Photosynthetic Pigments

The thylakoids of the chloroplast contain the pigments which absorb light of different wavelengths and carry out the photochemical reaction of photosynthesis. The role of the pigments is to absorb light energy, thereby converting it to chemical energy. These pigments are located on the thylakoid membranes and the chloroplasts are usually so arranged within the cells that the membranes are at right angles to the light source for maximum absorption.

The photosynthetic pigments of higher plants fall into two classes: chlorophyll and carotenoids.

The photosynthetic pigment chlorophyll is the principle pigment involved in photosynthesis. It is a large molecule and absorbs light maximally in the violet blue and in the red region of the visible spectrum and reflects green light and thus leaves appear green in colour.

Carotenoids (carotene and xanthophyll) absorb light in the regions of the spectrum not absorbed by the chlorophylls and transfer that energy to chlorophyll to be used in photosynthesis.

Chlorophyll-a (a special type of chlorophyll) is the main pigment that traps solar energy and converts it into chemical energy. Chlorophyll-a is present in all autotrophic plants except photosynthetic bacteria. Thus Chl-a is called the essential photosynthetic pigment responsible for representing the reaction centre.

All other pigments such as chlorophyll b and carotenoids are collectively called accessory pigments since they pass on the absorbed light energy to chlorophyll a (Chl-a) molecule to be utilized for photosynthesis. These pigments, that is the reaction centres (Chl-a) and the accessory pigments (harvesting centre) are packed into functional clusters called photosystems. Photosystems are of two types PSI and PSII.

About 250-400 Chl-a molecules constitute a single photosystem. Two different photosystems contain different forms of chlorophyll a in their reaction centres. In photosystem I (PSI), chlorophyll-a with maximum absorption at 700 nm (P700) and in photosystem II (PSII), chlorophyll-a with peak absorption at 680 nm (P680), act as reaction centres. (P stands for pigment).

The primary function of the two photosystems, which interact with each other is to trap the solar energy and convert it into the chemical energy also called assimilatory power (ATP and NADPH₂).