3 Major classes of photosynthetic pigments in plants
The primary site of photosynthesis is leaf. We have already discussed the exact site of lightdependent and light independent reaction of photosynthesis within the chloroplast in our last post
• An anabolic process by which chloroplast of green plants and other phototrophs synthesize carbohydrates (glucose) and evolve molecular O2 as by-product, using CO2, H2O and sunlight.
• It involves conversion of light energy into chemical energy
• Approx. 0.2 % of light falling on earth is used for photosynthesis
3 Major Classes of Photosynthetic pigments in Plants
- Photosynthetic pigments are located in the thylakoid membranes of the chloroplast. These pigments are capable of converting the energy of sunlight to chemical energy. The pigments absorbs light rays from the visible region of the electromagnetic spectrum
- The two major groups of pigments are a) Principal Pigments b) Accessory pigments
- The principal pigment in all photosynthetic plants is chlorophyll-a.
- Accessory pigments include chlorophyll-b, c, d, carotenoids and phycobilins.
- In green plants, chlorophyll-a is the primary pigment and chlorophyll-b and carotenoids are the accessory pigments.
- Chlorophylls are magnesium porphyrin derivatives.
- Chl-a is the primary pigment in photosynthesis and it forms the reaction centre of the photosystems (light harvesting complex, LHC).
- Chl-b, c, d, e and bacteriochlorophyll are the other type of chlorophyll molecules.
- Chl absorbs maximum at red and blue region and reflects green light.
- A chlorophyll molecule consists of a hydrophobic pyrole head and hydrophilic phytol tail with a central Mg atom. The head is formed of 4 pyrole molecules linked together by methane (CH3) group to from a ring called porphyrin ring. The side group of chl-a is methyl group (-CH3) and chl-b is an aldehyde group (-CHO).
- Chl-a is bluish green and chl b is yellow green.
- Carotenoids are fat soluble, accessory pigments with yellow, orange or red colour found in all photosynthetic plants. These pigments absorb blue-violet region of the visible spectrum.
- Carotenoids are of two groups, carotenes which are red or orange coloured (C40H56) and xanthophylls which are brown or yellow coloured (C40H56O2). The carotenes include α, β, carotene, lycopene, phytoene etc.
- Xanthophylls include lutein, violaxanthin, zeaxanthin etc.
- The functions of carotenoids are
- Absorb more solar energy and transfer it to reaction centre and protects chlorophyll molecules from photo-oxidation
- Water-soluble, red or blue accessory pigments present on cyanobacteria and red algae. The structure is similar to chlorophyll molecule but the central Mg is absent.
- The two types of phycobilins are phycoerythrins which are red coloured and phycocyanins, which are blue coloured.
- Thus the question how the light energy is converted to chemical energy in photosynthesis? The answer is these pigments can trap the energy of sunlight and the excited electron while moving through different electron acceptors releases some energy which is utilized for the phosphorylation of ADP to ATP.