20 : 4/ Photosynthesis 



369 



in that it is exothermic, liberating energy and apparently going even in 

 the absence of any specific enzyme. Reduced pyridine nucleotide is 

 used to convert 3-phospho-glyceric acid to 3-phosphoglyceraldehyde. 



Carboxydismutase 

 System 





Epimerase 



H 2 CO® 



Xylulose - 



5-P 



Sucrose 

 and Starch 



Figure 4. Some of the steps in photosynthesis. (P) stands for 

 phosphate in the diagram and P for phosphate in the names. 

 Note that all compounds with five or more carbons are sugars 

 and probably exist primarily in a ring isomer; straight-chain 

 formulas are for convenience only. After J. A. Bassham and 

 M. Calvin, The Path of Carbon in Photosynthesis (Englewood 

 Cliffs, N.J.: Prentice-Hall, Inc., 1957). 



ATP is used at this step and in the formation of the ribulose-l,5-diphos- 

 phate. 



Although the scheme shown in Figures 4 and 5 may seem extremely 

 complex, all evidence indicates that it is an oversimplification. None- 

 theless, the general cyclic character and the need for reduced pyridine 

 nucleotide (PNH) and ATP seem established beyond question. The 



