The Biosynthesis of Carbohydrates by Plants

The early workers in plant chemistry recognized that, as the result of photosynthesis, starch, sucrose, fructose, and glucose were formed. It was known that plants also synthesized fats, amino acids, proteins, organic 

New information on this problem has come from enzyme studies and isotope tracer studies. The work discussed in the preceding section disclosed that a great many different products were synthesized from carbon dioxide during photosynthesis. The nearest approach to a first product of photosynthesis is n-glyceric acid 3-phosphate. From this compound, through a whole series of enzyme-catalyzed reactions, the plant carries out the synthesis of the carbohydrates. 

As a consequence of the widespread occurrence of the enzyme aldolase in plants, Tewfik and Stumpf 64) concluded that this enzyme played a major role in the sugar transformations of plants. Aldolase from mammalian tissue had been shown 66) to catalyze the following reaction  

It was shown that the enzyme, although specific for dihydroxyacetone phosphate, would react with a large number of different aldehydes. Thus, dihydroxyacetone phosphate reacted with D-glyceraldehyde, L-glyceralde-hyde, and acetaldehyde to form D-fructose 1-phosphate, L-sorbose 1-phos-phate, and a 5-deoxypentose 1-phosphate, respectively. 

Additional evidence for the aldolase reaction being involved in monosaccharide biosynthesis came from the isotope studies of photosynthesis. During short-term photosynthesis, the radioactivity from carbon dioxide appeared in a number of compounds in the following order D-glyceric acid 3-phosphate, triose phosphate, hexose phosphates, and sucrose 70), The distribution of the tracer carbon in these compounds was compatible with 

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