Among the chlorophylls, chlorophyll a and b are best known and most widely distributed. These are absent in pigmented bacteria.
Chlorophylls c, d and e are found only in algae in combination with chlorophyll a. Bacteriochlorophylls and chlorobium chlorophylls are found only in photosynthetic bacteria.
The chlorophyll molecule is a complex structure basically made up of a ‘head’ and ‘tail’ resembling a tennis racquet. The ‘head’ is a porphyrin structure made of four Pyrrole rings attached to each other at the centre by ring containing Mg atom at the centre.
Extending from one of the pyrrole rings is the ‘tail’ – the alcoholic chain (phytol). The emperical formula for the chlorophyll molecule is CH, 0, N Mg. The phytol chain is esterified on the C atom of one of the pyrrole rings and has only one double bond.
Chlorophyll a and b differ structurally in having different atomic groupings at the C atom in the pyrrole ring. In chlorophyll a C atom has a methyl group, while chlorophyll b has an aldehyde group; besides the two pigments have different absorption spectra.
The peaks are as follows Chlorophyll a 429,410 and 660nm Chlorophyll b 430,453 and 442nm (The above spectra are for chlorophyll in vitro)
2. Carotenoid pigments:
These are lipid compounds ranging in colour from yellow to purple. They are widely distributed in both plants and animals. They are also present in microorganisms including red algae, cyanobacteria, photosynthetic bacteria, fungi, etc. Wackenroder (1931) isolated the first carotenoid – carotene from tissue.
Carotenoids are derivatives of lycopene a red pigment found in many plants. They (carotenoids) are known to have eight isoprene (CH2 = C [CHJ – CH = CH2) like residues.
The main carotenoids found in plants is the orange yellow coloured (5 carotene with some quantity of a carotene
The carotenoids are also located in the chloroplast. According to Goodwin (1960), carotenoids and chlorophylls may be combined with the same protein to form a complex known asphotosynthein carotenoids protect the chlorophyll form photo oxidation and transfer the light energy they absorb to chlorophyll a.
These are found only in algae. There are two types of pycobilins – the Phycoreythrin (red) and Phycocyanin (blue) these pigments are strongly associated with a protein and consequently it is difficult to isolate the pigments in the pure state.
Like carotenoids phycobilins are also involved in the transfer of light energy (they absorb) to the chlorophyll.
The absorption spectra of phycobilins present an interesting study considering the fact that they act as accessory pigments in photosynthesis. R – phycoerythrin has peaks at 495,540 and 545nm, while R – phycocyanin has peaks at 550 and 615nm.