Photosynthesis Part-2 || Photosynthetic Pigments || Structure of Chlorophyll | BSc,MSc,CSIR NET,NEET

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Chlorophylls

They are the green photosyn­thetic pigments. Five types of chlorophylls occur in plants other than bacteria— a, b, c, d and e. Out of these only two chlorophylls occur in the chloro­plasts of higher plants, a and b. The amount of chlorophyll b is roughly one fourth of total chloro­phyll content. Chlorophyll a is found in all photo­synthetic plants except bacteria. Hence, it is termed as universal photosynthetic pigment.

It is also called primary photosynthetic pigment because it performs primary reaction of photosynthesis which involves conversion of light into chemical or elec­trical energy. Other photosynthetic pigments are thence called accessory pigments.

They absorb light energy of different wavelengths, broaden the spec­trum of light absorption and hand over the energy to chlorophyll a through electron spin resonance. Some of the chlorophyll a molecules function as reaction centres (P700, P680).

Bacteria possess two types of related pigments— bacteriochlorophyll (further of several subtypes) and bacterioviridin (= chlorobium chlorophyll). Bacte­riochlorophyll a has an empirical formula of C55H74O6N4Mg and molecular weight of 911.

Chlorophyll a is bluish-green in the pure state. It has an empirical formula of C55H70O5N4Mg and molecular weight of 893. Chlorophyll b is olive green in the pure state with an empirical formula of C55H70O6N4Mg and molecular weight of 907. Both the chlorophylls are soluble in a number of organic solvents but chlorophyll a is more soluble in petroleum ether while chlorophyll b is more soluble in 92% methyl alcohol.

Chlorophyll structure was first studied by Wilstatter, Stoll and Fischer in 1912. It has a tadpole like configuration with a head called porphyrin and a tail made up of long chain alcohol called phytol (Fig. 13.8). Porphyrin head is made up of four pyrrole rings which are linked by methine bridges (—CH=). The skeleton of each pyrrole ring is made up of 5 atoms— 4 carbon and one nitrogen. The latter lies towards the centre.

Structure of Chlorophyll a.

A nonionic magnesium atom is held in the centre of porphyrin head by nitrogen atoms of pyrrole rings (through two covalent and two coordinate bonds).

The external carbon atoms of the pyrrole rings have been given specific numbers, 1-8. Carbon atoms 1, 3, 5 and 8 have methyl groups (__CH3). Carbon atom 2 possesses a vinyl group (—CH = CH2) while carbon atom 4 has an ethyl group (— CH2 — CH3). Carbon atom 6 is attached to next methine group by a fifth isocyclic ring called cyclopentanone.

Carbon atom 7 is connected to phytol tail through a propionic acid residue. Phytol is an insoluble long chain of carbon and hydrogen atoms with a formula of C20H39OH. It anchors the chlorophyll molecule into the lipid part of thylakoid membrane. Chlorophyll without its Mg-core is colourless and called phaeophytin. It is the early electron acceptor#photosynthesis #csirnetlifescience #botany #lifescienceexam