THE CHEMISTRY OE PHOTOSYNTHESIS 271 



CO2 + H2O + Light 



(If not removed, destroys) -^ Chlorophyll 



t 

 I 



H2O2 + CH2O 



I . ^ 



Enzymes— (If not removed, poisons) 



H2O -|- O Living protoplasm 



Carbohydrates 



Usher and Priestley point out that an accumulation of hydrogen per- 

 oxide would be harmful to the plant and a destruction of any excess is 

 accomplished by the enzyme catalase. Similarly an accumulation of 

 formaldehyde would poison the enzyme. 



The reports of Thunberg ^" that salts of carbonic acid can be reduced 

 to formaldehyde with hydrogen peroxide have been found to be erroneous. ^^ 

 Although Weigert^- partially bases his theory on Thunberg's results, the 

 fact that the latter have not been substantiated does not entirely annul 

 Weigert's theory. According to this view chlorophyll is to be regarded 

 as an optical sensitizer. The primary effect of the absorption of light by 

 quanta results in an internal photoelectric effect, i.e. a shifting of an 

 electron from one atom to another. The primary reaction in photosyn- 

 thesis is a loss from the chlorophyll molecule of an electron which is then 

 taken on by a molecule of oxygen, it being recognized that oxygen is essen- 

 tial for photosynthesis. The resulting negative oxygen ion transfers its 

 charge to the water with the breaking up of the latter into hydrogen and 

 hydroxyl ions. Whether it is the free hydrogen atoms or hydroxyl ions or 

 the H2O2 which reacts with carbonic acid Weigert does not decide. 



2 Chlorophyll -f 2 h, + 2 O. = 2 Chr + 2 Or 



2 O2- +2 H,0 m 2 H + 2 OH- -f 2 O2 



2 H + 2 OH- + 2 Chr = H2 + H2O2 + 2 Chi 



2 HoO -f 2h,= Ho + H2O2 



CO2 +H2 + H,02 = HCOH -f H2O + O2 



CO, + HoO + 2 h, = HCOH -f O2 



According to these equations two energy quanta suffice to reduce one 

 molecule of carbon dioxide, i.e. it is necessary that only one electron be 

 carried by the molecule of oxygen to each of the two water molecules. 

 Weigert further bases his theory upon certain energy relations which are 

 discussed in Chapter 6. The theory is clearly based upon the formal- 



Thunberg. Zcit. physik. Chcm., 106. 305 (1923). 



Bach and Monson, 'Bcr. chcm. Gcs.. 57, 735 (1924). 



Weigert, Zeit. physik. Chcm., 106, 313 (1923) ; 109, 79 (1924). 



