beta carbon atoms of PGA involves a randomization of the 

 label. This could mean that along this pathway there is a 

 symmetrical intermediate or that an intermediate is in rapid 

 reversible equilibrium with a symmetrical compound (see 

 Figure 7). 



When Wilson and Calvin (19) studied the effect of CO2 

 depletion following a period of photosynthesis with C^^02 

 by algae, they found that the lowering of CO2 pressure re- 

 sulted in a great increase in the amount of labeled glycolic 

 acid. This increase in labeled glycolic acid was sustained for 

 at least 10 minutes. Upon application of 1 per cent CO2 

 again, the level of labeled glycolic acid declined. 



Tolbert (22) found that glycolic acid formation from 

 C^^02 during 10 minutes photosynthesis in leaves of Sedum 

 alboresum is much higher at very low CO2 pressure than at 

 high CO2 pressure. As mentioned earlier, he also found that 

 glycolic acid is the predominant labeled compound excreted 

 into the medium by chloroplasts from Swiss chard photo- 

 synthesizing in the presence of HC^^Os-. He had shown 

 earlier (43) that glycolic labeled with C^* is excreted into 

 the medium by Chlorella photosynthesizing in C^^02. He 

 suggested that glycolate may function in ion balance with 

 HCO3- between cells and their medium or between chloro- 

 plasts and other cell compartments. He also proposed that 

 glycolate might be a carrier of "carbohydrate reserves" from 

 the chloroplasts to the cytoplasm. 



Moses and Calvin (44) exposed photosynthesizing Chlo- 

 rella pyrenoidosa to tritium-labeled water for various periods 

 from 5 seconds to 3 minutes. Analysis was made by the usual 

 extraction, two-dimensional paper chromatography, and 

 radioautography. The greatest darkening of the film by far 

 occurred where it was in contact with the glycolic acid area 

 of the chromatogram. This result, which we shall discuss 

 later, seems to agree with Tolbert's suggestion that the gly- 

 colic acid acts as a carrier of hydrogen. 



During normal photosynthesis (Figure 2), two-carbon 

 moieties (carbon atoms number 1 and 2 from a keto sugar 



40 



