Calvin cycle reactions

Balancing the Calvin Cycle Reactions To Account for Net Hexose Synthesis... 

The exploratory studies, as conducted, did not distinguish between effects Imposed on the stromal-associated CO2 fixation (Calvin cycle) reactions or on the light reactions associated with the thylakoids. Consequently, studies were conducted on light-induced electron transport and ATP synthesis associated with isolated spinach thylakold membranes. 

Calvin cycle reactions make sugar from 3-phosphoglycerate. 

The diagram shown in Fig. 13.12 is a simplified outline of the Calvin Cycle reactions which convert C02 to glucose and other sugars. 

The incorporation of COz into carbohydrate by eukaryotic photosynthesizing organisms, a process that occurs within chloroplast stroma, is often referred to as the Calvin cycle. Because the reactions of the Calvin cycle can occur without light if sufficient ATP and NADPH are supplied, they have often been called the dark reactions. The name dark reactions is somewhat misleading, however. The Calvin cycle reactions typically occur only when the plant is illuminated, because ATP and NADPH are produced by the light reactions. Therefore light-independent reactions is a more appropriate term. Because of the types of reactions that occur in the Calvin cycle, it is also referred to as the reductive pentose phosphate cycle (RPP cycle) and the photo synthetic carbon reduction cycle (PCR cycle). 

The Calvin cycle is a series of light-independent reactions in which C02 is incorporated into organic molecules. The Calvin cycle reactions occur in three phases carbon fixation, reduction, and regeneration. Photorespiration is a wasteful process in which photosynthesizing cells evolve C02. 

The Calvin cycle reactions that fix CO2 do not function in the dark. What are the likely reasons for this How are these reactions regulated by light ... 

See also Calvin Cycle Reactions, C4 Cycle, Basic Processes of Photosynthesis, Relationship of Gluconeogenesis to Glycolysis (from Chapter 16), Pentose Phosphate Pathway (from Chapter 14)... 

Unlike most other Calvin cycle reactions, this one has no counterpart in the pentose phosphate pathway. 

See also Calvin Cycle, Calvin Cycle Reactions... 

The set of reactions that transforms 3-phosphoglycerate into hexose is named the Calvin-Benson cycle (often referred to simply as the Calvin cycle) for its discoverers. The reaction series is indeed cyclic because not only must carbohydrate appear as an end product, but the 5-carbon acceptor, RuBP, must be regenerated to provide for continual COg fixation. Balanced equations that schematically represent this situation are... 

Each number in parentheses represents the number of carbon atoms in a compound, and the number preceding the parentheses indicates the stoichiometry of the reaction. Thus, 6(1), or 6 COg, condense with 6(5) or 6 RuBP to give 12 3-phosphoglycerates. These 12(3)s are then rearranged in the Calvin cycle to form one hexose, 1 (6), and regenerate the six 5-carbon (RuBP) acceptors. 

Most of the enzymes mediating the reactions of the Calvin cycle also participate in either glycolysis (Chapter 19) or the pentose phosphate pathway (Chapter 23). The aim of the Calvin scheme is to account for hexose formation from 3-phosphoglycerate. In the course of this metabolic sequence, the NADPH and ATP produced in the light reactions are consumed, as indicated earlier in Equation (22.3). 

Compartmentation of these reactions to prevent photorespiration involves the interaction of two cell types, mescrphyll cells and bundle sheath cells. The meso-phyll cells take up COg at the leaf surface, where Og is abundant, and use it to carboxylate phosphoenolpyruvate to yield OAA in a reaction catalyzed by PEP carboxylase (Figure 22.30). This four-carbon dicarboxylic acid is then either reduced to malate by an NADPH-specific malate dehydrogenase or transaminated to give aspartate in the mesophyll cells. The 4-C COg carrier (malate or aspartate) then is transported to the bundle sheath cells, where it is decarboxylated to yield COg and a 3-C product. The COg is then fixed into organic carbon by the Calvin cycle localized within the bundle sheath cells, and the 3-C product is returned to the mesophyll cells, where it is reconverted to PEP in preparation to accept another COg (Figure 22.30). Plants that use the C-4 pathway are termed C4 plants, in contrast to those plants with the conventional pathway of COg uptake (C3 plants). 

Write a balanced equation for the synthesis of a glucose molecule from ribulose-l,5-bisphosphate and COg that involves the first three reactions of the Calvin cycle and subsequent conversion of the two glyceraldehyde-3-P molecules into glucose. 

Fig. 6.3 The Calvin cycle or the dark reactions of photosynthesis see Cooper and also Stryer in Further Reading.

Much interest has recently been shown in artificial photosynthesis. Photosynthesis is a system for conversion or accumulation of energy. It is also interesting that some reactions occur simultaneously and continuously. Fujishima et al. [338] pointed out that a photocatalytic system resembles the process of photosynthesis in green plants. They described that there are three important parts of the overall process of photosynthesis (1) oxygen generation by the photolysis of water, (2) photophosphorylation, which accumulates energy, and (3) the Calvin cycle, which takes in and reduces carbon dioxide. The two reactions, reduction of C02 and generation of 02 from water, can occur simultaneously and continuously by a sonophotocatalytic reaction. 

Figure 38, Chapter 3. A bifurcation diagram for the model of the Calvin cycle with product and substrate saturation as global parameters. Left panel Upon variation of substrate and product saturation (as global parameter, set equalfor all irreversible reactions), the stable steady state is confined to a limited region in parameter space. All other parameters fixed to specific values (chosen randomly). Right panel Same as left panel, but with all other parameters sampled from their respective intervals. Shown is the percentage r of unstable models, with darker colors corresponding to a higher percentage of unstable models (see colorbar for numeric values).

The pathway is depicted in Fig. 35. The Calvin cycle, taking place in the chloroplast stroma of plants, is a primary source of carbon for all organisms and of central importance for a variety of biotechnological applications. The set of reactions, summarized in Table VIII, is adopted from the earlier models of... 

Figure 35. A model of the photosynthetic Calvin cycle, adapted from the earlier models of Petterson and Ryde Petterson [113] and Poolman et al. [124, 125, 331]. The pathway consists of r 20 reactions and m 18 metabolites. For metabolite abbreviations, see Table VIII.

As one of its characteristic features, the Calvin cycle leads to a net synthesis of its intermediates with significant implications for the stability of the cycle. Obviously, the balance between withdrawal of triosephosphates (TP) for biosynthesis and triosephosphates that are required for the recovery of the cycle is crucial. The overall reaction of the Calvin cycle is... 

The further complicated way of CO2 assimilation to form CH20 flows through series of intermediate compounds and reactions (Calvin cycle). Let us show some results of calculations of total spatial-energy assimilation processes of CO2. When C02 is reduced to the level of its structural formation in CH20, the chemical bonds are reconstructed on all stages of the cycle. Therefore, the additional activation energy from ATP and NADPN is required. 

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