Structure of photosynthetic Pigments


Chlorophylls :

* They are magnesium porphyrin compounds

* The porphyrin ring consists of four pyrrole rings joined together by CH bridges.

* A long chain of C atoms called as Phytol chain is attached to the porphyrin ring.

* Molecular formulae :

                      



 Both the chlorophylls (a and b) consist of :

Mg-Porphyrin head which is hydrophilic in nature and a

Phytol tail possessing lipophilic properly.

Difference between two chlorophylls

In chlorophyll b there is -CHO (aldehyde) group, instead of a _CH3 (acetyl) group in chlorophyll a molecule.

Synthesis of Chlorophyll

* Chlorophyll is formed from protochlorophyllide in the presence of light with the loss of two hydrogen atoms as

follows :

                      Protochlorophyll               →                Chlorophyll

                                                               -2H

Location of Chlorophyll Pigments in Chloroplast

The photosynthetic pigments are located in grana portions of the chloroplasts of higher plants.

Chlorophyll molecules form a monomolecular layer between the alternative protein and lipid layers in grana lamellae (thylakoids). The hydrophilic heads of the chlorophyll molecules remain embedded in the protein layer while the lipophylic phytol tails in the lipid layer.

Absorption Spectra of chlorophylls

The chlorophyll pigments chiefly absorb in the violet-blue and red parts of the spectrum. The absorption band shown by the chlorophylls in the violet-blue region is also called as Soret band.

In either solution, chlorophyll a shows a maximum absorption peak at 663 nm in the red region and at 435nm in the violet region-while chlorophyll b has a prominent absorption peak at 645nm and 453nm. The other absorption band for chlorophyll-a a could be 615, 578,533, and 410nm, and of chlorophyll b 595, 549, and 430nm.

Absorption Peaks of Different Chlorophylls

                     Type of                                        In vivo(nm)                                 In vivo(nm)

                Chlorophylls

                Chlorophylls a                          435,670,680 (several forms)                   410,660

                 Chlorophylls b                            480,650                                               452,642

                 Chlorophylls c                            645                                                        445,625

                 Chlorophylls d                              740                                                      450,690

in vivo: Within the living organisms or plants

in vitro: In glass vessels. This is usually applied to biological processes when they are experimentally made to

occur in isolation from the whole organism.

* Carotenoids include carotenes and xanthophylls. The latter is also called carotenols

* Different pigments absorb light of different wavelengths and show characteristic absorption peaks.

Photosynthetic Units

The photosynthetic units are defined as the smallest group of chlorophyll pigment molecules, which are necessary for the photochemical reaction during photosynthesis. They absorb and migrate the quantum of light to the trapping center where they help in the release of electrons.

Emerson and Arnold (1932) found that about 2500 chlorophyll molecules are necessary for fixing up one molecule of CO2 or for releasing one molecule of O2; they labeled this number as Chlorophyll Unit.

Subsequently, the name has been changed to Photosynthetic Unit.

Since the reduction or fixation of one molecule of CO2 requires about 10 quanta of light, it is assumed that 10 flashes of light are required to yield one molecule of O2 or for reduction of one CO2 molecule. Therefore, each unit would now contain only 1/10 of 2500, i.e., 250 chlorophyll molecules. It is the number 200-300 chlorophyll molecules per photosynthetic unit, which is widely accepted and is now considered as Physiological Unit of

Function.

Later, Park and Beggins (1964) found these photosynthetic units as distinct granular structures in the grana region of chloroplast and they named these structures as Quantasomes, which are considered as the morphological expression of the physiological photosynthetic units.

Quantasomes measure 180o A x 160o A and are 100 o A thick. Park and Biggins (1964) revealed that one quantasome contains about 230 chlorophyll molecules. This is very close to the number of chlorophyll (200-300) contained in the physiological photosynthetic units.

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