334 PHOTOSYNTHESIS 



of such primary photoelectric processes lead to an accumulation of a photo- 

 chemical reaction product will depend thermodynamically upon the energy 

 balance. 



The necessity of oxygen for the photosynthetic process has been re- 

 peatedly demonstrated. The formation of hydrogen peroxide on illumina- 

 tion of light sensitive substances in the presence of water is well known. 

 It is assumed that the primary reaction in photosynthesis is the loss of an 

 electron from the chlorophyll molecule. 



2 chlorophyll + 2 Jw + 2 O. == 2 chr + 2 0.- 



2 Oo- -f 2 H.O = 2 H + 2 OH- + 2 O. 

 2 H -h 2 OH- -f 2 chr = Ho + H.Oo + 2 chl 



' 2 HoO -f 2 hv = Ho + H2O2 



CO2 + Ho + HoOo = HCOH + HoO + Oo 



CO2 -f HoO ^2hv = HCOH + O2 



It is undetermined whether in this scheme the free hydrogen atoms 

 and hydroxyl groups or the H-2 and HoOo react with carbonic acid. 



According to these equations 2 energy quanta suffice to reduce 1 mole 

 of carbon dioxide to formaldehyde, i.e., it is necessary that only one electron 

 be carried by the molecular oxygen to each of the two water molecules. 

 This will occur only with low light intensities ; with increased intensity 

 there will be a reduction in efficiency. 



The thermochemical reactions are as follows : 



2 H2O = Ho + HoOo — 92,000 calories 

 COo + Ho + HoOo = HCOH + H2O + Oo — 18,000 calories 



CO2 + HoO = HCOH + Oo — 110,000 calories 



It is evident that this is an endothermic reaction, and the required 

 energy is supplied by 2 N hv in which N is the Avagadro constant. The 

 magnitude of N /;z/ increases with decreasing wave length, i.e., for 700 \i\y. 

 it is 40,500 calories and for 600 ^n, 47,500 calories. These values must 

 be doubled for the equation given above in which case there is still a 

 deficit, vis.: 



CO2 -f H2O + 2 N hv = HCOH + O, — 15,000 (600 ^^i) 



— 29,000 (700 \i\i) 



A complete balancing of the energy in the equation is attained only at 



517 |i[i when 



2 N hv = 110,000 calories 



This lessens the validity of ihe equations upon which the theory is 

 based. Direct evidence hereto can be supplied only through experimenta- 

 tion in which the efficiency of the leaf is determined, i.e., the ratio of the 

 radiant energy absorbed to that converted into potential chemical energy. 

 The experiments of Warburg and Negelein are the most recent and prob- 



