Photosynthesis, Calvin cycle

Ribulose (— a-D-Ribulose) (pentose monosaccharide) Universal photosynthetic Calvin Cycle intermediate (phosphorylated) Melvin Calvin (USA, Nobel Prize, 1961, Chemistry, photosynthesis Calvin cycle) Sweet... 

TPP involved in reactions catalysed by pyruvate decarboxylase (alcoholic fermentation), pyruvate dehydrogenase a-ketoglutarate dehydrogenase (TCA cycle), transketolase (photosynthesis Calvin cycle) acetolactate synthetase (Val, Leu biosynthesis)... 

Chloroplasts Light capturing processes and electron transport oxidative phosphorylation for photosynthesis Calvin cycle (dark reactions of photosynthesis). 

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 ... 

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. 

The conversions conducted in both steps are currently based on empirical relationships that are more or less robust. For example, the relationship between the chlorophyll and carbon content in an average phytoplankton cell is dependent on factors that influence cell metabolism, including nutrient arailability, temperature, and light. The temperature dependence of photosynthesis is associated with an enzyme-mediated step in the Calvin cycle (Figure 7.6a). 

On p. 407, the initial step of the dark reactions in plant photosynthesis (in the Calvin cycle) is shown at the top left. 

The dark reaction of photosynthesis (Figure 2.3) is so-called as it does not require light to proceed. It does, however, require the products of the light reaction to operate and it will not, therefore, take place in the absence of light. It was discovered by Calvin and is often known as the Calvin cycle. 

Figure 2.3 The dark reaction (the Calvin cycle) of photosynthesis...

NADPH and ATP are produced in the light-dependent reactions of photosynthesis in about the same ratio (2 3) as they are consumed in the Calvin cycle. Nine ATP molecules are converted to ADP and phosphate in the generation of a molecule of triose phosphate eight of the phosphates are released as Pj and combined with eight ADP to regenerate ATP. The ninth phosphate is incorporated into the triose phosphate itself. To convert the ninth ADP to ATP, a molecule of Pj must be imported from the cytosol, as we shall see. 

Photosynthesis in vascular plants takes place in chloroplasts. In the C02-assimilating reactions (the Calvin cycle), ATP and NADPH are used to reduce C02 to triose phosphates. These reactions occur in three stages the fixation reaction itself, catalyzed by rubisco reduction of the resulting 3-phosphoglycerate to glyceraldehyde 3-phosphate and regeneration of ribulose 1,5-bisphosphate from triose phosphates. 

The photosynthetic process in green plants occurs in subcellular organelles called chloroplasts. These organelles resemble mitochondria they have two outer membranes and a folded inner membrane called the thy-lakoid. The apparatus for photosynthesis, including the chlorophyll reaction centers and electron carriers, is in the thylakoid membrane. The chemical reactions of the Calvin cycle take place in the stroma, the region around the thylakoid membrane. 

In contrast to the previous results, Weigel (1985 a, b) reported that in mesophyll protoplasts of Valerianella locusta and in intact chloroplasts of Spinacea oleraceae cadmium affects photosynthesis by inhibition of several reaction steps of the Calvin cycle and not by interaction with the electron transport or photophosphorylation (cf. section on photosynthetic C02 fixation). 

The primary inhibitory effect of cadmium on photosynthesis of excised leaves was proposed to be metal-induced stomatal closure (Bazzaz et al., 1974 Lamoreaux and Chaney, 1978). In epidermal peels floating on a metal-containing solution, stomatal closure was reported (Bazzaz et al., 1974). Inhibition of transpiration by several metals, especially by cadmium, is well documented (Poschenrieder et al., 1989), but is not necessarily at the stomatal level. Metal-induced stomatal closure can indirectly be responsible for a decrease of photosynthetic C02 fixation indeed. However, several enzymes of the Calvin cycle are directly affected by metals. 

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. 

The discussion of the light reactions of photosynthesis in Chapter 19 leads naturally into a discussion of the dark reactions—that is, the components of the Calvin cycle—in Chapter 20. This pathway is naturally linked to the pentose phosphate pathway, also covered in Chapter 20, because in both pathways common enzymes interconvert three-, four-, five-, six-, and seven-carbon sugars. 

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. 

Figure 20.12. Calvin Cycle. The diagram shows the reactions necessary with the correct stoichiometry to convert three molecules of CO2 into one molecule of DHAP. The cycle is not as simple as presented in Figure 20,1 rather, it entails many reactions that lead ultimately to the synthesis of glucose and the regeneration of ribulose 1,5-bisphosphate. [After J. R. Bowyer and R. C. Leegood. "Photosynthesis," in Plant Biochemistry, P. M. Dey and J. B. Harbome, Eds. (Academic Press, 1997), p. 85.]...

Figure 20.14. Light Regulation of the Calvin Cycle. The light reactions of photosynthesis transfer electrons out of the... 

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