Evolution of Photosynthetic Mechanisms 19 



the whole sequence of enzymatic reactions that are involved in this 

 reduction are dark reactions. Once we have availahle the products of 

 the light reaction, namely, a reducing agent and some type of "high 

 energy" phosphate, the whole carhon cycle can be operated and car- 



CARBOHYDRATES 

 Xu5P 



CHjOH 



CO 

 I 

 HC-OH 



HC-OH 



3f HjC-OPC^H 



I Ru5P 



® 



ATP" 



HgC-OPOjH" 



C=0 

 I 



HC-OH 



ASPARTIC 

 ACID 



FIG. 1. Carbon reduction cycle (simplified version) : (1) Ribulose diphosphate 

 reacts with CG\> to give an unstable 6-carbon compound which splits to give two 

 3-carbon compounds, one of which is 3-phosphoglyceric acid. The other 3-carbon 

 compounds might be either 3-PGA, as it is known to be in the isolated enzyme 

 system, or some other 3-carbon compound such as a triose phosphate ( dashed 

 arrow). (2) PGA is reduced to triose phosphate with ATP and TPNH derived from 

 the light reaction and water. (3) Various condensations and rearrangements convert 

 the triose phosphate to pentose phosphates. (4) Pentose phosphate is phosphoryl- 

 ated with ATP to give ribulose diphosphate. Further carbon reduction occurs via 

 conversion of PGA to phosphoenolpyruvic acid (5) and earboxylation (6). to form 

 a 4-carbon compound (probably oxaloacetic acid). Reactions leading to the forma- 

 tion of some of the secondary intermediates in carbon reduction are also shown. 



bon can be taken from CO L » into a variety of compounds, among them 

 sugar. The sugar can be taken out of the cycle. Every time the cycle 

 turns six times, for example, we can take out a hexose sugar molecule 

 and still have the cycle molecules left. This, indeed, is what happens. 



