THE ENERGY OF PHOTOSYNTHESIS 



49 



Table 3.V 



Energies (A//c) and Free Energies (AFc) of Combustion of Carbohydrates 

 TO Liquid H2O and CO2 Gas, at 25° C. 



"D. = DeMpine (1897), E.B. = Emery and Benedict (1911), K.F. = Karrer and Fiorom (1923), 

 K.N.H.W. = Karrer, Nageli, Hurwitz, Walti (1921), N.H.J. = Neuberg, Hoffmann, Jacoby (1931), 

 S.K.L. = Stohmann, Kleber, Langbein (1S90), S.L. = Stohmann, Langbein (1892), V.K. = Verkade, 

 Koops (1923), W.L. = v. Wartenberg, Lerner-Steinberg (1925), and W.M.R. = v. Wartenberg, Much- 

 lewski, Riedler (1924). „ ^ . ^ 



fc Figures from G. S. Parks and H. M. Huffman, The Free Energies of Some Organic Compounds. 



hexoses, the comparatively high energy of the keto group is rapidly 

 "diluted" by the lower energies of the alcoholic groups, until a Kmiting 

 value of about 112 kcal per link is reached. (The heats of combustion 

 of the inositols, the only compounds consisting entirely of HCOH groups, 

 have not yet been determined.) 



The standard bond energy table (c/. Table 9. II) shows that the 

 endothermal character of photosynthesis has- a double origin. In the 

 first place, the C— H bond is less stable (by 12 kcal) than the O— H 

 bond, so that the hydrogen atoms have^^to^be transferred, in photosyn- 

 thesis, from a stronger to a weaker bond. In the second place, the 

 C=0 double bond in carbon dioxide (which has to be "opened" in 

 photosynthesis) is more stable (by as much as 72 kcal) than the 0^0 

 double bond formed in this process. The weakness of the 0=0 double 

 bond is the most important cause of the tendency of most elements for 



