THE ENERGY OF PHOTOSYNTHESIS 47 



In addition to "common-sense" arguments against the direct forma- 

 tion of starch by photosynthesis, the fact that many plants do not 

 contain any leaf starch at all, also favors this conclusion. It has been 

 known since Meyer (1885) that starch is less common in the leaves of the 

 monocotyledons, than in those of the dicotyledons. Another argument 

 in favor of starch formation as a secondary process which is not a part 

 of photosynthesis proper, is the capacity of plants to convert artificially 

 supplied sugars (or similar compounds) into starch, without the help of 

 light. 



The starch synthesis and starch dissolution (amylolysis) in leaves must be consid- 

 ered a part of the "second stage" of plant nutrition, which follows photosynthesis proper. 

 Important advances in this field have become possible by the successful enzymatic 

 synthesis of glycogen from glucose phosphate by Cori and coworkers. However, it 

 would lead us too far to enter here into this complex matter. Spoehr and Milner 

 (1939) have studied the effects of oxygen, carbon dioxide, water and temperature on 

 the rate of dissolution of starch amylolysis in vivo. (These effects are important for 

 photosynthesis because they influence the mechanism by which the stomata of the 

 higher plants are opened and closed, thus regulating the supply of carbon dioxide to 

 the chloroplasts, cf. page 334.) Spoehr and coworkers (1940) also initiated a study of 

 the organic nutrition of albino plants to estabhsh the food requirements of plants which 

 have been denied the possibihty of preparing their own food from the air. Obviously, 

 studies of this kind can indirectly help in the identification of the first product of photo- 

 synthesis. Glucose and other sugars can supply the plants with all their food require- 

 ments (except for nitrogen and mineral elements assimilated through the roots), so that 

 the formation of hexoses constitutes an entirely sufficient interpretation of the over-all 

 reaction of photosynthesis; but whether it is also the minimum possible explanation, 

 is another question. 



At the present stage of our knowledge, all processes from the moment 

 of the entrance of carbon dioxide into the plant to the completion of 

 sugar synthesis must be included into the "over-all reaction of photo- 

 synthesis," which thus becomes 



(3.6) 6 CO2 + 6 H2O > CeHisOe + 6 Oa 



We will often use the abbreviated equation 



(3.7) CO2 + H2O > ICH2O} + O2 



where {CH2O} stands for a generalized link in a carbohydrate chain, 



C. The Energy of Photosynthesis * 



From the time of Robert Mayer, it was known that photosynthesis 

 converts light into chemical energy. The energy stored in this way is 

 equal to the heat of combustion of the primary products of photosynthesis. 

 In the first approximation, the heat of combustion of organic compounds 

 containing carbon, hydrogen and oxygen, depends only on their level of 



* BibUography, page 59. 



