Photosynthesis dihydroxyacetone phosphate

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 dihydroxyacetone phosphate (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, Harborne, Eds. (Academic Press, 1997), p. 85.]...

Because glyceraldehyde-3-phosphate and dihydroxyacetone phosphate are readily interconverted, these two molecules (referred to the triose phosphates) are both considered to be Calvin cycle products. The synthesis of triose phosphate is sometimes referred to as the C3 pathway. Plants that produce triose phosphates during photosynthesis are called C3 plants. Triose phosphate molecules are used by plant cells in such biosynthetic processes as the formation of polysaccharides, fatty acids, and amino acids. Initially, most triose phosphate is used in the synthesis of starch and sucrose (Figure 13A). The metabolism of each of these molecules is briefly discussed below. 

PGA+NADPH+H +ATP <-> DHAP+NADP +ADP+Pj (1) in which ATP is consumed uid NADPH oxidized, is close to thermodyncunic equilibrium even when photosynthesis is fast therefore possible to calculate F from the ratio of dihydroxyacetone-phosphate (DHAP) to 3-phosphoglycerate (PGA) ... 

Sugar Phosphates. Among the first sugar phosphates identified were fructose-6-phosphate (VI) and glucose-6-phosphate. Soon thereafter dihydroxyacetone phosphate (VII) and fructose-1,6-diphosphate (VIII), both of which occur in rather small concentrations, were identified. The finding of these compounds led Calvin and Benson (1948, 1949) to conclude that the path of carbon dioxide reduction in photosynthesis included a reversal of several steps of the glycolytic pathway which leads from hexose phosphate to PGA. 

Fig. 35. Experiment to ascertain the path of carbon in the secondary processes of photosynthesis. C 02 was supplied to algal suspensions. The algae were then killed at specified times and extracts from them were separated in two-dimensional paper chromatography. PGA = 3-phosphoglyceric acid, triose = 3-phosphoglycerinaldehyde + dihydroxyacetone phosphate. The blackened spots on the copies of the chromatograms shown indicate that is present in the respective compounds (modified after Baron 1967).

Isomerases catalyze the isomerization of one compound into another. There are many important isomerization reactions in the metabolism of carbohydrates. D-Glucose-6-phosphate is converted into D-fructose-6-phosphate by phosphoglu-coisomerase. Dihydroxyacetone phosphate is converted into 3-phospho-D-glyceraldehyde by the enzyme triose phosphate isomerase. In the Calvin cycle of photosynthesis, this same enzyme converts 3-phospho-D-glyceraldehyde into dihydroxyacetone phosphate. 

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