Photosynthesis light stage/reactions

In land plants and green algae (chlorophytes), photosynthesis has two distinct stages, the light reactions, which convert light energy to ATP and NADPH and the dark reactions, which convert CO2 to carbohydrate using ATP and NADPH. Both occur in the chloroplasts (Fig. 13.4). 

Calvin cycle Discovered by chemist Melvin Calvin (1911-97), it is the second major stage in photosynthesis after light reactions whereby carbon molecules from... 

The photosynthetic process in plants can be divided into four stages, each localized to a defined area of the chloro-plast (1) absorption of light, (2) electron transport leading to formation of O2 from H2O, reduction of NADP to NADPH, and generation of a proton-motive force, (3) synthesis of ATP, and (4) conversion of CO2 into carbohydrates, commonly referred to as carbon fixation. All four stages of photosynthesis are tightly coupled and controlled so as to produce the amount of carbohydrate required by the plant. All the reactions in stages 1-3 are catalyzed by proteins in the thylakoid membrane. The enzymes that incorporate CO2 into chemical intermediates and then convert them to starch are soluble constituents of the chloroplast stroma. The enzymes that form sucrose from three-carbon intermediates are in the cytosol. 

These methods have been applied to studying various natural light-induced biomolecular processes such as photosynthesis and vision. With modem pulsed-laser techniques one can use light pulses as short as a few femtoseconds, or less, and this gives information about the very earliest stages of important photochemical reactions. 

At this stage, we must make the following important observation regarding the Z-scheme for photosynthesis (summarizing all the primary biochemical reactions of the metabolism, ie, the light reactions of photosynthesis). If we consider only the stoichiometric photons involved in this scheme, then we can simply define the mean quantum yield using the data on the stoichiometric coefficient v o-x, direcdy Unked by the structured equations (eg, Eq. (121)) to the stoichiometric coefficient Vmadph,h -x and to the value of the P/2e ratio from... 

Figure 25 An artificial antenna-reaction-center complex that mimics the early stages of photosynthesis. The central hexaphenylbenzene core provides structure and rigidity for the surrounding wheel of five bis(phenylethynyl)anthracene antennas that gather light at 430 nm. The energy is transferred to a porph) complex in 1-lOps (green arrows), whra-e it excites an electron that is transferred to the fullerene acceptor in 80 ps the resulting charge-separated state has a lifetime of 15 ns. Complexes such as the one shown provide the first steps in artificial photosynthesis. They have the potential to drive furtha- chanical reactions, such as the oxidation of water to produce H2 or the reduction of CO2 to CH4, alcohol, or other fuels. (Adapted with permission from Ref. 69. American Chemical Society, 2006.)...

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