Pigments are the substances which absorb certain wavelength of visible light. Different colours of pigments are due to the visible spectrum which they reflect or transmit. Chlorophylls and carotenoids are the two major groups of pigments found in higher plants.
Chlorophylls, the green pigments of plants, are the most important pigments active in the photosynthetic process. Green substance in producers that traps light energy from the sun, which is then used to combine carbon dioxide and water into sugars in the process of photosynthesis Chlorophyll is vital for photosynthesis, which helps plants get energy from light.
Chlorophyll a and chl. b are the two main pigments which occur in higher plants. They occur in the approximate ratio of 2:1.
The total weight of chlorophyll in green leaves varies from 0.20% to 0.55% of tie fresh weight. The empirical formulae for chl. A is CS5H72OSN4Mg and it is usually blue-green in colour and that of chl. b is CS5H70O6N4Mg which is yellow-green.
A chlorophyll molecule has a cyclic tetrapyrrolic structure (porphyrin) with an isocyclic ring, and contains a magnesium atom in the centre. A phytol chain is attached to one of the pyrrole rings.The phytol chain, which is esterified with the carboxyl group on the seventh carbon of porphyrin, is a long hydrophobic chain that contains one double bond. It extends into the interior of the chloroplast's membranes and interacts with other hydrophobic lipid molecules.
Chlorophyll performs metabolic functions in plants such as respiration and growth. Amusingly, the chlorophyll molecule is chemically similar to the blood of human, except that its central atom is magnesium, whereas that of human blood is iron. Chlorophyll is a chlorine pigment. Its structure is very similar to heme, but at the center of the chlorine ring is a magnesium ion Mg. it may be noted that hemes are most commonly recognized in their presence as components of hemoglobin. As we know, hemoglobin does very effectively deliver oxygen to human tissues, it’s in every mammal’s blood - that's the reason we talk about a great plan in Nature.
Chlorophyll a has a methyl group (-CH3) attached to the third carbon, which is replaced by an aldehyde group (-CHO)in chl. b. Both chl. a and chl. b show absorption maxima in the blue-violet and orange-red regions of the visible spectrum. The structure of chlorophyll is related to its function.
The long hydrophobic phytol tail is lipid soluble and it is anchored in the thylakoid membrane. The head is hydrophilic (water loving) and generally lies in the surface of the membrane next to the aqueous solution of the stroma and assists in light absorption. Modifications in side groups on the head bring changes in the absorption spectrum so that different energies of light are absorbed. Absorption of light energy by the head causes changes in the energy levels of electrons within the head.