Photosynthetic pigments
Pigments are the organic molecules that absorb light of specific wavelengths in the visible region due to presence of conjugated double bonds in their structures. The chloroplast pigments are fat soluble and are located in the lipid part of the thylakoid membranes. There is a wide range of chloroplastic pigments which constitute more than 5% of the total dry weight of the chloroplast. They are grouped under two main categories : (i) Chlorophylls and (ii) Carotenoids
The other photosynthetic pigments present in some algae and cyanobacteria are phycobilins.
# (i) Chlorophylls : The chlorophylls, the green pigments in chloroplast are of seven types i.e., chlorophyll a, b, c, d, e, bacteriochlorophyll and bacterioviridin.
Of all, only two types i.e., chlorophyll a and chlorophyll b are widely distributed in green algae and higher plants.
Chlorophyll 'a' is found in all the oxygen evolving photosynthetic plants except photosynthetic bacteria. Reaction centre of photosynthesis is formed of chlorophyll a. It occurs in several spectrally distinct forms which perform distinct roles in photosynthesis (e.g., Chl a680 or P680, Chl a700 or P700, etc.). It directly takes part in photochemical reaction. Hence, it is termed as primary photosynthetic pigment. Other photosynthetic pigments including chlorophyll b, c, d and e ; carotenoids and phycobilins are called accessory pigments because they do not directly take part in photochemical act. They absorb specific wavelengths of light and transfer energy finally to chlorophyll a through electron spin resonance.
- Chlorophyll a is blue black while chlorophyll b is green black. Both are soluble in organic solvents like alcohol, acetone etc. chlorophyll a appears red in reflected light and bright green in transmitted light as compared to chlorophyll b which looks brownish red in reflected light and yellow green in transmitted light. Chlorophyll is a green pigment because it does not absorb green light (but reflect green light) Chlorophyll a possesses — CH3 (methyl group), which is replaced by — CHO (an aldehyde) group in chlorophyll b. Chlorophyll molecule is made up of a squarish tetrapyrrolic ring known as head and a phytol alcohol called tail. The magnesium atom is present in the central position of tetrapyrrolic ring. The four pyrrole rings of porphyrin head is linked together by methine (CH=) groups forming a ring system. Each pyrrole ring is made up of four carbon and one nitrogen. The porphyrin head bears many characteristic side groups at many points. Different side groups are indicative of various types of chlorophylls.
Phytol tail is made up of 20 carbon alcohol attached to carbon 7 position of pyrrole ring IV with a propionic acid ester bond. The basic structure of all chlorophyll comprises of porphyrin system.
When central Mg is replaced by Fe, the chlorophyll becomes a green pigment called 'cytochrome' which is used in photosynthesis (Photophosphorylation) and respiration both.
Chlorophyll synthesis is a reduction process occurring in light. In gymnosperm seedlings, chlorophyll synthesis takes place in darkness in presence of enzyme called 'chlorophyllase'. The precursor of chlorophyll is chlorophyllide.
# (ii) Carotenoids : The carotenoids are unsaturated polyhydrocarbons being composed of eight isoprene (C5H8) units. They are made up of two six-membered rings having a hydrocarbon chain in between. They are sometimes called lipochromes due to their fat soluble nature. They are lipids and found in non-green parts of plants. Light is not necessary for their biosynthesis. Carotenoids absorb light energy and transfer it to Chl. a and thus act as accessory pigments. They protect the chlorophyll molecules from photo-oxidation by picking up nascent oxygen and converting it into harmless molecular stage. Carotenoids can be classified into two groups namely carotenes and xanthophyll.
(a) Carotenes : They are orange red in colour and have general formula C40H56. They are isolated from carrot.
They are found in all groups of plants i.e., from algae to angiosperms. Some of the common carotenes are alpha, beta, gamma and delta carotene; phytotene, lycopene, neurosporene etc. The lycopene is a red pigment found in ripe tomato and red pepper fruits. The alpha-carotene on hydrolysis gives vitamin A, hence the carotenes are also called provitamin A. beta-carotene is black yellow pigment of carrot roots.
(b) Xanthophylls : They are yellow coloured carotenoid also called xanthols or carotenols. They contains oxygen also along with carbon and hydrogen and have general formula C40H56O2.
Lutein a widely distributed xanthophyll which is responsible for yellow colour in autumn foliage. Fucoxanthin is another important xanthophyll present in Phaeophyceae (Brown algae).
# (iii) Phycobilins : These pigments are mainly found in blue-green algae (Cyanobacteria) and red algae. These pigments have open tetrapyrrolic in structure and do not bear magnesium and phytol chain.
# Blue-green algae have more quantity of phycocyanin and red algae have more phycoerythrin. Phycocyanin and phycoerythrin together form phycobilins. These water soluble pigments are thought to be associated with small granules attached with lamellae. Like carotenoids, phycobilins are accessory pigments i.e. they absorb light and transfer it to chlorophyll a.
The other photosynthetic pigments present in some algae and cyanobacteria are phycobilins.
# (i) Chlorophylls : The chlorophylls, the green pigments in chloroplast are of seven types i.e., chlorophyll a, b, c, d, e, bacteriochlorophyll and bacterioviridin.
Of all, only two types i.e., chlorophyll a and chlorophyll b are widely distributed in green algae and higher plants.
Chlorophyll 'a' is found in all the oxygen evolving photosynthetic plants except photosynthetic bacteria. Reaction centre of photosynthesis is formed of chlorophyll a. It occurs in several spectrally distinct forms which perform distinct roles in photosynthesis (e.g., Chl a680 or P680, Chl a700 or P700, etc.). It directly takes part in photochemical reaction. Hence, it is termed as primary photosynthetic pigment. Other photosynthetic pigments including chlorophyll b, c, d and e ; carotenoids and phycobilins are called accessory pigments because they do not directly take part in photochemical act. They absorb specific wavelengths of light and transfer energy finally to chlorophyll a through electron spin resonance.
- Chlorophyll a is blue black while chlorophyll b is green black. Both are soluble in organic solvents like alcohol, acetone etc. chlorophyll a appears red in reflected light and bright green in transmitted light as compared to chlorophyll b which looks brownish red in reflected light and yellow green in transmitted light. Chlorophyll is a green pigment because it does not absorb green light (but reflect green light) Chlorophyll a possesses — CH3 (methyl group), which is replaced by — CHO (an aldehyde) group in chlorophyll b. Chlorophyll molecule is made up of a squarish tetrapyrrolic ring known as head and a phytol alcohol called tail. The magnesium atom is present in the central position of tetrapyrrolic ring. The four pyrrole rings of porphyrin head is linked together by methine (CH=) groups forming a ring system. Each pyrrole ring is made up of four carbon and one nitrogen. The porphyrin head bears many characteristic side groups at many points. Different side groups are indicative of various types of chlorophylls.
Phytol tail is made up of 20 carbon alcohol attached to carbon 7 position of pyrrole ring IV with a propionic acid ester bond. The basic structure of all chlorophyll comprises of porphyrin system.
When central Mg is replaced by Fe, the chlorophyll becomes a green pigment called 'cytochrome' which is used in photosynthesis (Photophosphorylation) and respiration both.
Chlorophyll synthesis is a reduction process occurring in light. In gymnosperm seedlings, chlorophyll synthesis takes place in darkness in presence of enzyme called 'chlorophyllase'. The precursor of chlorophyll is chlorophyllide.
# (ii) Carotenoids : The carotenoids are unsaturated polyhydrocarbons being composed of eight isoprene (C5H8) units. They are made up of two six-membered rings having a hydrocarbon chain in between. They are sometimes called lipochromes due to their fat soluble nature. They are lipids and found in non-green parts of plants. Light is not necessary for their biosynthesis. Carotenoids absorb light energy and transfer it to Chl. a and thus act as accessory pigments. They protect the chlorophyll molecules from photo-oxidation by picking up nascent oxygen and converting it into harmless molecular stage. Carotenoids can be classified into two groups namely carotenes and xanthophyll.
(a) Carotenes : They are orange red in colour and have general formula C40H56. They are isolated from carrot.
They are found in all groups of plants i.e., from algae to angiosperms. Some of the common carotenes are alpha, beta, gamma and delta carotene; phytotene, lycopene, neurosporene etc. The lycopene is a red pigment found in ripe tomato and red pepper fruits. The alpha-carotene on hydrolysis gives vitamin A, hence the carotenes are also called provitamin A. beta-carotene is black yellow pigment of carrot roots.
(b) Xanthophylls : They are yellow coloured carotenoid also called xanthols or carotenols. They contains oxygen also along with carbon and hydrogen and have general formula C40H56O2.
Lutein a widely distributed xanthophyll which is responsible for yellow colour in autumn foliage. Fucoxanthin is another important xanthophyll present in Phaeophyceae (Brown algae).
# (iii) Phycobilins : These pigments are mainly found in blue-green algae (Cyanobacteria) and red algae. These pigments have open tetrapyrrolic in structure and do not bear magnesium and phytol chain.
# Blue-green algae have more quantity of phycocyanin and red algae have more phycoerythrin. Phycocyanin and phycoerythrin together form phycobilins. These water soluble pigments are thought to be associated with small granules attached with lamellae. Like carotenoids, phycobilins are accessory pigments i.e. they absorb light and transfer it to chlorophyll a.