QUANTUM YIELD MEASUREMENTS BY THE MANOMETRIC METHOD 1089 



When theorists tried to devise a detailed mechanism of photosynthesis, 

 plausible not only from the point of view of chemistry, but also from that of 

 energetics, they found themselves badly hampered by the straight-jacket 

 into which the limitation to four quanta had put them. 



Franck and Herzfeld (1941), in particular, pointed out that, if the reduction of 

 carbon dioxide has to be brought about by several consecutive photochemical steps (as 

 in scheme 7.VA), the intermediates must have a certain degree of stabiUty in order to 

 avoid reoxidatiou while waiting for the stipply of another quantum of light energy. Con- 

 K(>quently, the ".stabilization energy" of several intermediate products must be added to 

 the energj' requirements of photosynthesis; and a further allowance must be made for 

 the heat of formation of the {CO2} complex and the heat of decomposition of a per- 

 oxide (which is the probable precursor of free oxygen evolved in photosynthesis). 



2 [HjOo] + (H; CO} 



AW. ^ 10 kcal 



hHj ^ 10 kcal 



Lhp I ^,5 kcal 



— Oo + (CH2O} + 3H2O 



AHj^ a 10 kcal 



CO2 + H2O 1 



(CO2} + H 



AHr= 112 kccl/mole 



^^•(co^r^-"'^^'^' 



Fig. 29.2, Energy requirements of a linear four-stage mechanism of 

 photosynthesis (after Franck 1941). Total energy required Ai? + A/fc + 

 MI[C02\ + Ai/ii + Ai7i2 + A//13 + Ai/p = 210 kcal. /mole. 



The energy relations in photosynthesis, according to Franck and Herzfeld, are illus- 

 trated by figure 29.2, in which Afl'i cOs) (the energy of formation of the carbon dioxide- 

 acceptor complex), A///j, Aff/j, Aff^/^ (the "stabilization energies" of three intermedi- 

 ates) and A/7p, the (>nergy of stabilization of the end products (which includes the de- 

 composition energy of the peroxide, {II2O2}) are shown as additional energy terms, which 

 together with the accumulated chemical energy, A^c, must be supplied by light. 



The stabilization energy required for the prevention of "backsliding" of intermedi- 

 ates in periods of several minutes (which must pass, in weak light, in a dense suspension 

 of green cells, between the absorption of two light quanta by one and the same chloro- 

 phyll molecule) must be of the order of 10 kcal /mole; the heat of formation of the { CO2I 

 complex was estimated in chapter 27 as >20 kcal/mole. Altogether, three intermediates, 

 the ICO2} complex and the primary peroxide, must increase the energy requirements of 

 photosynthesis from 112 kcal/mole to probably as much as 210 kcal/mole — whereas 4 

 einsteins of red light (X = 660 m^) provide only 170 kcal. 



