PLANT METABOLISM 215 



reversal of ihc pathways of synthesis. Therefore, a separate discussion 

 of these two processes is (juite artificial. Nevertheless, for purposes 

 of emphasizing the dilferent goals concerned, such a division will be 

 made here. Obviously many cross references will be necessary in 

 avoiding extensive repetition. 



Synthesis 



Plants synthesize carbohydrates from carbon dioxide by fixing this 

 substance and then reducing the product to carbohydrate derivatives. 

 Some of the interconversions of these derivatives (phosphate esters) 

 have been presented as part of the pathway of glycolysis (page 168) 

 and as part of the system for the reduction of carbon dioxide during 

 photosynthesis (page 213). Since the glycolytic reactions between 

 glucose and 3-phosphoglycerate are known to be reversible, formation 

 of the latter as a major intermediate in photosynthesis assures forma- 

 tion of glucose. All the intervening compounds thus become available 

 for conversion to still other carbohydrate derivatives. 



A consideration of the two figures referred to above reveals reactions 

 for the formation of derivatives of the following monosaccharides: 

 glycerose (glyceraldehyde), dihydroxyacetone, erythrose, xylulose, ribu- 

 lose, ribose, fructose, glucose, and sedohepttdose. All these sugars be- 

 long to the D family except, of course, the optically inactive dihydroxy- 

 acetone. They are here listed in order of increasing size, two trioses, 

 one tetrose, three pentoses, two hexoses, and one heptose. Only the last 

 three of these occur in nature to any appreciable extent as the free 

 monosaccharides, sedoheptulose being most abundant in succulent 

 plants, for example, the Sediim group for which this sugar was named. 



Free monosaccharides are formed from the corresponding phos- 

 phates when the ester linkage is split by enzymes called phosphatases. 

 Although the substrate specificity of this group has not been clarified, 

 the enzymes appear to be of great importance. Note the phosphatases 

 at two steps in the scheme of photosynthetic reduction of carbon 

 dioxide. The hydrolyses catalyzed yield inorganic phosphate and 

 energy. They go rapidly and irreversibly so far as is known, as shown 

 by the following: 



CHaOPOaH" CH2OH 



H 



+ H2PO4- 



H /^ V H H A 0. H 



H \ . TT /-. phosphatase 



OH H/l ■*■ "2O ^ 



Y OH HO N K OH 



H OH H OH AF = -3000caL 



D-glucose-6-phosphate D-glucose 



