148 PHOTOSYNTHESIS 



prepared in the dark under practically anaerobic conditions. 

 Liquid nitrogen was used to freeze the leaf tissue initially. 

 The extracts contained chloroplast material plus the soluble 

 cell constituents together, no doubt, with particles derived 

 from the mitochondria. The insoluble residue alone was 

 found to be inactive but activity could be restored by 

 addition of the soluble portion. The soluble portion itself 

 could be fractionated by precipitation with acetone and a 

 fraction added to the insoluble which would restore the 

 activity to the same extent as the untreated cell extract. 

 With this in vitro system PGA was formed in the dark but 

 the effect could be doubled by exposure to light. The 

 activity of this in vitro system when compared with the rate 

 of photosynthesis shown by a plant was extremely small. 

 Nevertheless this discovery of Fager may well lead to a 

 more complete knowledge of the nature of the process of 

 formation of PGA from CO2. This would be important for 

 the following reason. The rapid labelling of PGA found by 

 Calvin to always occur with normally assimilating cells 

 requires the postulation of something equivalent to a two- 

 carbon compound which has to be supplied by a cycle of 

 chemical transformations. Such a cycle of chemical changes 

 is found in the widely present transketolase and transaldo- 

 lase mechanisms for the oxidation of carbohydrate through 

 6-phosphogluconic acid. If these group transfer mechanisms 

 lead to an acceptor for the uptake of COg with phospho- 

 glyceric acid as the product, the only reduction step required 

 to bring one equivalent of COg to the state of carbohydrate 

 would be the reduction of two equivalents of phospho- 

 glyceric acid. With present knowledge of the plant enzymes 

 this reduction step seems possible in the presence of con- 

 tinuously reduced TPN and DPN together with a phos- 

 phorylation mechanism. This pathway for carbon assimila- 

 tion in the green plant could thus depend on the reduction 

 of the coenzymes by the photochemical system as originally 

 suggested by Ochoa. In addition to the single carboxylation 

 step postulated by Calvin, however, it has to be admitted 

 that other possibilities exist for the entry of carbon dioxide 

 into the metabolism of plants. 



