82 THE SYNTHESIS OF PROTEINS 



It must not be concluded from these observations that the 

 making of proteins by the green plant is necessarily a photo- 

 synthesis. For the process to take place there must be avail- 

 able the appropriate raw materials and of these carbohydrate 

 and various elements, especially nitrogen, in suitable com- 

 binations, are the most significant. The carbohydrate pre- 

 sumably must be a sugar, and provided it be available, protein 

 synthesis is independent of light * and can take place in all 

 parts of the plants although in different degrees, f Since 

 the photosynthesis of carbohydrates is a daylight process, 

 it is not surprising to find that the production of pro- 

 teins may be four or five times as great in the light as in 

 darkness. % 



Schimper,§ experimenting with cut leaves of Sambucus 

 and with potted plants of Pelargonium zonale, found that 

 nitrates were destroyed in green leaves exposed to daylight 

 but were not so destroyed if the leaves were kept in the 

 dark, and in agreement with this, shade leaves were found 

 to be richer in nitrates than sun leaves. Furthermore, no 

 destruction of nitrate occurs in etiolated leaves exposed to 

 sunlight. 



The investigations of Ullrich || indicate that the chloro- 

 plasts, the centres of carbohydrate formation, play a part 

 in protein synthesis. Detached leaves of Brassica, Cucurbita, 

 Lactuca, Phaseolus, Tropceolum and Zea, grown in the light 

 and supplied with nitrate, produced protein which probably 

 had its origin in the plastids, for these showed an increase 

 in size in comparison with the plastids of leaves grown in 

 similar conditions but deprived of nitrogen. When protein- 

 containing plastids were digested with pepsin, the appearance 

 of the remaining structure was like the chloroplasts of the 

 nitrogen starved leaves. 



As has been indicated above, protein synthesis can take 

 place in darkness provided that an adequate supply of carbo- 



* Jost : " Biol. Centrlbl.," 1900, 20, 625. 



f Bombacioni : "Rend. Ace. Lincei Roma," 1923, 32, ii., 108. 



X Montemartini : " Atti. R. Inst. Bot. Pavia," 1905, II., 10, 20. 



§ Schimper : " Bot. Zeit.," 1888, 46, 128. 



|| Ullrich: "Zeit. Bot.," 1924, 16,513. 



