222 



PLANT BIOCHEMISTRY 



consideration of the metabolism of carbohydrates by animals, where 

 important quantities of energy are known to be involved. 



When formed from any source, part of the glucose-1-phosphate 

 undergoes the reactions of the glycolytic or Embden-Meyerhof pathway 

 diagrammed on page 168, leading to pyruvate. The pyruvate is then 

 converted to carbon dioxide and water via the tricarboxylic acid cycle 

 shown on page 171. In the higher plants and animals most of the 

 energy obtained during respiration comes from the oxidations of the 

 DPNH and TPNH formed during the degradation of pyruvate. 



Not all the glucose-6-phosphate formed follows the Embden-Meyer- 

 hof-tricarboxylic acid cycle sequence. Most, perhaps all, higher plants, 

 including pea, cucumber, and many seeds, convert part of this hexose- 

 phosphate to carbon dioxide by a different route. On reaching ribu- 



TPN' 



TPNH 



CH^OFOg^ 



Glucose - 6- phosphate 



glucose-6- phosphate 



dehydrogenase 



=0 



H OH 



6-phosphogluconolactone 



gUiconolactonase 



CH2OPO3" 



TPNH 



TPN 



CHaOPOyH' 



6-phosphogluconate 



dehydrogenase 



-CH2OH 



CO., 



COO" 



rlbulose-5- phosphate 6-phosphogluconate 



lose-5-phosphate, the system must involve several of the reactions 

 written for the photosynthetic fixation of carbon dioxide. Although 

 unsettled, the pentosephosphate pathway may be represented tenta- 

 tively as in Figure 9-6. 



As a central compound, the 3-phosphoglyceraldehyde may be metab- 

 olized by way of pyruvate and the tricarboxylic acid cycle. Or 3-phos- 

 phoglyceraldehyde and dihydroxyacetonephosphate may be joined by 

 the adolase to form fructose- 1,6-diphosphate. This last compound is 

 readily converted to fructose-6-phosphate by a phosphatase, and the 

 latter substrate converted to glucose-6-phosphate for another turn 

 around the pentosephosphate pathway. 



This latter arrangement eliminates one molecule of carbon dioxide 

 for each turn around and provides two molecules of reduced coenzyme 



