88 



PROBLEMS IN PHOTOSYNTHESIS 



For all further reductions, e.g., from carbohydrate to protein, 7 = 0, so that 7 

 decreases when photosynthesis is accompanied by protein formation. 



It may well be imagined that some of the energy of the induced respiration 

 is used for secondary metabolic processes. More Oo will react back when 

 carbohydrate acts as a hydrogen acceptor (in the case of protein formation) 

 than when CO., is the hydrogen acceptor. The photosynthetically produced 

 O2 will then be decreased and l/(^ will increase. However, this explanation 

 is not established. In any case, the quantum requirement measured is the 

 correct quantum requirement of photosynthesis only when 7 lies between 

 — 1 .0 and — 2.0, i.e., when growth need not be taken into consideration. For 

 reliable photosynthetic measurements use must be made of cells which produce 

 as litde protein as possible under the experimental conditions. Tolbert and 

 Zill (34, 35) and Calvin et al. (15) showed in 'C-O. experiments that ChloreUa 

 forms glycolic acid in photosynthesis (see § 57) 



2COo + 2HoO -> CH.OH-COOH + IV2 O2 



According to this reaction, 7 = - 1 .33. It may be concluded that in glycollic 

 acid formation, i.e. when CO2 serves as the only hydrogen acceptor, the quan- 

 tum requirement would have the best value. In the photosynthetic forma- 

 tion of phosphoglyceric acid 



3C0.2 + 3H,0 -> CH2OHCHOHCOOH + 2V2O2 



the photosynthetic quotient is only — 1 .2. 



§ 37 Splitting of Photosynthesis Into Light and Back Reactions 



Respiration and back reaction are closely connected processes. It is 

 possible to separate them if respiration is compensated before the beginning 

 of the experiment. Compensadng light must be of low intensity, so that 

 manometric pressure changes will be zero. On additional illumination with 

 green light, the positive pressure changes observed are due to the one-quantum 

 reaction (light reaction) as well as to the back reaction. Negative pressures 

 found after removal of the measured light must be ascribed to the back reaction 

 only. If the quantum requirement for the complete cycle (light reaction + 

 back reaction) is to be determined, the positive pressure changes on intermit- 

 tent illumination at intervals of one minute are read every hour. However, 

 if the quantum requirement of the light reaction alone is to be determined, 

 the measured light is added at intermittent intervals of three minutes. The 

 light reaction occurs almost without back reaction in the first minute of the 

 cycle, but at the end of the third minute the back reaction is nearly complete. 

 The true back reaction is observed after removal of the measured light and has 

 nearly expired after a further period of three minutes. Thus, in the cycle of 

 6 minutes (3 minutes with and 3 minutes without measured light) it is possible 

 to split photosynthesis into its components. Figure 36 shows the average 

 values of five experiments (49). The quantum yield of the light reaction 

 is 1 and that of the whole cycle 0.268. 



