Plants assimilate CO2 from the atmosphere and reduce it to the level of sugar phosphates through a pathway commonly referred to as Calvin-Benson cycle or reductive pentose phosphate pathway. However, a number of plants have evolved adaptations in which CO2 is first fixed by a supplementary pathway and then released for the operation of Calvin cycle. One of the supplementary pathways, the C4 pathway involves special leaf anatomy and a division of biochemical labour between cell types, i.e. CO2 is initially fixed in the cytosol of mesophyll cells by the enzyme phosphoenol pyruvate (PEP) carboxylase to form four carbon compound (malate or aspartate), which is then translocated to the bundle sheath cells and gets decarboxylated in the chloroplasts. The CO2 produced is then refixed by Rubisco. Based on the mechanism of decarboxylation, the plants possessing this pathway have been sub-grouped into NAD-ME type, NADP-ME type and PCK-type. Plants endorsed with this pathway have no or very little photorespiration, greater efficiency and are able to grow under conditions of high light intensity and elevated temperatures. A second supplementry pathway found in species of the Crassulaceae is called Crassulacean acid metabolism (CAM). The plants possessing this pathway fix CO2 in the night into C4 acids. During the day, CO2 released from decarboxylation of C4 acids is converted to sugar phosphates by Calvin cycle. Since Calvin cycle is universal and ultimate pathway for CO2 fixation in all plants, only the reactions of this pathway are discussed here in this chapter. Further, the regulation of CO2 assimilation through this pathway is also described.
Starch and sucrose are the major end-products of photosynthesis. Carbon fixed during photosynthesis is either retained in the chloroplast and converted to starch or translocated to the cytosol in the form of triose phosphates, mainly DHAP and converted to sucrose. Starch plays an important role in plant metabolism as a temporary reserve form of reduced carbon, whereas, sucrose plays a central role in translocation as the transport form of reduced carbon. Sucrose synthesized in the cytosol can either be translocated from the leaf or temporarily stored within the leaf. In the former case, sucrose is released from the mesophyll cell to the leaf apoplast prior to active uptake into the companion cell/sieve element complex by a H+: sucrose symport. Transported sucrose is then either retained in sink tissues or further metabolized to sustain cell maintenance and fuel growth or converted to alternative storage compounds (starch, fat etc). All these aspects related to synthesis and transport of sucrose have been discussed here. Attempt has also been made to enlist the important characteristics of enzymes involved in the pathway of sucrose biosynthesis. Regulatory role of recently discovered metabolite i.e. fructose-2,6-
bisphosphate (fru-2,6-P2) in carbon partitioning between sucrose and starch has also been described.
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