i. Mainly leaves, though all green parts containing chloroplasts have the potential to carry on photosynthesis.
ii. Chlorophyll (green pigment) is present in chloroplasts i.e., localised.
Photosynthetic Pigments: 3 Types
(1) Chlorophyll:
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Total 9 types; chlorophyll ‘a’ and chlorophyll ‘b’ are important types of chlorophyll.
Chlorophyll:
Molecule consists of a porphyrin ring (hydrophilic) and a phytol tail (lipophilic).
Porphyrin ring:
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Flat, square structure having smaller pyrrole rings with a magnessium atom at the centre. Chiorophyll a (C55H7,O5N4Mg) possess a CH, (methyl) group and chlorophyll b (C55H70O6N4Mg) possess a – CHO (aldehyde) group. The head is joined to a long’ hydrocarbon tail i.e., Phytol.
Chlorophyll a:
Most abundant, only pigment in all photosynthetic plants.
Chlorophyll absorbs light near both ends of the visible spectrum i.e., blue and red light and transmit or reflect green light i.e., why looks green in colour.
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Chlorophyll synthesis in flowering plants requires light and iron.
Algae, bryophytes, pteridophytes and gymnosperms develop chlorophyll even in absence of light.
Raw materials for chlorophyll synthesis are glycine (amino acid) and succinyl co-A.
Fluorescence:
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Isolated chlorophyll a in pure form emits red colour. It is known as fluorescence.
Phosphorescence:
The delayed emission of long wave radiation from an activated molecule is called phosphorescence.
(2) Carotenoids:
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i. Yellow to orange pigments absorb light strongly in the blue-violet range.
ii. Called shield pigments because protect chlorophyll from photo oxidation by light of high intensity and also from oxygen produced during photosynthesis.
iii. Absorb light and transfer’ it to chlorophyll for use in photosynthesis.
iv. Two types: (a) Carotenes: (Orange coloured) – hydrocarbons (tetrapenes) (b) Xanthophylls: (Yellow) – contain oxygen
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(3) Phycobilin:
Water soluble, open tetrapyrrole pigments found in red algae and blue green algae.
Found in phycobilisomes attached to thylakoids and transform light energy to chlorophyll. 3 types i.e., Phycocyanin (blue), Allophycocyanin (blue) and Phycoerythrin (red).
Quantasome:
i. Present as small units on the membranes of thylakoids.
ii. This term coined by Park and Biggins (1964) for a group of pigment molecules required for carrying out a photochemical reaction.
iii. Each quantasome consists of about 250-300 chlorophyll molecules, carotenoids, quinone compounds, sulpholipids, phospholipids, proteins etc.
Action Spectra:
Graph exhibiting the rate of photosynthesis at different wavelengths of light.
Absorption Spectra:
Graph depicting amount of light of different wavelengths absorbed by a pigment.
Close similarity between action spectrum of photosynthesis and absorption spectrum of chlorophyll exhibits that chlorophylls are responsible for absorption of light in photosynthesis.