PLANT METABOLISM 397 



they have unraveled many of the complexities of the early photosynthetic 

 reactions and have revealed the importance of phosphoglyceric acid as 

 an intermediate. 



Figure 15-6 gives reactions postulated by Calvin and co-workers as 

 occurring between the initial fixation of carbon dioxide and its final 

 reduction to sugar. ^ The initial reaction is the carboxylation of the 

 hypothetical vinyl phosphate (1) to 2-phosphogly eerie aeid (2), which 

 is then converted to 3-phosphoglyeerie acid (3). In extremely short 

 periods (5 seconds) of photosynthesis by algal cells in the presence of 

 C^'^Oo, it has been shown that up to 85 per cent of the C^"' fixed appears 

 in 2- and 3-phosphoglyccric acids. By reversing the glycolytic scheme 

 (see Figs. 13-1 and 13-3), 3-phosphoglyceric acid can be converted to 

 sugar. It first is reduced to 3-phosphoglyceraldehyde (4) , which is in 

 equilibrium with diliydroxyacetone phosphate (5). Condensation of 

 these compounds yields fructose-l,6-diphosphate (6) , which in turn yields 

 fructose-6-phosphate (7) and glucose-6-phosphate (8). Free fructose 

 and glucose can be obtained from these by the removal of phosphate. 

 The method by which plants form sucrose is unknown, and the sucrose 

 phosphate shown is a hypothetical intermediate. However, all the gly- 

 colytic steps included in Fig. 15-6 have been demonstrated to occur in 

 plants (preparations from pea seedlings). 



The left part of the scheme represents a suggested method for regen- 

 erating the Co fragment, which must serve as the acceptor of carbon 

 dioxide when phosphoglyceric acid is formed; this remains the least well- 

 defined and most controversial portion of the scheme. In the first formu- 

 lation shown, one molecule of 2-phosphoglyceric acid is used in sugar 

 formation, while water is removed from the other molecule to yield 

 2-phosphoenolpyruvic acid (9). Addition of another molecule of carbon 

 dioxide yields enol-oxalacetic acid (10). This is converted to an unde- 

 fined C4 acid, w'hicli splits to give 2 molecules of glyoxylic acid (11) ; 

 upon reduction this would yield glycolic acid (12). Several Co com- 

 pounds have been suggested between glycolic acid and the hypothetical 

 vinyl phosphate (1). 



An alternative method, postulated by Calvin and co-workers, for form- 

 ing a triose to serve in the reversed glycolytic mechanism and for regen- 

 erating the C2 fragments is shown in skeleton form in the lower portion 

 of Fig. 15-6. A C4 and a C3 compound are condensed to form the C7 

 sugar, sedoheptulose. This compound is split to yield a Co fragment 

 which recycles and a C5 keto-sugar, ribidose. The ribulose in turn is 

 split to a triose and a Co compound, which also recycles. The complete 

 cycle, involving 3 carboxylations and subsequent reductions, yields a 

 molecule of triose. Most of the specific intermediates in this reaction 

 sequence remain to be defined. 



^ In Fig. 15-6 "P" indicates either a phosphate grouj) or a molecule of phosphoric acid. 



