Photosynthetic pigments

Photosynthetic pigments
    Photosynthetic pigments
    1. Chlorophylls
    2. Carotenoids and
    3. Phycobillins.
    • Chlorophylls and carotenoids are insoluble in water and can be extracted only with organic solvents such as acetone, petroleum ether and alcohol.
    • Phycobilins are soluble in water
    • Carotenoids include carotenes and xanthophylls. The xanthophylls are also called as carotenols.
       


    Chlorophylls (green pigments)
    • Chlorophylls are magnesium porphyrin compounds.
    • The porphyrin ring consists of four pyrrol rings joined together by CH bridges.
    • Long chain C atoms called as phytol chain is attached to porphyrin ring at pyrrol ring IV.
    • The chemical structure of chlorophyll a and chlorophyll b are well established.
    • The molecular formula for chlorophyll a: C55H72O5N4 Mg and chlorophyll b: C55H70O6N4 Mg. Both of them consist of Mg porphyrin head which is hydrophilic and a phytol tail which is lipophilic.
    • The two chlorophylls differ because in chlorophyll b there is a –CHO group instead of CH3 group at the 3rd C atom in pyrrol ring II.
    • Chlorophyll is formed from protochlorophyll in light.
    • The protochlorophyll lacks 2H atoms one each at 7th and 8th C atoms in pyrrol ring IV.
    • Difference between two chlorophylls is in chlorophyll b there is -CHO (aldehyde) group, instead of a _CH3 (acetyl) group in chlorophyll a molecule.

    Carotenoids (yellow or orange pigments)
    1. Carotenes: Carotenes are hydrocarbons with a molecular formula C40H56
    2. Xanthophylls (carotenols)
    • They are similar to carotenes but differ in having two oxygen atoms in the form of hydroxyl or carboxyl group.
    • The molecular formula is C40H56O2.
    • The role of carotenoids is absorption of light energy and transfer the light energy to chlorophyll a molecules.
    • They also play a very important role in preventing photodynamic damage within the photosynthetic apparatus.
    • Photodynamic damage is caused by O2 molecules which is very reactive and is capable of oxidizing whole range of organic compounds such as chlorophylls and thereby making them unfit for their normal physiological function.

    Phycobilins (red and blue pigments)
    • These also contain four pyrrol rings but lack Mg and the phytol chain.


Last modified: Tuesday, 26 June 2012, 4:38 AM