50 OVER-ALL REACTION OF PHOTOSYNTHESIS CHAP. 3 



oxidation, and of the difficulty of reversing this oxidation and expelling 

 oxygen from oxides or organic oxygen compounds. 



The values of AFc in the last two columns of table 3.V, serve to illus- 

 trate the statement, made in chapter 1 (page 3) that the increase in free 

 energy in photosynthesis is even larger than the increase in total energy, 

 particularly if AF is calculated not for the "standard" pressure of one 

 atmosphere, but for the actual partial pressure of carbon dioxide in the 

 air, 3 X 10~* atm. (Only for formaldehyde vapor AFc is smaller than 

 AHc, because in this case two gases, H2CO and O2, are converted into 

 one gas, CO2, and a liquid, H2O, thus decreasing the molecular disorder.) 



Of course, an increase in free energy is possible only because photo- 

 synthesis is not a spontaneous process in a closed system, but a photo- 

 chemical reaction, maintained by a continuous supply of light energy 

 (to make the system complete, the sun should be included in it). 



To sum up, table 3.V makes it probable that photosynthesis proceeds 

 with an accumulation of at least 112 kcal per mole of reduced carbon 

 dioxide: 



light 

 (3.8) CO2 + H2O > {CH2OI + O2 - 112 kcal 



plant 



and, in the free atmosphere, with an increase in free energy by about 120 

 kcal per mole. 



In thermochemical equations, we will conform to the usage and designate the 

 absorbed energy by a minus sign, and the released energy, by a plus sign. On the other 

 hand, the heat effect AH of a reaction shall be considered as positive for endothermal 

 and negative for exothermal reactions, in accordance with the notation of Lewis and 

 Randall. In other words, the figure — 112 kcal in equation (3.8) represents minus AHc. 



The efficiency of photosynthesis as an energy-converting process 

 depends on the amount of light required for the reduction of one mole 

 of the substrate. Much study has been devoted to this problem (which 

 will be discussed in Vol. II, Chapters 28 and 29). Anticipating the results 

 we can state that the average conversion yield in direct sunlight is of the 

 order of 3% of the absorbed light energy, or 2% of the incident visible 

 light; but in weak light and in presence of ample carbon dioxide it may 

 rise to as much as 30%. This indicates that the low energy conversion 

 observed under natural conditions is caused by a limited capacity of the 

 photosynthetic apparatus and consequent dissipation of energy absorbed 

 in excess of this capacity, rather than by an obligatory utilization of a 

 large part of light energy for the activation of the chemical process of 

 photosynthesis. Fundamentally, the photosynthetic mechanism is cap- 

 able of converting light into chemical energy with an efficiency of not less 

 than 30%, and perhaps more. This is a much higher efficiency than has 

 ever been achieved in photochemical processes in the laboratory (except 

 for reactions which take place only in ultraviolet light). 



