84 PROBLEMS IN PHOTOSYNTHESIS 



chemical process must take place in the two vessels which are of different 

 size. When the Oo production is measured simultaneously in both vessels 

 with a divided light beam, there is no COo outburst from living cells. In 

 accordance with true photosynthesis, there can only be a O2 outburst. 



§ 35 The One-Quantum Reaction of Photosynthesis 



From what has already been said, it follows that the quantum requirement 

 of photosynthesis is not 8, 10 or 12; under the proper experimental conditions, 

 it is 3. For the reduction of one molecule CO., the action of 3 quanta is 

 necessary, but according to Einstein's law one molecule should be activated 

 by one quantum only. If more quanta seem to be necessary, the existence 

 of dark reactions must be envisaged (see § 6) . The question therefore arises 

 whether dark reactions are also involved in photosynthesis. As a result of 

 considerable experimental work, Burk and Warburg (13, 14, 42) succeeded 

 in solving this problem. They found that on intermittent illumination with 

 high intensities, when one-minute periods of light and dark alternated, the 

 calculated quantum requirement in each light minute was 1 . In each light 

 minute one absorbed quantum therefore caused the reduction of only one 

 molecule COo and the production of nearly one molecule Oo, the photo- 

 synthetic quotient varying between —1.0 and —1.2. By contrast, in each 

 dark minute the production of COo and the removal of Oo (respiration) was 

 noted. Burk and Warburg see the solution of the quantum problem in the 

 coupling of light and dark reactions. The light reaction in which Oc is 

 produced and CO2 is fixed is a one-quantum reaction. However, it can only 

 occur when coupled with an exergonic oxidation reaction, the so-called back 

 reaction or induced respiration. Thus, we have a cyclic process. When 

 112000 cal are needed for the reduction of one mole CO2, the light reaction 

 contributes only 43000 cal/mole (in red). The remaining energy, about 

 70000 cal/mole, is supplied by the induced respiration. The reactions can be 

 represented as follows 



Reaction 7 (light) : 



(ChlCOs)* + N/w + C02^ (ChlCOo) + C + Oo 



Reaction 2 (dark) : 



2/3 C + V3 Os -^ 2/3 COo + 70000 cal 



Reaction 3 (dark) : 



(ChlCO?) -^ (ChlCOo)* - 70000 cal 



The over-all reaction is 



V3 COo + Nhu -> 1/3 G + V3 Oo 



In the light reaction one molecule O2 is produced per molecule chlorophyll 

 with the quantum requirement 1 . In the dark, Vs of the O2 produced in the 

 lioht are used for the induced respiration (reaction 2). The over-all reaction 



