In this article we will discuss about the functions of reaction centre chlorophylls.
In eukaryotic microorganisms, there are chloroplasts (special intracellular/organelles) that function as photosynthetic apparatus.
The chlorophyll pigments are attached to lamellar (sheet-like) membrane structures of the chloroplast called thylakoids. The stacks of thylakoids are called grana. Contrary to it, the chloroplasts are absent in prokaryotic microorganisms and the photosynthetic pigments remain integrated into internal membrane system.
The latter arise:
(i) From invagination of the plasma membrane (purple bacteria),
(ii) In both the plasma membrane and specialized non-unit membrane-enclosed structures called chlorosomes (green sulphur bacteria), or
(iii) In thylakoid membranes (cyanobacteria).
Chlorophyll or bactenochlorophyll molecules are associated with proteins to form complexes consisting of anywhere from 50 to 300 molecules. Only a very small number of these pigment molecules participate directly in the conversion of light energy to chemical energy (ATP), and are called reaction centre chlorophylls or bacteriochlorophylls (Fig. 25.3).
The latter are surrounded by the more numerous light-harvesting or antenna chlorophylls or bacteriochlorophylls. The antenna pigments capture light and transfer the energy of light to the reaction centre.
Unlike purple bacteria wherein the bacteriochlorophyll molecules are associated with proteins, the chlorosomes of green sulphur bacteria are not associated with proteins and instead function much like a solid state circuit.
They absorb light of low intensities and funnel the energy of light to reaction centre bacteriochlorophyll. Because of having ability to capture very low intensity light with the help of chlorosomes, the green sulphur bacteria can grow at the lowest light intensity of any known phototrophic microorganism.