Photosynthesis first light reaction

Photosystem I (PS I) in the cyanobacterium Synechococcus elongatus is the first system of this type for which the structure has been solved in atomic detail. Although the bacterial photosystem differs slightly from the systems in higher plants, the structure provides valuable hints about the course of the light reactions in photosynthesis (see p. 128). The functioning of the photosystem is discussed in greater detail on p. 130. 

The first series of steps in photosynthesis, known as the light reaction, involve the absorption of photons, the evolution of O2, and the formation of high energy compounds (ATP) and reductants (NADPH). The absorption of photons and the transfer of electrons from H2O to NADPH are carried out by two photosynthetic... 

In photosynthesis there are two types of processes. The first are light reactions, divided into photosystems I and II, which are directly dependent on solar energy the others are dark reactions, which can take place without light. In the dark reactions, the products of the light reactions, ATP and NaDPH-ase, are used for the reduction of CO2 to form carbohydrates. The dark reactions have been mapped in great detail. 

The operation of tandem cells bears a close similarity to the processes that take place in photosynthesis where there are also two photosystems connected in series. In the first, light is absorbed by chlorophyll and this acts as a mediator to oxidize water to oxygen [reaction (4.21)], while in the second, the organic compound nicotinamide adenine dinucleotide phosphate (NADP) is reduced by electrons to a state generally designated NADPH. 

The path of electrons in the light reactions of photosynthesis can be considered to have three parts. The first is the transfer of electrons from water to the reaction-center chlorophyll of photosystem II. 

The impact of light is represented by the lumped yield coefEcient. This represents the first linear part in the so-caUed PI curve, see below. Of course this does not go at infinitum, but ends at a specific maximum rate represented by the second more or less constant part of the PI curve. This maximum value can be determined either by maximum capacity of the light reaction in photosynthesis or by another limiting step, may be capacity of RuBisCo for CO2 fixation or nutrient availability. Also other intracellular botdenecks in metabolism cannot be excluded a priori. 

In the primary processes of photosynthesis ATP and NADPH + H+ are formed (and, in addition, O2 is liberated, a fact which is of no interest in the present context). In which chemical reaction or reactions of the secondary processes are these two substances utilized This question could be approached experimentally in that the reaction concerned must be indirectly light-dependent. Indirect in the sense that light directly excites the chlorophylls in the first and second light reactions which have been discussed. 

What molecular architecture couples the absorption of light energy to rapid electron-transfer events, in turn coupling these e transfers to proton translocations so that ATP synthesis is possible Part of the answer to this question lies in the membrane-associated nature of the photosystems. Membrane proteins have been difficult to study due to their insolubility in the usual aqueous solvents employed in protein biochemistry. A major breakthrough occurred in 1984 when Johann Deisenhofer, Hartmut Michel, and Robert Huber reported the first X-ray crystallographic analysis of a membrane protein. To the great benefit of photosynthesis research, this protein was the reaction center from the photosynthetic purple bacterium Rhodopseudomonas viridis. This research earned these three scientists the 1984 Nobel Prize in chemistry. 

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

Reaction centers of purple bacteria. The exact composition varies, but the properties of reaction centers from several genera of purple bacteria are similar. In Rhodopseudomonas viridis there are three peptide chains designated H, M, and L (for heavy, medium and light) with molecular masses of 33,28, and 24 kDa, respectively. Together with a 38-kDa tetraheme cytochrome (which is absent from isolated reaction centers of other species) they form a 1 1 1 1 complex. This constitutes reaction center P870. The three-dimensional structure of this entire complex has been determined to 0.23-nm resolution288 319 323 (Fig. 23-31). In addition to the 1182 amino acid residues there are four molecules of bacteriochlorophyll (BChl), two of bacteriopheophytin (BPh), a molecule of menaquinone-9, an atom of nonheme iron, and four molecules of heme in the c type cytochrome. In 1984, when the structure was determined by Deisenhofer and Michel, this was the largest and most complex object whose atomic structure had been described. It was also one of the first known structures for a membrane protein. The accomplishment spurred an enormous rush of new photosynthesis research, only a tiny fraction of which can be mentioned here. 

The idea that light drives the formation of oxidants and reductants was first advanced by C. B. van Niel in the 1920s. It was strengthened through experiments done by Robin Hill in 1939. Hill discovered that isolated chloro-plasts evolved 02 if illuminated in the presence of an added electron acceptor such as ferricyanide, Fe(CN)63-. The electron acceptor became reduced in the process. Because no fixation of C02 occurred under these conditions, this experiment demonstrated that the photochemical reactions of photosynthesis can be separated from the reactions that involve C02 fixation. 

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