Photosynthetic Carbohydrate Synthesis

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The synthesis of carbohydrates in animal cells always employs precursors having at least three carbons, all of which are less oxidized than the carbon in CO2. Plants

752 Chapter 20 Carbohydrate Biosynthesis in Plants and Bacteria

Sucrose (transport)

Starch (storage)

Hexose phosphates

Cellulose (cell wall)

Pentose phosphates

Triose phosphates

Metabolic ■ intermediates

DNA RNA Protein Lipid

ADP, NADP

ATP, NADPH

Light-dependent reactions of photosynthesis

ADP, NADP

ATP, NADPH

DNA RNA Protein Lipid

CO2, H2O

FIGURE 20-1 Assimilation of CO2 into biomass in plants. The light-driven synthesis of ATP and NADPH, described in Chapter 19, provides energy and reducing power for the fixation of CO2 into trioses, from which all the carbon-containing compounds of the plant cell are synthesized. The processes shown with red arrows are the focus of this chapter.

and photosynthetic microorganisms, by contrast, can synthesize carbohydrates from CO2 and water, reducing CO2 at the expense of the energy and reducing power furnished by the ATP and NADPH that are generated by the light-dependent reactions of photosynthesis (Fig. 20-1). 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.

Green plants contain in their chloroplasts unique enzymatic machinery that catalyzes the conversion of CO2 to simple (reduced) organic compounds, a process called CO2 assimilation. This process has also been called CO2 fixation or carbon fixation, but we reserve these terms for the specific reaction in which CO2 is incorporated (fixed) into a three-carbon organic compound, the trióse phosphate 3-phosphoglycerate. This simple product of photosynthesis is the precursor of more complex biomolecules, including sugars, polysaccharides, and the metabolites derived from them, all of which are synthesized by metabolic pathways similar to those of animal tissues. Carbon dioxide is assimilated via a cyclic pathway, its key intermediates constantly regenerated. The pathway was elucidated in the early 1950s by Melvin Calvin, Andrew Benson, and James A. Bassham, and is often called the Calvin cycle or, more descriptively, the photosynthetic carbon reduction cycle.

Melvin Calvin, 1911-1997

Carbohydrate metabolism is more complex in plant cells than in animal cells or in nonphotosynthetic microorganisms. In addition to the universal pathways of glycolysis and gluconeogenesis, plants have the unique reaction sequences for reduction of CO2 to triose phosphates and the associated reductive pentose phosphate pathway—all of which must be coordinately regulated to ensure proper allocation of carbon to energy production and synthesis of starch and sucrose. Key enzymes are regulated, as we shall see, by (1) reduction of disulfide bonds by electrons flowing from photosystem I and (2) changes in pH and Mg2+ concentration that result from illumination. When we look at other aspects of plant carbohydrate metabolism, we also find enzymes that are modulated by (3) conventional allosteric regulation by one or more metabolic intermediates and (4) covalent modification (phosphorylation).

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