Calvin Benson cycle

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... 

FIGURE 22.23 The Calvin-Benson cycle of reactions. The number associated with the arrow at each step indicates the number of molecules reacting in a turn of the cycle that produces one molecule of glucose. Reactions are numbered as in Table 22.1. 

All the O atoms evolved as Og come from water none comes from carbon dioxide. But 12 O atoms are evolved as 6 Og, and only 6 O atoms appear as 6 HgO in the equation. Also, 6 COg have 12 O atoms, yet there are only 6 O atoms in CgHigOg. How can yon account for these discrepancies Hint Consider the partial reactions of photosynthesis ATP synthesis, NADP reduction, photolysis of water, and the overall reaction for hexose synthesis in the Calvin-Benson cycle.)... 

Purple sulfur bacteria fix carbon dioxide using the Calvin-Benson cycle, but green sulfur bacteria use a completely different pathway, the reverse tricarboxylic acid cycle. Other photosynthetic bacteria use still different pathways for CO2 fixation (Perry and Staley, 1997). 

Fig. 3-5 The Calvin-Benson cycle. This is the series of reactions by which CO2 is fixed into organic material.

The light independent reactions take place in the stroma with the help of ATP and NADPH. In a process called the Calvin-Benson cycle, or carbon fixation, carbon dioxide from the atmosphere is captured and converted into carbohydrates [135]. The reaction is catalyzed by the enzyme RuBisCO (ribulose-1,5-biphosphate... 

A quantitatively much more important pathway of C02 fixation is the reductive pentose phosphate pathway (ribulose bisphosphate cycle or Calvin-Benson cycle Fig. 17-14). This sequence of reactions, which takes place in the chloroplasts of green plants and also in many chemiautotrophic bacteria, is essentially a way of reversing the oxidative pentose phosphate pathway (Fig. 17-8). The latter accomplishes the complete oxidation of glucose or of glucose 1-phosphate by NADP+ (Eq. 17-48) ... 

The overall reaction of carbon dioxide reduction in the Calvin-Benson cycle (Fig. 17-14) becomes... 

Figure 17-14 (A) The reductive carboxylation system used in reductive pentose phosphate pathway (Calvin-Benson cycle). The essential reactions of this system are enclosed within the dashed box. Typical subsequent reactions follow. The phosphatase action completes the phosphorylation-dephosphorylation cycle. (B) The reductive pentose phosphate cycle arranged to show the combining of three C02 molecules to form one molecule of triose phosphate. Abbreviations are RCS, reductive carboxylation system (from above) A, aldolase, Pase, specific phosphatase and TK, transketolase.

The Calvin-Benson cycle and the pentose phosphate pathway (Eq. 17-12) have many features in common but run in opposite directions. Since the synthesis of glucose from C02 requires energy, the energy expenditure for the two processes will obviously differ. Describe the points in each pathway where a Gibbs energy difference is used to drive the reaction in the desired direction. 

Two molecules of NADPH are required to reduce one molecule of C02 via the Calvin-Benson cycle (Fig. 17-14), and three molecules of ATP are also needed. How are these formed The Z scheme provides part of the answer. There is enough drop in potential between the upper end of PSI and the lower end of PSII to permit synthesis of ATP by electron transport. It is likely that only one molecule of ATP is formed for each pair of electrons passing through this chain. Since, according to Fig. 17-14, one and a half molecules of ATP are needed per NADPH, some other mechanism must exist for the synthesis of additional ATP. Furthermore, many other processes in chloroplasts depend upon ATP so that the actual need for photogenerated ATP may be larger than this. 

The key reaction of the Calvin-Benson cycle of C02 reduction is the carboxylation of ribulose bisphos-phate to form two molecules of 3-phosphoglycerate (Eq. 13-48). The properties of ribulose bisphosphate carboxylase (rubisco, Figs. 13-10 to 13-12), which catalyzes this reaction, are discussed in Chapter 13. It... 

The overall effect is to transport C02 from the mesophyll cells into the bundle sheath cells along with two reducing equivalents, which appear as NADPH following the action of the malic enzyme. The C02, the NADPH, and additional NADPH generated in the chloroplasts of the bundle sheath cells are then used in the Calvin-Benson cycle reactions to synthesize 3-phospho-glycerate and other materials. Of the C02 used in the bundle sheath cells, it is estimated that 85% comes via the C4 cycle and only 15% enters by direct diffusion. The advantage to the cell is a higher C02 tension, less competition with 02, and a marked reduction in photorespiration. 

This pathway is sometimes called the Calvin-Benson cycle, after the biochemists who elucidated it. The 5-carbon, doubly phosphorylated carbohydrate, ribulose bisphosphate is the acceptor for CO2 the enzyme is called ribulose-bisphosphate carboxylase/oxygenase (called Rubisco). 

Glycine is then transported to the mitochondrial matrix where the conversion of two glycines to one serine occurs with the loss of CO2 and NH3 from the pool of fixed molecules. The serine is transported into the peroxisome, where it is deaminated to glycerate. The glycer-ate is transported back to the chloroplast, where it is phosphorylated to 3-phosphoglycerate for the Calvin-Benson cycle. 

Calvin cycle (aka Calvin-Benson Cycle or Carbon Fixation) Series of biochemical, enzyme-mediated reactions during which atmospheric carbon dioxide is reduced and incorporated into organic molecules, eventually some of this forms sugars. In eukaryotes, this occurs in the stroma of the chloroplast. 

PGA (phosphoglycerate) A three-carbon molecule formed when carbon dioxide is added to ribulose biphosphate (RuBP) during the dark reaction of photosynthesis (Calvin, or Calvin-Benson Cycle). PGA is converted to PGAL, using ATP and NADPH. 

Felbeck H, Childress JJ, Somero GN. Calvin-Benson cycle and 56. sulfide oxidation enzymes in animals from sulphide-rich habitats. 

Plant biomass species that use the Calvin-Benson cycle are called C3 plants. The cycle is common in many fruits, legumes, grains, and vegetables. C3 plants usually exhibit low rates of photosynthesis at light saturation, low light... 

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