Carbon photosynthetic reduction

FIGURE 20-4 The three stages of C02 assimilation in photosynthetic organisms. Stoichiometries of three key intermediates (numbers in parentheses) reveal the fate of carbon atoms entering and leaving the cycle. As shown here, three C02 are fixed for the net synthesis of one molecule of glyceraldehyde 3-phosphate. This cycle is the photosynthetic carbon reduction cycle, or the Calvin cycle. 

Bassham, J. A., and Krause, G. H. 1969. Free energy changes and metabolic regulation in steady-state photosynthetic carbon reduction. Biochim. Biophys. Acta 153, 211-218. 

Calvin M. The photosynthetic carbon reduction cycle. J. Chem 40. Soc. 1956 1895-1915. 

Discovered sedoheptulose and ribulose phosphates in the intermediates of the photosynthetic carbon reductive cycle. Discovered and identified... 

Photosynthetic Carbon Reduction Cycle (Calvin Cycle)... 

On the other hand, the evidence ha vs gradually been accumulated indicating that IE like photosynthetic carbon reduction is closely connected with chloroplast functioning (1, 4-6, 14-16), and in 1982 it was shown that isolated poplar chloroplasts are capable of isoprene photobiosynthesis (17). 

Previous work in our laboratory indicated a consistant, differential, pattern of variable chlorophyll fluorescence (Ft) between S and R over the course of a diel, i.e. R is a chronomutant (3). What remains unknown is the relationship of this differential pattern to terminal fluorescence and changes in carbon fixation during the diurnal. Therefore, we hypothesized that the mutation to the psbA plastid gene that confers s-triazine resistance also results in an altered diurnal pattern of photosynthetic carbon reduction relative to that of the susceptible biotype. We present data that supports this hypothesis. 

Photosynthetic carbon reduction can be highly efficient at low F values. Under all conditions investigated, reduced photosynthesis was accompanied by high F values. F increased, when stress conditions limited photosynthesis. 

Now let us turn our attention to the reductive and synthetic reactions of photosynthesis. The basic carbon reduction cycle by which carbon dioxide is reduced to sugar phosphate involves at least twelve intermediate compounds. Some of these substances are found in very small concentrations. Many similar compounds are also present in the photosynthetic cell. In some cases they are closely linked by metabolism to the intermediates in the carbon reduction cycle. In order to understand the mechanism of photosynthetic carbon reduction, one must know the identity of the in-... 

The fundamental basis of photosynthetic carbon metabolism is the incorporation of carbon dioxide by ribulose-bisphosphate carboxylase (rubisco). This leads to the synthesis of three-carbon sugars which are either exported from the chloroplast or metabolized to regenerate the acceptor ribulose bisphosphate. Rubisco is a bifunctional enzyme in that, in parallel to carboxylation, it catalyzes an oxygenation reaction that leads to phospho-glycolate. This is the starting point for photorespiratory metabolism, which will be discussed below (Section 1.6.2). In C4 plants, the conventional C3 pattern of the photosynthetic carbon reduction Calvin cycle is confined to the bundle sheath cells. The surrounding mesophyll cells act as an ancillary carbon dioxide pump, fixing carbon dioxide via phosphoenolpyruvate carboxylase into C4 acids. These are transported to the bundle sheath for decarboxylation.In this way, photorespiration is limited because of the elevated carbon dioxide levels. 

Inhibitors of the Photosynthetic Carbon Reduction (Calvin) Cycle... 

Quantitatively, the most important enzyme of the photosynthetic carbon reduction cycle is rubisco, which represents around 50% of soluble leaf protein. This has a major disadvantage in that inhibition of a large amount of leaf protein could require logistically difficult and probably prohibitively expensive herbicide levels. 

He contributed to discoveries of the photosynthetic carbon reduction cycle. In addition, the opal glass method for the spectral characterization of chloroplasts was firmly established during this stay at Berkeley. Invited by Stacy French, he spent 1955-56 at the Carnegie Foundation Research Laboratories, Stanford, California studying the greening of etiolated... 

The other two enzymes of this part of the photosynthetic carbon reduction cycle respond differently to illumination. Ribulose-phosphate kinase increases in activity by about 66%, but only after about 12 hours of illumination. Ribose-phosphate isomerase increases only by 50% after about 18 hours. These changes in activity do not occur in chloramphenicol-treated leaves. 

To summarize these observations First, the enzymes of this segment of the photosynthetic carbon reduction cycle are not controlled co-ordinately a single inductive step does not appear to affect the activity (and presumably the production) of these three enzymes in the same way. Second, the kinds of control mechanisms which appear to occur here are (1) a direct and prompt effect of illumination, reflected in the rapid increase in ribulose-diphosphate carboxylase activity, and (2) a more indirect kind of control, possibly involving induction of the other enzymes (e.g., ribose-phosphate isomerase) by small molecules produced in photosynthesis. Unfortunately, it has not been possible to demonstrate an increase in the level of either the isomerase or kinase by administration of glucose and some other carbohydrates to etiolated leaves in darkness. 

Subsequent research has shown that the pentose phosphate so produced is further metabolized in reactions utilizing the enzymes transaldolase and transketolase and involving as intermediates pentose, heptulose, tetrose and triose (5, 7, 4 and 3 carbon sugars) phosphates which are also involved in the photosynthetic carbon reduction cycle. A version of the pentose phosphate pathway is shown in Fig. 4.7. 

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