288 PHOTOSYNTHESIS 



taken up the notion of Pringsheim ^' that the oxygen is not liberated in 

 the chloroplasts. It will be recalled that Pringsheim on the basis of his 

 studies of photosynthetic asphyxiation came to the conclusion that carbon 

 dioxide reduction and oxygen liberation do not occur at the same place 

 in the cell, that oxygen is not formed within the cell, but that during 

 photosynthesis a substance is formed which migrates away from the 

 chloroplasts, and oxygen is liberated from this substance only when it 

 reaches the surface of the cells. Pringsheim's ideas were apparently re- 

 futed by the discovery of gas vacuoles and of oxygen bubbles in the 

 cells. Wurmser now stresses the great susceptibility of chlorophyll to 

 photo-oxidation and concludes that oxygen cannot possibly be liberated 

 at the surface of the pigment ; he returns to the conception of Pringsheim, 

 though in a somewhat modified form. 



Wurmser considers that the reduction of carbonic acid takes place 

 in the protoplasm. This reduction is brought about by the absorption 

 of energy from other reactions, apparently oxidation reactions. In photo- 

 synthesis a primary transformation takes place at the surface of the 

 chlorophyll granules. He supposes that in the chloroplast there is a sub- 

 stance, the nature of which is yet unknown, which undergoes a chemical 

 reaction with the absorption of energy: 



A + Light > A' — X cal. (I) 



The product A' dififuses in the protoplasm and undergoes a spontane- 

 ous reaction with the regeneration of A and the liberation of energy: 



A' > A + X' cal. (II) 



It is the energy liberated in the second reaction which makes possible 

 the reduction of carbonic acid : 



CO, > C + O, — 94,300 cal. (Ill) 



CO2 4- H^b > CH,6 + O2 — 127,200 cal. (IV) 



Reaction I is a photochemical reaction ; the quantity of A' formed 

 in unity of time is proportional to the quantity of light absorbed. But 

 the rate of photosynthesis is not dependent only upon the concentration 

 of A', but also on reaction II which depends upon the ability of the 

 protoplasm to utilize the energy in A'. The substance A is not chloro- 

 phyll, for Wurmser has shown that the only known photochemical re- 

 action of chlorophyll, namely that of photo-oxidation, shows maximum 

 and minimum reaction in portions of the spectrum which are quite differ- 

 ent from those in which photosynthesis exhibits its greatest activity. 

 Wurmser considers that the substance A is therefore colorless. Accord- 

 ing to this view the reduction of carbonic acid is not a photochemical 

 reaction and this reduction is not directly associated with chlorophyll. 



•"Pringsheim, Sitzber, Preussischcn Akad. Wiss., 38, 763 (1887). 



