PLANT METABOLISM 



^ erythrose-4P 



223 



fructose-6P 



phosphogluco- 

 isomerase 



glucose-6P 



/ 





reactions 



of page 



222 



CO2 



xylulose-5P 



\ 



^phosphopentose 

 epimerase 



transketolase 



ribulose-5P 



dihydroxyacetone 

 phosphate 



aldolase 



^ triosephosphate 

 isomerase 



3-phospho glycer- 

 aldehyde 



transketolase 



sedoheptulose-1, 7-diP 



-^ADP 

 kinase 

 -^ATP 



sedoheptulose-7P 



ribose-5P 



FIGURE 9-6. An outline (1959) of the transformations of the pentoscphosphate path- 

 way. Except for perhaps the decarboxvlation step, these reactions are probably 

 reversible. The kinase reaction is not reversible of itself, but a phosphatase can 

 convert sedoheptulose-1, 7-diphosphate to sedoheptulose-7-phosphate. Since ADP is 

 not changed to ATP, this stage is not reversible in a strict sense. 



(TPNH). Hence the energy yield is probably similar by both meta- 

 bolic systems. Undoubtedly both are used, but their relative im- 

 portances are unknown. Obviously the inetabolism of pentoses must 

 at least start with the pentoscphosphate pathway. 



METABOLISM OF NITROGEN COMPOUNDS 



Much less is known about the metabolism of nitrogen compounds 

 in plants than about that of carbohydrates. It is not yet possible to 

 write general reaction patterns with any degree of certainty. Neverthe- 

 less, it is possible to discuss certain aspects of nitrogen metabolism 

 from a broad point of view. Solely for convenience in this treatment 

 the subject matter is divided into sections on inorganic and organic 

 nitrogen, respectively. 



Inorganic Nitrogen 



The early atmosphere of the earth is believed to have been rich in 

 ammonia. Species then probably depended upon the utilization of 



