Dark reaction carbon dioxide assimilation

Fig.2.7 Assimilation of carbon dioxide. The key reaction of the dark reaction is the assimilation of carbon dioxide by ribulose l 5-bisphosphate carboxylase (Simplified from Fig. 26-31 in Biochemistry. L. Stryer, 4th Ed. 1995. W.H. Freeman Co., New York)...

Photosynthetic earboxylation the enzymatic fixation of carbon dioxide in photosynthesis. In C-3 plants, the photosynthetic earboxylation enzyme is ri-bulose iirphosphate carboxylase (EC 4.1.1.39). In C-4 plants it is phosphoeno/pyruvate carboxylase (EC 41.1.31). P. c. is the first step of carbon dioxide assimilation in photosynthesis, and one of the dark reactions. 

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. 

Carbon dioxide is also fixed in the dark by photosynthetic organisms by the so-called Wood-Werkman reaction (Wood and Stjemholm, 1962). The CO2 assimilated, however, rarely exceeds that formed by dark respiration i.e. there is no net CO2 uptake. On the other hand, the amount of organic carbon derived from CO2 may be as high as 30% in heterotrophic bacteria and 90% in mixotrophic organisms. In the natural environment, non-photo-synthetic CO2 fixation by these organisms, together with the above-mentioned dark fixation by photosynthetic organisms, may under some condi-... 

The utilization of carbon dioxide by ATP and NADPH occurs in the chloroplast matrix, (outside the thylakoid lumen). A series of reactions assimilates carbon dioxide (Fig. 2.6), the Calvin cycle or dark reaction, and generates fructose 6-phosphate. Fructose 6-phosphate is the immediate precursor of glucose 6-phosphate for the synthesis of starch in the... 

---