OXYGEN LIBERATION BY ISOLATED CHLOROPLASTS 



1129 



species. These yields could be measured at light intensities up to 7000 

 erg/ cm.- sec; at the higher intensities, transition to ordinary photosyn- 

 thesis occurred too rapidly. 



4. Quantum Yield of Oxygen Liberation by Isolated Chloroplasts 



French and Rabideau (1945) measured the quantum yield of the "Hill 

 reaction" (photochemical oxygen production from ferric oxalate solution, 

 sensitized by a chloroplast suspension). This reaction was described in 

 chapter 4 (Vol. I) as possibly representing ''one half of photosynthesis" — 

 namely, photoxidation of water, with the ferric salt instead of carbon di- 

 oxide serving as oxidant. Chloroplast suspensions were obtained from 

 spinach, or from Tradescantia, by maceration and centrifugation, and ad- 

 ded to 0.5 M K2C2O4 + 0.01 M FeNH4(S04)2 + 0.02 M K3Fe(CN)G. The 

 solution also contained 0.20 M sucrose and 0.17 M sodium sorbitol borate 

 buffer. A 10% NaOH solution was present in a side arm of the manome- 

 tric vessel to absorb carl)on dioxide (which could be produced by respira- 

 tion). Figure 29.7 shows the course of pressure changes in a Warburg ap- 



LU 

 (T 



CO 



cr 



Q- 



UJ 



o 



X 



o 



20 



TIME, min. 



30 



40 



Fig. 29.7. Gas liberation in light from suspension of 

 spinach chloroplasts (after French and Rabideau 1945). 



paratus filled with this mixture. The light used was a red band at 660- 

 720 mM, with a maximum at 685 m^. The measurements were made at 

 10° C. Table 29.X gives some typical results obtained with chloroplasts 

 from spinach. Material from Tradescantia gave somewhat lower yields. 

 The quantum yields varied between 0.013 and 0.080, or between 12 and 

 78 quanta per molecule of liberated oxygen. The average was 7 = 0.042 

 for chloroplasts from spinach, and 7 = 0.030 for chloroplasts from Trades- 



