The present chapter has presented an overall view of carbon fixation through Calvin-Bensen cycle, its subsequent conversion to sucrose in source leaves and further transport for the reserve synthesis in sink tissues. Most of the enzymes of these pathways as well as proteins involved in transport and regulation etc. have been purified and their properties studied under in vitro and reconstituted systems. These studies have yielded valuable information regarding the regulatory aspects of the metabolic pathways discussed here. Important advances made in recent years in the area of molecular biology have helped scientists to clone many of these proteins and have made it possible to better analyze the structure, function and expression of different enzymes, carriers and regulatory proteins. Notable progress has been made during the last decade in the understanding of redox regulation mechanism of chloroplast enzymes at molecular level. Furthermore, availability of transgenic plants with under/ over expression of one or the other enzyme of metabolic pathways of carbon fixation and sucrose biosynthesis would help us to determine flux of carbon through these pathways. However, whether or not this knowledge would translate into increased crop yield still remains unresolved. Furthermore, a clear understanding of whole process of yield build up of crop plants still awaits great investigations under field conditions. This becomes more important in view of the fact that the plant in the open field is to interact with a large number of biotic and abiotic factors.

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