called phycobilins. Seven types of phycobilins are found. Cyanobacteria and red algae contain phycobilins. Phycobilins store light energy and help in photosynthesis. The red colored phycobilin is called phycoerythrin. It absorbs blue-green light. Its chemical symbol is C34H46O8N4. The blue colored phycobilin is called phycocyanin. It absorbs orange light. Its chemical symbol is C34H46O8N4.