MECHANISM OF PHOTOSYNTHESIS I47 



Still a controversial one. The study of partial reactions which 

 depend on light absorption may resolve the difficulties in 

 the interpretation of experiments with living cells. 



(d) path of carbon in photosynthesis 



It seems, then, that the study of photosynthesis has 

 opened up a biochemical field of oxidations and reductions 

 comparable with that of respiration. There is, however, a 

 different problem awaiting solution, namely the determina- 

 tion of the sequence of carbon compounds which belong 

 specifically to the process of photosynthesis. The knowledge 

 we now possess is due to a brilliant series of investigations 

 by Calvin and co-workers (1949, 195 1) and of Gaffron and 

 colleagues (Fager and Gaffron) discussed in Chapter 6. 



One of the difficulties of this approach is that when these 

 methods are applied to the living cell the intermediates of 

 greatest interest are likely to be present in the lowest con- 

 centration. Nevertheless the initial findings in Calvin's work 

 of the extreme rapidity with which the carbon of the assimi- 

 lated CO2 was incorporated into a large variety of cellular 

 metabolites, gave a new impetus to the study of metabolism 

 in the plant. For example, it was found that in Scenedesmus 

 fats and not carbohydrate were formed from acetate in the 

 light. With CO2 over a period of a few minutes labelled 

 amino acids and corresponding carboxylic acids as well as 

 carbohydrate were formed. In the dark there was a similar 

 incorporation of the CO2 though at a much slower rate. 



With the exposure of preilluminated cells to ^^COg for 

 only a few seconds most of the radioactivity is found in the 

 phosphate ester fraction. Using a method of extrapolation 

 back to zero time it was concluded that phosphoglyceric acid 

 was the first stable derivative of labelled carbon dioxide and 

 that the carboxyl group was labelled first. The same con- 

 clusion was reached later by Gaffron in independent experi- 

 ments. This finding was consistent with the initial labelling 

 of the hexose found in the 3- and 4-positions. Recently 

 Fager (1952) has shown, using the tracer method, that 

 phosphoglyceric acid (PGA) is formed in vitro by cell free 

 preparations of spinach leaves. For this a cell extract was 



