I06 PHOTOSYNTHESIS 



reduction of nitrate to the level of ammonia has been shown 

 to occur in the dark, the rapid reduction of nitrate observed 

 in illuminated green tissue or cells seems to depend upon 

 the photosynthetic activity. This may be because one of the 

 intermediary products of photosynthesis is utilized speci- 

 fically for the reduction of nitrate, in addition to the process 

 occurring in most plants independently of light. Indeed, 

 Gorham found that with one strain of Lemna reduction of 

 nitrate was only possible in the light. Since in the plant 

 sulphur is absorbed as sulphate, reduction to the level of 

 HgS (or nearly so) is an essential step in protein synthesis; 

 there is however no evidence that this reduction is dependent 

 upon light. 



Generally, it is best to regard as the product of photo- 

 synthesis the whole of the addition to the cell in carbon 

 compounds which takes place during the period of illumina- 

 tion. Thus in a sunflower leaf the first starch to be formed 

 in the presence of tracer COg may have no labelled carbon, 

 yet for a physiological description the starch formed may 

 be considered a product of photosynthesis. 



At the present time a definition of a first product of photo- 

 synthesis is that of a stable compound in which the carbon 

 atom derived from the CO2 is first found. The studies made 

 with tracer COg have rendered this line of approach far 

 more possible than did the former methods of direct 

 chemical analysis of the plant material exposed to different 

 external conditions. 



Experiments with radioactive carbon 



Investigations of the path of carbon during photosynthesis 

 were initiated using radioactive carbon by Ruben, Kamen, 

 and Hassid (1940) at a time when the only tracer available 

 was the short-lived ^^C. The short life of the tracer necessi- 

 tated the use of brief periods of illumination of the order of 

 minutes; nevertheless as we shall see in the light of subse- 

 quent work during such relatively brief periods the tracer 

 carbon becomes distributed amongst many compounds. 

 Thus it is not surprising that these workers were unable 

 to identify any single compound in which the radioactivity 



