SEQUENCE OF SUGARS 1685 



be the exposures to lead to reliable discrimination between primary and 

 secondary products of photosynthesis, this earlier chemical evidence can- 

 not be considered as very significant. 



The C(14) work, described in the preceding section, proved that the 

 first products on the reduction level of carbohydrates (L = 1.0) are phos- 

 phate esters. These include mono- and diphosphates of trioses, pentoses, 

 hexoses and heptoses. The heptose and pentose phosphates seem to func- 

 tion as intermediates in the regeneration of PGA, rather than in the forma- 

 tion of the stable products of photosynthesis — sugars and proteins. The 

 path to the latter leads from triose phosphates to hexose phosphates, and 

 from these to sucrose; while free glucose and free fructose appear only 

 later, as hydrolysis products of sucrose, or of their own phosphate esters. 

 This sequence represents an approximate, but not exact, reversal of the 

 common mechanism of glycolysis, which begins with free glucose and pro- 

 ceeds via glucose diphosphate to fructose diphosphate, and thence to triose 

 monophosphates. 



Following are the successive observations of Calvin and coworkers on 

 the chemical mechanism of sugar transformations in photosynthesis. 



On their first paper chromatograms, Calvin and Benson (1949) noted 

 that, of the three nonesterified sugars, sucrose was the first to appear; they 

 suggested that it was formed directly from glucose monophosphate and fruc- 

 tose monophosphates (which — as well as hexose diphosphate — regularly 

 appeared earUer than the free sugars) ; and that free hexoses were formed 

 afterwards by hydrolysis of sucrose. Of the two hexose monophosphates, 

 the fructose-6-phosphate appeared to precede the glucose-1-phosphate; 

 in agreement with this, sucrose at first contained more activity (after 30 

 sec, twice as much) in the fructose moiety than in the glucose moiety. 



Aronoff and Vernon (1950) made similar experiments with soybean leaves, and con- 

 firmed most of the findings of Calvin and Benson ; they found, however, that glucose-1- 

 phosphate appeared before the fructose-6-phosphate and suggested that the reactions 

 leading to these relatively late products of photosynthesis do not need to proceed in 

 exactly the same way in different plants. They also noted that tagged glyceric acid 

 was more abundant than tagged phosphoglyceric acid after several minutes of photo- 

 synthesis, but that PGA was predominant in the first seconds of photosynthesis. 



Bean and Hassid (c/. Hassid 1951) found that leaves (of barley, sugar beet or soy- 

 bean) killed by being dropped into boiUng ethanol, after exposure to CO2, confirmed 

 Calvin's conclusions: the tagging of sucrose precedes that of free hexoses. Extraction 

 methods not leading to equally rapid inactivation of enzymes resulted in tagged hexoses 

 being found before the tagged sucrose. It could be proved, however, that these free 

 hexoses were the product of decomposition of phosphate esters. For example, if the 

 cells were dropped into liquid nitrogen, ground in the cold, and extracted with ethanol at 

 20° C, the radioactivity was found predominantly in nonphosphorylated compounds- 

 including glyceric acid and glucose; but if extraction was done with boiling ethanol, 

 all activity was found in the phosphate esters. 



