Carbon dioxide Calvin cycle

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. 

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). 

Photosynthesis in green plants occurs in two basic processes. In the dark (the Calvin cycle) carbon dioxide is reduced by a strong reducing agent, the reduced form of nicotinamide adeninedinucleotide phosphate, NADPH2, with the help of energy obtained from the conversion of ATP to ADP ... 

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. 

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... 

The dark reaction involves the fixation of carbon dioxide to form carbohydrates. The ATP and the NADPH produced in the light reaction drive this carbon fixation. It might be thought that the interruption of the Calvin cycle would also produce effective herbicides but this is not the case. There are two reasons why. First, the reaction is not an energetic reaction whose interruption would lead to the destruction of cellular components and second, the enzymes involved in the process are present in very high amounts. If an enzyme is to be targeted as a key step in the metabolism of a plant, it is important that it is present in small amounts and that it is not turned over very quickly. If an enzyme is abundant,... 

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

The Calvin cycle is completed by the phosphorylation of D-ribulose 5-phosphate with ATP. The resulting D-ribulose 1,5-diphosphate then is used to start the cycle again by combining with carbon dioxide. There is one sixth more fructose generated per cycle than is used to reform the ribulose 1,5-diphosphate. This fructose is used to build other carbohydrates, notably glucose, starch, and cellulose. 

The resultant NADPH and ATP provide the reducing power and free energy to drive the reduction of carbon dioxide (the dark reactions) via the pentose phosphate pathway (or Calvin cycle l7>) and lead ultimately to the synthesis of glucose according to the overall stoichiometry below. 

The dark reactions (carbon-fixation reactions) use the ATP and NADPH produced by the light reactions to fix carbon dioxide as carbohydrate sucrose and starch. The reactions form a cycle (the Calvin cycle) in which the enzyme ribulose bisphosphate carboxylase (rubisco), located in the stroma, condenses a C02 molecule with ribulose 1,5-bisphosphate to produce two molecules of 3-phosphoglycerate. Other reactions then regenerate the ribulose... 

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. 

RuBP Ribulose biphosphate the 5-carbon chemical that combines with carbon dioxide at the beginning of the Calvin Cycle. 

Photosynthesis proceeds in two parts the light reachons and the dark reactions. The light reactions, which were discussed in Chapter 19. transform light energy into ATP and biosynthetic reducing power, NADPH. The dark reactions, which constitute the Calvin cycle, named after Melvin Calvin, the biochemist who elucidated the pathway, reduce carbon atoms from their fully oxidized state as carbon dioxide to the more reduced state as a hexose. The components of the Calvin cycle and called the dark reactions because, in contrast with the light reactions, these reactions do not directly depend on the presence of light. 

Atmospheric carbon dioxide measurements as Manna Loa, Hawaii. These measurements show annual cycles resulting from seasonal variation in carbon dioxide fixation by the Calvin cycle in terrestrial plants. Much of this fixation takes place in rain forests, which account for approximately 50% of terrestrial fixation. [Dennis Potokar/Photo Researchers.]... 

The Calvin Cycle Synthesizes Hexoses from Carbon Dioxide and Water... 

Although we will focus on the Calvin cycle, other means of fixing carbon dioxide into hexose sugars exist in the photosynthetic world, notably a version of the citric acid cycle running in reverse. 

Figure 20.2. Tracing the Fate of Carbon Dioxide. Radioactivity from C02 is incorporated into 3-phosphoglycerate within 5 s in irradiated cultures of algae. After 60 s, the radioactivity appears in many compounds, the intermediates within the Calvin cycle. [Courtesy of Dr. J. A. Bassham.]...

The dark reaction, known as the Calvin cycle, uses the reducing power of NADPH as well as the free energy stored in the ATP to assimilate carbon dioxide in the form of carbohydrates. The way by which Nature achieves carbon fixation is via the reaction of CO2 with ribulosebiphosphate (RuBP) to give two molecules of 3-phosphoglycerate, a process which is catalyzed by the enzyme RuBP-carboxylase. The phosphogylcerate is converted further to fructose 6-phosphate, the final product of the Calvin cycle. The overall reaction, despite its complex mechanism, corresponds to the simple Eq. (16) above. 

The dark phase is dark because it does not require light. The dark phase is also called the Calvin cycle for the chemist who first described it, Melvin Calvin. In the dark phase, carbon, from carbon dioxide gas, is fixed, that is, converted into a less volatile form, sugar, that can be used by plants and animals as a source of energy. Oxygen gas is produced in this step but given off to the atmosphere as a by-product ... 

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