Calvin-Benson pathway

It is obvious now since the pioneering work of Wintermans (1960) that galactolipids in plants having the Calvin-Benson pathway of photosynthesis (Cs plants) or the C4 dicarboxylic pathway of photosynthesis (C4 plants) appear to be concentrated within the chloroplast membranes (Lichtenthaler and Park, 1963 Nichols, 1963 Allen et al., 1966a,b Appelqvist et al.. 

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). 

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) ... 

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. 

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). 

Most plants reduce CO2 to carbohydrate according to the well-known Calvin-Benson or C3 pathway, where the initial product of photosynthesis is the 3C compound phosphoglycerate. Fixation of CO 2 to phosphoglycerate occurs with the assistance of the enzyme ribulose bisphosphate (RuBP) carboxylase, which discriminates heavily against C02 (11). Consequently, plants with C3 photosynthesis have 6 values that average -27.0 (12). Plants with the Hatch-Slack or Ci,... 

All oxygenic (oxygen-evolving) organisms from the simplest prokaryotic cyanobacteria to the most complicated land plants have a common pathway for the reduction of CO2 to sugar phosphates. This pathway is known as the reductive pentose phosphate (RPP), Calvin-Benson or C3 cycle. 

Elucidation of the pathway was chiefly the work of Calvin, Benson, Bassham and co-workers, although there were important contributions by others. In their experiments they used green algae, Chlorella and Scenedesmm, but since that time their results have been confirmed many times in a wide variety of higher plants. 

Some plants employ a photosynthetic pathway creating at first a three-carbon phosphoglyceric acid (C3 or Calvin-Benson photosynthesis). These plants fractionate isotopes more intensely, and so have more negative values (-33%o to —22%o PDB) than plants which use a photosynthetic pathway creating at first a four-carbon malic and aspartic acid (C4 or Hatch-Slack photosynthesis -16%o to -9%o PDB). Crassulacean acid metabolism (CAM) is yet another photosynthetic pathway, which creates organic matter of intermediate isotopic composition (-35%o to -ll%o PDB). Methanogenic microbes are even more extreme in their fractionation of the light isotope (5 C down to -110%o and typically -60%o PDB ... 

Calvin-Benson photosynthetic pathway, a maximum mechanism... 

Calvin-Benson or three-carbon pathway catalyzed by ribulose biphospahate carboxilase oxygenase (Rubisco)... 

The lithoautotrophs have to form cellular materials from carbon dioxide. The process to change carbon dioxide into organic compounds is called fixation of carbon dioxide. On the basis of the knowledge to date, all algae and cyanobacteria, and many of the plants, fix carbon dioxide through the Calvin-Benson cycle (or reductive pentose phosphate cycle) (Bassham et al., 1954), while the plants of 20 families and 1200 species have been known to fix carbon dioxide through the Hatch-Slack pathway (or C4 dicarboxylate pathway) (Hatch et al., 1967). 

As a result, oxaloacetate (OAA, C4-compound) is formed unlike the case of the Calvin-Benson cycle in which 3-phosphoglycerate (C3-compound) is formed. The pathway in the fixation of carbon dioxide by the catalysis of PEP-carboxylase is observed in sugar cane, corn, etc., and is called the Hatch-Slack pathway (Hatch et al., 1967). The plants having the Hatch-Slack pathway have chloroplasts both in mesophyll cells and in vascular bundle sheath cells, and the Hatch-Slack pathway occurs in the mesophyll cells. Oxaloacetate formed by the fixation of carbon dioxide in the mesophyll cells is reduced to malate. Malate thus formed moves to the vascular bundle sheath cells and releases carbon dioxide there. Carbon dioxide released is fixed by the catalysis of Rubisco, and the organic compounds are formed through the Calvin-Benson cycle. (Fig. 6.3). 

The plants producing organic compounds from carbon dioxide through the Calvin-Benson cycle are called C3-plants, while the plants producing organic compounds from carbon dioxide through the Hatch-Slack pathway are called C4-plants. 

Carbon Dioxide-Fixing Pathways Other than the Calvin-Benson Cycle in the Lithoautotrophs... 

As already mentioned, cyanobacteria and most of the chemolithoautotrophic bacteria fix carbon dioxide through the Calvin-Benson cycle, but some litho-autotrophic bacteria fix carbon dioxide through other pathways. When the green phototrophic bacterium Chloroflexus aurantiacus grows lithoautotrophically, the bacterium fixes carbon dioxide through the 3-hydroxypropionate cycle (Ivanovsky... 

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