the carbon reduction cycle are saturated (about 3 minutes in 

 this experiment) and long before they themselves, or any 

 other amino acids, are saturated with radiocarbon (30). Prob- 

 ably glutamic acid is a primary amino acid also, but kinetic 

 data alone cannot prove this at the moment. Glutamine is 

 generally supposed to arise from glutamic acid, but there is 

 some evidence to indicate that it may arise as a primary 

 amino acid amide (32,30). 



In any event, the rates of synthesis of alanine, serine, and 

 aspartic acid in reservoirs we believe to be closely associated 

 with the chloroplasts in Chlorella are great enough to permit 

 the following conclusions. 



1 . An appreciable fraction of the carbon assimilated dur- 

 ing photosynthesis in Chlorella is used directly in the syn- 

 thesis of amino acids without the intermediacy of sugars or 

 any other class of compounds except acid phosphates and 

 carboxylic acids. 



2. Since this amino acid synthesis accounts for a major 

 portion of the inorganic nitrogen uptake, these amino acids 

 must be used to a large extent in protein synthesis. However, 

 some important amino acids (i.e., glycine) are so slowly 

 labeled that they probably do not supply a major part of 

 the carbon for protein synthesis. Instead, the carbon skeletons 

 corresponding to these amino acids must be incorporated into 

 protein in some form other than as the free amino acid. 



Before considering synthetic routes to specific amino 

 acids, we wish to reiterate our belief that photosynthetic re- 

 actions need not follow the same course as the better-known 

 synthetic reactions of other nonphotosynthetic organisms. 

 Also note that few if any enzymes involved in amino acid 

 synthesis have ever been isolated from chloroplasts. Thus 

 we are forced to suggest new and untested hypothetical paths. 

 Our guiding principles will be that chemical potential should 

 be used to drive the reactions rapidly in the forward direction 

 and that loss of carbon or reduction level should be avoided 

 wherever possible. 



32 



