Plant cells starch synthesis

Down ton and Hawker (1973), showed that starch, starch synthase, and ADPGlc PPase were much higher in bundle-sheath cells than in mesophyll cells on a protein basis or on a chlorophyll basis. The mesophyll cell is able to synthesize starch on exposure of the leaf to continuous light for approximately 2.5 days. Under these conditions, starch synthase levels in the mesophyll cell increased. Thus the mesophyll cell is capable of starch synthesis under certain conditions. Later reports on other C4 plants (e.g., nutsledge leaves, Chen et al., 1974 Digitaria pentzii, Mbaku et aL, 1978) also indicate that both tissues are capable of starch synthesis. 

Libessart, N., Maddelein, M. L., Van den Koomhuise, N., Decq., A., Deirue, B., and Ball, S. 1995. Storage, photosynthesis and growth The conditional nature of mutations affecting starch synthesis and structure in Chalamydomonas reinhardtii. Plant Cell 7, 1117-1127. 

In green algae and in leaf cells of higher plants, ADP-Glc PPase has been demonstrated to reside in the chloroplast (82). More recently, using plastids isolated from maize and barley endosperm (83-85), the existence of two ADP-Glc PPases, a plastidial form, and a major cytosolic form were found. Subsequently, cytosolic forms of ADP-glucose pyrophosphorylase have been found in wheat (86, 87) and rice (88). Because starch synthesis occurs in plastids, it was proposed that in cereal endosperms, synthesis of ADP-Glc in the cytosol requires the involvement of an ADP-Glc carrier in the amyloplast envelope (85). Subsequently, characterization of the ADP-Glc transporter has been reported for maize endosperm (89, 90), barley endosperm (91), and wheat endosperm (92). 

EM, Geigenberger P. Starch synthesis in potato tubers is reg- 92. ulated by post-translational redox modification of ADP-glucose pyrophosphorylase a novel regulatory mechanism linking starch synthesis to the sucrose supply. Plant Cell 2002 14 2191-2213. 93. 

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. 

Thus, the current data show the importance of the plant ADP-Glc PPase in regulating starch synthesis. Moreover, the allosteric effectors, 3-PGA and Pi, are important in vivo effectors in photosynthetic as well as in nonphotosynthetic cells for regulating starch synthesis. 

Plants (and other autotrophs) can use CO2 as the sole source of the carbon atoms required for the biosynthesis of cellulose and starch, lipids and proteins, and the many other organic components of plant cells. By contrast, heterotrophs cannot bring about the net reduction of CO2 to achieve a net synthesis of glucose. 

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