THE ENERGETICS OF PHOTOSYNTHESIS 85 



shows that apparently 3 quanta are necessary for the reduction of one molecule 

 CO2. In reality, however, only one quantum is involved, in accordance with 

 Einstein's law. The substance obtained during illumination — indicated by 

 C — is partly oxidized in the dark (2/3 according to reaction 2). Thus, the 

 remaining gain (' '3) is not identical with the gain in the light reaction, but is 

 equal to the difference between the gain during illumination and the con- 

 sumption in the dark. 



The remaining gain cannot be more than \ '3 of the light gain, as the energy 

 of one mole quanta in red is about 43000 cal, i.e., Vsof the energy requirement 

 of photosynthesis (1 12000 cal'mole COo). A higher gain is thermodynami- 

 cally impossible. When - 3 of the O2 produced are used for the induced res- 

 piration — as shown in reaction 2 — the quantum requirement of the over-all 

 reaction is 3. When '' '4 of the Oo react back the quantum requirement is 4. 

 When ^^12 of the Oo are used for the induced respiration, the quantum re- 

 quirement will have the high value of 12, a value Ehrmantraut and Rabino- 

 witch (18) found in their experiments in alkaline medium. When all the 

 O2 is used, the quantum requirement will be infinite. Daniels (33) in 1938, 

 obtained values of nearly 500. 



These considerations show that the over-all reaction of photosynthesis 

 depends upon relatively small changes in the back-reacting amounts of O2. 

 The photochemical process and the back reaction mask one another. The 

 slower the back reaction, the more apparent the light reaction. 



In the dark reaction the oxygen of the CO2 molecule is loosened in such a 

 way that the action of one quantum is sufficient to break the bonds and to 

 produce one molecule O2. The CO2 derivative with the loosened oxygen is 

 probably a peroxide, indicated by (ChlGOa) *. Warburg calls it the photolyte 

 of photosynthesis (43, 64). The Oo is developed from the photolyte, inactive 

 CO2 combining with the chlorophyllproteid (reaction 1). Reaction 3 shows 

 how the inactive COo is changed into the photolyte with the aid of the energy 

 of the induced respiration. The formula of the photolyte is written as if 

 it were a simple COo derivative of chlorophyll. This is certainly not the case, 

 as the CO2 is probably bound to the protein part of the chlorophyllproteid. 

 However, the formula correctly shows that the Oo developed from the 

 photolyte under the influence of light is equivalent to the chlorophyll content 

 of the cells. This stoichiometric relationship will be discussed in § 39. To 

 avoid misunderstanding, it may be better to write the formula of the photolyte 

 as CO2* no indication being given of the site of the COo in the chlorophyll- 

 proteid. The process is represented as follows (see also § 60) 



Reaclion J (lighl) : 



COo* + Xhv + COo -> CO, + C + O2 



Reactiuit 2 {dark) : 



V3 C + V3 Oo -^ 73 COo + 70000 cal 



