KINETICS 1593 



Quantum Yield. French and Rabideau (1945) measured the quantum 

 yield of the oxygen hberation by chloroplasts from Spinacia and Trades- 

 cantia (with ferric oxalate as oxidant), and obtained figures ranging from a 

 minimum of 15, up to 50 or more quanta per mole oxygen (c/. chapter 29, 

 Table 29.X). The variations must have been due to differences in the 

 state of the chloroplasts; subsequent experiments (sec. 2 above) made it 

 appear possible that duration of illumination prior to the grinding of the 

 leaves has been the most important factor. The quantum yield was < }/2 

 of that obtained in a parallel set of experiments with live Chlorella cells in 

 carbon dioxide. 



In Chapter 29 we also tabulated the quantum yields found by Ehrman- 

 traut and Rabinowitch (1952). Most of these were for whole Chlorella 

 cells (c/. part C of this chapter) . Measurements were made, however, also 

 with chloroplasts from Phytolacca, using quinone, Hill's mixture and ferri- 

 cyanide as oxidant. The results were close to those obtained with live 

 cells (I/t = 9 to 12, cf. table 29.XI). Since similar yields were obtained 

 also for the oxygen production by Chlorella cells in carbonate buffers, they 

 were considered as supporting the hypothesis that the primary photochemi- 

 cal process in the Hill reaction is the same as in photosynthesis. 



Warburg (1952) arrived at entirely different conclusions. He measured the quan- 

 tum yield of the Hill reaction in spinach chloroplasts with quinone as oxidant, at different 

 wave lengths. His preparations (which he designated as "green grana," cf. section 

 2(6) above) gave I/7 = 65, 73, 85 and 101 quanta per O2 molecule for \ = 366, 436, 480 

 and 644 uifj., respectively. Warburg noted that these quantum yields were proportional 

 to wave lengths, so that the energy yields (e on p. 1083) were constant (about 1%) 

 throughout the spectrum (including the region of strong absorption by the carotenoids). 

 He concluded that the Hill reaction is different from photosynthesis not only in that it 

 has a very low photochemical efficiency, but also in that it has a X-independent energy 

 yield, «, while photosynthesis has an extremely high efficiency and a X-independent 

 quantum yield, 7. It will be noted that the disagreement is in this case opposite to that 

 obtained in the study of the quantum efficiency of photosynthesis, Warburg's efficiencies 

 being much lower than those found by other observers. He seems to have used chloro- 

 plast preparations of low activity — as witnessed by their failure to give any reduction of 

 Hill's mixture at all. 



Light Curves. Hill and Scarisbrick (1940) gave the earliest hght curve 

 of the Hill reaction; it was reproduced in fig. 6 (p. 65, Vol. I), and showed 

 saturation in the region of 20 klux. Aronoff (1946^) found that the rate of 

 oxygen liberation from spinach "grana," with quinone as oxidant, was pro- 

 portional to hght intensity up to 13 klux; (cf. fig. 35.21). Spikes, Lumry, 

 Eyring and Wayrynen (1950) reported that the light curve for spinach 

 chloroplasts, with ferricyanide as oxidant, is a rectangular hj'-perbola. 



Holt, Brooks and Arnold (1951) gave a light curve of the oxygen pro- 

 duction by a chloroplast suspension from Phytolacca americana with an 



