Photosynthetic units

Photosynthetic Units Consist of Many Chlorophyll Molecules hut Only a Single Reaction Center... 

Figure 12.7 The photosynthetic unit, in which an antenna chlorophyll molecule is excited by photon absorption and the energy is transferred to the chlorophyll dimer at the reaction centre...

In this section the recent literature on EPR applications of the bRC and the related light-harvesting systems (LH1 and LH2) are covered which form the photosynthetic unit in photosynthetic bacteria.53... 

ATP synthase, whose knobs also protrude into the stroma. A photosynthetic unit can also be defined chemically by the number of various types of molecules present in a chloroplast membrane for each four manganese atoms (Table 23-2). Separate units contain PSI and PSII. These reaction centers appear to have a different distribution within the thylakoids, the PSI units being located principally in the unstacked membranes and the PSII units in the grana stacks.255 259... 

Approximate Composition of Photosynthetic Units in a Spinach Chloroplast3... 

A better understanding of the photosynthetic units of the in vivo system are essential for both the in vitro and in vivo approaches but if one is looking for practical applications the emphasis of the work changes from one approach to the other. If one is to modify photosynthesis in vivo one is almost certainly going to use the methods of genetic engineering so that immediately one needs to know the specific protein complexes involved, their sequences and those of their genomes. 

Crystallization in a form suitable for X-ray structure analysis of the protein complex containing this part of the photosynthetic unit has resisted all efforts until Deisenhofer et al. solved such a structure for a complex isolated from rhodopseudomonas viridis (Deisenhofer et al., 1984). In this complex the special pair ((BC)2) and the pheophytin (BP) were found to be spatially separated by a centre-to-centre distance of no less than 17 A, while the centre-to-centre distance between the pheophytin and the menaquinone (MQ)... 

In conclusion the molecules l(n) consitute a series of model systems that not only provides donor-acceptor separations similar to those found in the photosynthetic unit, but also mimics the high rate and the temperature independence of the primary steps in photosynthesis. 

Wild, A., and Ball, R. 1997. Photosynthetic Unit and Photosynthesis. Backhuys, Leiden, The Netherlands. 

The vast majority of the pigments in a photosynthetic organism are not chemically active, but function primarily as an antenna. They absorb photons and transfer energy to a highly organized integral membrane protein called a reaction center. The antenna system, its associated reaction center and ancillary electron transfer carriers are collectively known as a photosynthetic unit. 

Nonetheless, photosynthesis did not evolve immediately at the origin of life. The failure to discover photosynthesis in the domain of Archaea implies that photosynthesis evolved exclusively in the domain of Bacteria. Eukaryotes appropriated through endosymbiosis the basic photosynthetic units that were the products of bacterial evolution. All domains of life do have electron-transport chains in common, however. As we have seen, components such as the ubiquinone-cytochrome c oxidoreductase and cytochrome hf family are present in both respiratory and photosynthetic electron-transport chains. These components were the foundations on which light-energy-capturing systems evolved. 

V. Sundstrom, T. Pullerits, R. van Grondelle, Photosynthetic light-harvesting reconciling dynamics and structure of purple bacterial LH2 reveals function of photosynthetic unit. J. Phys. Chem. B 103, 2327-2346 (1999)... 

C. Hofmann, F. Prancia, G. Venturoli, D. Oesterhelt, J. Kdhler, Energy transfer in a single self-aggregated photosynthetic unit. FEBS Lett. 546, 345-348 (2003a)... 

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