670 



Martin Gibbs 



I 



functioning reductive COp cycle is present in the isolated sys- 

 tems. The conclusion of Arnon was tested by supplying intact | 

 spinach chlorop lasts and reconstituted spinach. and pea chloro- 

 plast, preparations with C-'-^Op, light, and PPNR^-*-"^. In addition 

 to C Op, broken spinach chloroplasts were supplied with D- 

 ribose-5-P or ribulose-l,5-diP. With the pea, material prepared 

 by- the method of Stocking, glycerate-3-P-l-C replaced the 

 C Op and pentose-P^^^v Similar experiments have been reported by 

 Trebst and Fiedler^ -"' . 



Table k. DISTRIBUTION OF C IN GLUC0SE-6-P AND GLYCERATE-3-P 

 FORMED DURING C Op FIXATION BY INTACT SPINACH CHLOROPLASTS 



Carbons of glucose-6-P K min. 10 min. hO min. 



C-1 Ih 79 92 



C-2 73 75 82 



C-3 97 86 9^ 



c-h 100 100 100 



C-5 — 61 92 



C-6 51 62 



Carbons of 3 -PGA 



COOH 100 100 100 



CH OH 55 63 75 



CH^OP 62 63 8o 



Carbon content is based on C-^ or C00H=100 



The most striking results of these data (Tables k and 5) was 

 the rapid spread of tracer which had taken place in the compounds 

 derived from the whole chloroplast experiments. In sharp con- 

 trast, the spread of C was low in those compounds isolated from 

 the reconstituted systems. It is of significance that this dif- 

 ference could not have been predicted on the basis of detecting 

 which compounds were labeled and which were unlabeled since C-"- 

 appeared in essentially the same compounds in the experiments 

 with both whole chloroplasts and reconstituted systems. It is 

 concluded from these data that the intact chloroplasts possess a 

 complete reductive pentose-P cycle even though the rate of COp 

 assimilation was only 2^ of that reported for intact photosyn- 

 thetic tissue. This cycle was operating at a limited rate, if at 

 all, in the broken preparations. This inability of the recon- 

 stituted system to carry out a complete cycle indicates that the 

 present methods of preparing reconstituted chloroplast systems 

 must be modified. Perhaps this failure should have been expected 

 since the intact chloroplasts were able to reduce COp at a small 



