of all the C^* found in compounds on the chromatograph. 

 Thus the incorporation of hydrogen into nonexchangeable 

 positions on glycolic acid seems to occur at ten times or 

 more the rate of incorporation of C^* into the same com- 

 pound. The simplest interpretation is that glycolic acid 

 plays a much more important role in the transport of hy- 

 drogen or reducing power than it does as an intermediate in 

 carbon-compound formation from CO2. If any carbon di- 

 oxide is reduced directly to glycolic acid during photosyn- 

 thesis by Chlorella, it would seem to be a minor part of the 

 total. 



A special role for glycolic acid in hydrogen transport is 

 suggested by a combination of experimental findings from 

 several laboratories. To Moses' finding of extremely rapid 

 tritium labeling of glycolic acid and Tanner's implication 

 of the role of glycolic acid with the requirement for man- 

 ganese, we may add Delavin and Benson's report (60) of the 

 light stimulation of the oxidation of glycolic acid with O2 

 to glyoxylate and peroxide in isolated chloroplasts. Further, 

 we must mention that manganese is thought by Kessler (61) 

 to play some part in the formation of peroxide or O2 from 

 water during the early stages of photosynthesis. Some form 

 of peroxide is commonly postulated as an intermediate be- 

 tween water and O2 during photosynthesis, and it may be 

 that the plant has some mechanism for conserving the chem- 

 ical potential energy that would be lost if peroxide were 

 permitted to decompose to water and oxygen by a catalase 

 mechanism. 



The decrease in labeled glycolate in algae grown in 

 Mn++ -deficient media (58,59) may be due to (1) some in- 

 crease in the level of an intermediate in the oxygen-evolution 

 pathway which is also capable of oxidizing glycolate to gly- 

 oxylate (presumably Mn++ might be required for the break- 

 down of this oxidant to O2); (2) a decrease in reduced pyri- 

 dine nucleotide concentration, owing to impairment of the 

 oxygen-evolving pathway; or (3) some enzymic requirement 



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