This glycolic acid is then oxidized by 2 molecules of MnCl- 

 (OH)2 (produced in the first step) to give glyoxylic acid. 

 According to Tanner, the greater labeling of glycolic acid 

 at low CO2 pressure during photosynthesis with C^^02 is 

 due to the first step being first order with respect to the utili- 

 zation of CO2 and the production of trivalent manganese, 

 whereas the second step is second order with respect to the 

 utilization of trivalent manganese. 



Whether or not Tanner's suggested route from CO2 to 

 glycolic acid will be borne out by experiment remains to 

 be seen. In all our experiments with C^*02, labeled glycolic 

 acid has been a relatively minor product of the photosyn- 

 thesis, except in those cases where the CO2 pressure has been 

 permitted to drop to a very low level. Glycolic acid is some- 

 what volatile, but it is a curious characteristic of this com- 

 pound on paper chromatograms that, although 20 to 85 per 

 cent may evaporate from the paper during development of the 

 chromatogram, the remainder disappears only very slowly 

 from the papers. This statement is based on measurement 

 of radioactivity following chromatography of synthetic C^*- 

 labeled glycolic acid. Thus it would seem that if a pathway 

 leading directly from CO2 to glycolic acid (that is, with no 

 isolable intermediates) were quantitatively important, we 

 should have seen much more labeled glycolic acid following 

 short periods of photosynthesis with C^^02, It could be that, 

 under normal conditions of photosynthesis (say with 1 per 

 cent CO2 in air), the reservoir size or concentration of gly- 

 colic acid is very small, so that it would not appear to be 

 strongly labeled, even though carbon from C^^02 enters it 

 very rapidly. 



However, Moses and Calvin (44) conducted parallel 

 experiments (3 minutes photosynthesis by Chlorella in the 

 presence of C^*02 in one case and T2O in the other). The 

 tritium-labeled glycolic acid accounted for more than 50 

 per cent of the darkening of the radioautograph in the sub- 

 sequent analysis by chromatography, whereas in the parallel 

 experiment the glycolic acid contained less than 5 per cent 



45 



