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N. E. Tolbert 



several of the first ones in the sequal are absent in etiolate 

 plants. Glycolate synthesis is associated with the chloroplasts 

 and C-'^'* is incorporated into its products very rapidly during 

 photosynthesis. The glycolate is rapidly converted to sucrose in 

 the 1 i gh t . 



Function of Glycolate Pathway 



Amino acid synthesis : The glycolate pathway does not exist 

 solely for production of glycine and serine for protein synthesis. 

 During €■'■^02 photos jnithes is glycine and serine become labeled 

 much more rapidly than other amino acids, and in turn they lose 

 C^"* equally rapidly if the C^^^Oa is exchanged for C^^Oa- 



CO? fixation : In previous sections reasons were given why the 

 glycolate is not thought to be an alternate pathway for CO2 fixa- 

 tion. Most pertinent was phosphoglycolate production by chloro- 

 plasts in the dark from sugar phosphates of the photosynthetic 

 carbon cycle. 



Anion exchange : We advanced the concept in 1956 that glycol- 

 ate excretion occurred for the purpose of anion exchange with 

 bicarbonate (26). Since that time the requirement for low CO2 

 partial pressure has been well documented for glycolate synthesis 

 and excretion. This requirement is consistent with the bicar- 

 bonate exchange theory. At high GO2 concentration sufficient 

 diffusion of bicarbonate and CO2 could occur for photosynthesis, 

 and the need would not exist for an exchange mechanism to move 

 bicarbonate rapidly into the chloroplasts. I do not feel that 

 this hypothesis has been either refuted or confirmed to date. 



Metabolic link between chloroplasts and cytoplasm : Since gly- 

 colate is rapidly metabolized to sucrose, and since it appears 

 outside of chloroplasts, the glycolate pathway may constitute a 

 link between the photosynthetic carbon cycle of the chloroplast 

 and the synthetic pathways of the cytoplasm (1, 15, 16). Thus 

 there exist two reservoirs of serine. One, inside the chloro- 

 plast, is carboxyl labeled and similar to photosynthetically pro- 

 duced phosphoglycerate , and one, outside the chloroplast or in 

 the cytoplasm, is uniformly labeled as dictated by the glycolate 

 pathway. Similarly there should exist two reservoirs of sucrose 

 with similar labeling differences. An experiment to test this 

 would be to determine the C^"* labeling patterns of sucrose of the 

 chloroplast in comparison with labeling in sucrose of the phloem. 



The glycolate lost by the chloroplasts is converted to glycer- 

 ate. The excretion of glycerate by the chloroplasts, as already 

 noted, would not alter the basic function of a carbon transport 



