QUANTUM YIELD 1955 



mum over-all requirement was 2.7 to 2.8 — the smallest thermochemically 

 possible value. After a "bright" period of 30 min., the change in the 

 rate of oxygen consumption in the first 3 min. of "dimness" was the same as 

 that observed in the first 3 min. of "dark" after 3 min. of "Hght." 



The minimum quantum requirement for oxygen liberation at the be- 

 ginning of a light period — calculated from the initial slope of the curve in 

 fig. 33. 6A — was the same (about 1.0) for carbon dioxide concentrations 

 between 5 and 50%; however, the oxygen loss in the back-reaction be- 

 came "larger than needed" above 5% CO2, thus causing a decline in the 

 over-all quantum yield. The oxygen gulp after an illumination period took 

 the same time (3 min.) at the higher CO2 pressures as at 5% CO2, but the 

 amount of oxygen consumed in this gulp was larger. At 50% CO2, the net 

 gain in the cycle was down to zero. 



The back-reaction appeared to be much faster in alkahne media; 

 at least, no oxygen gulp could be observed in carbonate buffers, even those 

 that gave an over-all quantum requirement of about 3.0 (0.1 N bicarbon- 

 ate saturated with 10% CO2, pH 7.7; or 0.2 N bicarbonate saturated with 

 2% CO2, pH 8.9). However, Burk (1952) reported that, using Chlami- 

 domonas, he was able to observe the separation of the two reaction stages 

 also in alkaline buffers. 



Assuming a first order back-reaction, Warburg and co-workers derived the equation: 



(37D.5) AOi/At = A(l - e)t + eA(l - e-^')/k 



for the rate of net oxygen Uberation, with A designating the number of absorbed ein- 

 steins of Hght, e the oxygen fraction that reacts back, and k the rate constant of the back- 

 reaction. When 6 = 1 (as in 50% CO2), no net oxj^gen production occurs. 



The experiments of other observers on the relation of photosynthesis 

 and respiration, summarized in section 3 of this chapter — in particular, the 

 unambiguous mass-spectroscopic data — taken in conjunction with the ob- 

 servations of transients by Emerson et al. (described in chapter 33, section 

 2 and in section ib) below) and by Brackett et al. (cf. section 3 above, and 

 section 4(c) below), and with the newly confirmed capacity of algae to 

 photosynthesize under completely anaerobic conditions (referred to in sec- 

 tion 2(6) above), leave httle doubt that Warburg and Burk's "one quantum 

 process" is an unwarranted generalization of arbitrarily selected data. It 

 needs hardly to be repeated that this criticism does not apply to the con- 

 cepts of a single primary photochemical process, and of energy-supplying 

 dark back reactions in photosynthesis as such (both concepts antedate the 

 investigations of Warburg and Burk, and have been made increasingly 

 plausible by recent biochemical studies), but only to the specific form these 

 concepts were given by Warburg and Burk (including the slo^vness of the 

 energy-supplying back reaction, permitting its separation from the light 



