Harvesting Pigment Molecules

The photosynthetic apparatus is found in and on membrane structures, which, in plant cells and algae, are located in chloroplasts and are called thylakoids. In bacteria the photosynthetic membrane is derived by complex invagination of the cytoplasmic membrane. The photosynthetic apparatus is made up of antennae, which contain light-harvesting pigment molecules (usually chlorophylls or bacteriochlorophylls) and photochemical reaction centres, which also contain pigments, together with the necessary enzymes and coenzymes. 

Absorption of Light Energy and the Fate of the Electronically Excited States Light-Harvesting Pigment Molecules Development of Some Modern Concepts of Photosynthesis The Thylakoid Membrane of Chloroplasts... 

The unique water-soluble peridinin- Chi a-protein (PCP) complexes are found in many dynoflagellates in addition to intrinsic membrane complexes. [64] It contains Chi a and the unusual carotenoid peridinin in stoichiometric ratio of 1 4. Unlike other families of antennas, the main light-harvesting pigments are carotenoids, not chlorophylls. The structure of the PCP consists of a protein that folds into four domains, each of which embeds four peridinin molecules and a single Chi a. The protein then forms trimers, suggested to be located in the lumen [64] in contact with both LHCI and LHCII [66], allowing efficient EET to occur. 

What happens if there were only one light-harvesting chlorophyll molecule per reaction center This single pigment molecule would be excited about once per second at a PPF of 200 pmol m-2 s 1. If the chemical reactions required 5 ms as used previously, the excitation could easily be processed by the chemical reactions. However, the photochemical step plus the subsequent enzymatic reactions leading to CO2 fixation would be working at only 0.5% of capacity —(5 x 10-3 s)/(l s), or 0.005, is the fraction of time they could be used. In other words, although all the absorbed photons would be used for photosynthesis, even the slowest of the biochemical steps would be idle more than 99% of the time. 

The two photochemical reactions are performed by two photosystems. Each photosystem consists of a so-called reaction centre, where the primary energy conversion takes place, associated with a few hundred pigment molecules (chlorophylls and carotenoids see Fig. 2) serving as light-harvesting antennas, which transfer the absorbed energy as electronic excitation energy to the reaction centres. 

The minimum structural unit of each ofthe three light-harvesting pigment-protein complexes consists of two BChl molecules and one carotenoid molecule, each noncovalently bound to two low molecular-weight (5-7 kDa) polypeptides designated as a and (3. In the core antenna complex B1015 of Rp. viridis and other BChl / -containing species, however, there is also a y-polypeptide. 

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