220 PHOTOSYNTHESIS 



other quite independently of the photosynthetic process and even the 

 identity of the hexoses is not fixed, but they may undergo stereoisomeric 

 change. Unquestionably it would be very helpful if it were known just 

 what the first sugar is that is produced in photosynthesis. In the present 

 state of our knowledge glucose fits the theoretical requirements most 

 adequately. Yet the fact cannot be entirely disregarded that the demon- 

 stration of glucose actually being the first sugar formed is still wanting. 



4. Are Other Substances Besides Carbohydrates Formed in 



Photosynthesis? 



CO, . 

 The fact that the photosynthetic quotient, -y^ , is very close to unity 



has been very generally accepted as evidence that the product of the re- 

 duction of carbon dioxide is a carbohydrate. There are, however, some 

 experimental observations, which, while not in direct contradiction to 

 the fact just stated are not in entire accord therewith and may neverthe- 

 less be of considerable importance in throwing some light on the chemical 

 steps involved in the process. 



Saposchnikofif "* endeavored to determine experimentally whether the 

 amount of carbohydrate formed in a leaf corresponds to the amount of 

 carbon dioxide which the leaf absorbs during the time it is illuminated. 

 This was undertaken with the following equation as expressing the course 

 of the photosynthetic reaction: 



6CO2 + 6H2O > CeHiaOs + 6O0. 



Leaves of Helianthus anniius were cut into two portions : one portion was 

 used for the determination of the gaseous exchange, the other for analysis 

 of the carbohydrate content. In this manner Saposchnikofif found that 

 only 64-87 per cent of the carbon dioxide absorbed appears in the leaves 

 in the form of carbohydrates, the latter being calculated as glucose. 

 These results are apparently not of the highest degree of accuracy; the 

 experiments were carried out over 35 years ago, before the methods of 

 analysis of leaf components had received any critical study. Too much 

 weight can therefore not be placed upon the numerical values given. If, 

 however, we accept the results in principle, it follows that other products 

 besides carbohydrates are formed simultaneously. These are probably 

 fats and proteins. There is nothing surprising in this conclusion ; it has 

 long been known that proteins are formed in the leaf when there is an 

 adequate supply of carbohydrates and there is some evidence that the 

 rate of protein synthesis in leaves is greater in light than in the dark."^ 

 The results of Saposchnikofif further demonstrate the error of endeavor- 

 ing to determine the rate of photosynthesis from the amount of carbo- 

 hydrate formed in the leaves of a plant during illumination.^^^ 



'"Saposchnikofif, Ber. bot. Ges., 8, 240 (1890). 



"' Czapek, "Biochemie dcr Pflanzen," 2nd Edition, II, 298, 1920. 



"'Maze, Contpt. rend., 180, 306 (1925). 



