146 



PROBLEMS OF PHOTOSYNTHESIS 



V 



2e + 2H" + H202 



Y 



1 



/SH 

 \SH 



1 



2H2O 



Fig. 52. Electron transfer of excited chlorophyll. 



photosynthetic 

 carbon cycle 



pyruvic acid 



Fig. 53. The status of lipoic acid. 



Acetyl CoA 



I 



tricarboxylic 

 acid cycle 



in the dark. Thus, it seems that the action of light blocks the link between 

 the photosynthetic carbon cycle and the tricarboxylic acid cycle. The photo- 

 reduction of lipoic acid would strongly inhibit the conversion of pyruvic acid 

 to acetylcoenzyme A. Figure 53 shows the position of lipoic acid, schemati- 

 cally. This means that respiration proceeding in the dark through the tri- 

 carboxylic acid cycle — a process producing CO2 — would be diverted in light 

 to proceed through another cycle — the carbon cycle — showing uptake of 

 CO2. According to Steward and Thomp.son (49), it is physiologically im- 

 probable that the tricarboxylic acid cycle is suppressed in the light. First of 

 all, there is no proof that respiration proceeds only through this cycle. As 

 there is no photosynthesis without respiration (see § 42), it is impossible to 

 admit that green cells in the light do not respire. Secondly, Steward and 

 Thompson assume that the tricarboxylic acid cycle may well proceed in the 

 light with substrates other than those used in the dark, and utilize carbon 

 sources drawn from amino-acids (e.g., glutamic acid) and synthesize pro- 

 teins. 



