Pigments involved in Photosynthesis – Class 11 | Chapter – 13 | Biology Short Notes Series PDF
Pigments involved in Photosynthesis: Photosynthetic organisms contain light-absorbing molecules called pigments. These photosynthetic pigments absorbs only specific wavelengths of visible light while reflecting others. The set of wavelengths absorbed by a pigment is its absorption spectrum.
Different photosynthetic organisms have a variety of different pigments, so they can absorb energy from a wide range of wavelengths.
Pigments involved in Photosynthesis
Chlorophyll
The photosynthetic plants have a primary light-absorbing pigment known as chlorophylls.
Chlorophyll is a water-insoluble magnesium porphyrin compound.
It can absorb light at a wavelength below 480 nm and between 550 and 700 nm.
As white sunlight falls on a chlorophyll layer, the green light with a wavelength between 480 and 550 nm is not absorbed but is reflected which is why plant chlorophylls and whole leaves are green.
Chlorophyll always occurs bound to proteins.
It is a green pigment with polycyclic and planar structure.
Chlorophyll is formed from proto-chlorophyll in the light. Addition of two H-atom to protochlorophyll gives chlorophyll.
Chlorophyll-a
It is a bluish green colored pigment with molecular formula C55H72O5N4Mg.
In reflected light chl-a shows blood red color while in transmitted light, it shows blue green light.
It is an universal pigment in all photosynthetic organism except bacteria.
Chlorophyll-b
It is a yellowish green color pigment with molecular formula C55H70O6N4Mg.
It appears dull brown in reflected light and yellowish green color in transmitted light.
Chl-b is absent in green algae, brown algae, red algae, diatoms, etc.
Carotenoids
They are secondary light-absorbing pigments or accessory pigments occurring in the thylakoid membranes.
They may be yellow, red, or purple colored pigments. The yellow colored carotenoids are lutein while red-orange is β-carotene
The carotenoid pigments absorb light at wavelengths which are not absorbed by the chlorophylls so they are supplementary light receptors.
There are 2 types of carotenoids; carotenes and xanthophylls.
Carotene is in orange color while xanthophylls are yellow in color.
These accessory pigments protect the chlorophyll against photo-oxidation and then transfer light energy to chlorophylls.
It also helps in preventing photodynamic damage.
Also, carotenoids are able to express themselves in mature leaves even if chlorophyll begins to disintegrate.
Phycobilins
These are open chain tetra pyrroles which lacks Mg++ and the pytol tail.
Green algae and red algae posses phycobilins such as phycoerythrobilin and phycocyanobilin respectively as their light-harvesting pigments.
Phycobilins are covalently linked to specific binding proteins, forming phycobiliproteins, which associate to form highly ordered complexes called phycobilisomes that constitute the primary light-harvesting structures in the microorganisms like green algae and red algae