QUANTUM YIELD MEASUREMENTS BY THE MANOMETRIC METHOD 1085 



The inverse of the quantum yield (or quantum efficiency, which is the 

 same thing), is called the quantum requirement. Kegrettably, the first 

 term is often used when the second one would be appropriate — for example, 

 it is said that "the quantum yield of photosynthesis is 4" (or 8, or some 

 other number, where n > 1), instead of saying that it is V4 (or Vs, or, gen- 

 erally, 1/w). 



1. Quantum Yield Measurements by the Manometric Method 



INIost quantum yield determinations of photos>aithesis were carried out 

 by the manometric method described in chapter 25 (see fig. 25. 3 A). In 

 this method, the change of gas pressure is measui'cd first above a dai-kened, 

 and then al)ove an illuminated cell suspension. The net effects obser\'ed 

 are the result of pressure changes due to the production and consumption 

 of both carbon dioxide and oxygen. If both the respiratory quotient and 

 the photosynthetic quotient are unity, the net pressure changes are different 

 from zero only because of the greater solubility of carbon dioxide in water 

 (or, still more, in alkaline buffers), as compared with that of oxygen. To 

 obtain a check on the two quotients, the measurement can be repeated, in 

 darkness and in light, with a different ratio of gas-filled and liquid-filled 

 volumes (cf. fig. 25. 3H). 



{a) Investigations of Warburg and Negelein 



Warburg and Negelein (1922, 1923) were the first to apply the mano- 

 metric method. They worked with suspensions of the unicellular green 

 alga Chlorella. (The species was described by them as Chlorella vulgaris, 

 but subsequent experience makes it uncertain whether it was this species, 

 or C. pyrenoidosa.) To avoid the difficulties of the measurement of light 

 absorption in plants caused by scattering (cf. chapter 22), they used dense 

 suspensions, absorbing practically all the incident light. Consequently, 

 at any given moment, most of the cells were shaded, and their contribution 

 to photosynthesis was small; on the other hand, all cells contributed 

 equally to respiration. For this reason and because of the low light in- 

 tensities used (of the order of 1000 erg/cm. ^ sec), the total volume of 

 respiration was larger than that of photosynthesis. (In other words, 

 Warburg and Negelein worked below the compensation point.) They 

 noted that the respiration of Chlorella was markedly stimulated by pro- 

 longed exposure to light (cf. chapter 20, page 564) . In order to avoid such 

 changes in respiration during the experiment, Warburg used illumination 

 periods of not more than 10 minutes, separated by equal or longer periods 

 of darkness. Because of the sluggish response of the manometer to changes 

 of gas concentration in the liquid, the determination of the gas exchange 



