Photosynthesis second light reaction

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

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. 

Once the light reactions have produced ATP and NADPH, the second series of photosynthesis reactions can occur. As shown in Figure 20.17, the Calvin cycle, which takes place in the stroma of chloroplasts, makes use of NADPH from the light reactions, as well as carbon dioxide taken in from the surroimdings. Simple sugars, such as glucose, result. 

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. 

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 oxidation state of the ocean interior is a consequence of a second energy source light, which drives photosynthesis. Photosynthesis is a redox reaction of the general form ... 

Molecules cannot move fast enough to keep the reaction as reported at equilibrium. For example, a recent paper on ammonia photosynthesis over oxide promoters by Augugliaro et al. [67] reports that ammonia was formed at 0.04 imol h-) in their system, and so this sets the minimal reverse rate at equilibrium. Nitrogen flowed into their system at 0.4 cm3 s-1 and so we can ask ourselves how fast ammonia molecules would have to move toward the surface of the catalyst in order to maintain the reverse rate. It transpires that the ammonia molecules would have to travel fast enough to cross the known universe (i.e., 2 x 1010 light years) about 1021 times in one second. This absurd result is intended only as another way of demonstrating that reaction 2, if it occurred, could not come to equilibrium. 

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. 

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