1126 THE LIGHT FACTOR. II. QUANTUM YIELD CHAP. 29 



saturated (because of supply limitations, or because of the limited amount 

 of a necessary enzyme) ; the normal reduction of carbon dioxide with ex- 

 ternal reductants then comes into its own. The manometrically deter- 

 mined quantum yield of the photochemical process using intercellular sub- 

 strates can be smaller than that of normal photoreduction, for two reasons: 



(a) When hydrogen is used as external reductant, any utilization of non- 

 volatile, intracellular reductants will diminish the rate of gas consumption, 

 even if the quantum yield of carbon dioxide reduction is the same with both 

 types of reductants. In French's experiments (1937) with Streptococcus 

 varians, the light curve had total gas consumption, AH2 + ACO2 as 

 ordinate; its sigmoid shape may have been due pntirely to an initial de- 

 ficiency in the consumption of hydrogen alone. In the experiments of the 

 same author with Spirillum ruhrum (1937^) a nonvolatile reductant was 

 used, and the curve showing ACO2, as function of light intensity, showed no 

 inflection. However, in the more recent experiments of Wassink, Katz and 

 Dorrestein (1942) with Chromatium, sigmoid curves were obtained not only 

 with hydrogen but often also with thiosulfate as reductant {cf. fig. 28.11). 

 A different explanation is needed in this case. 



(6) It was described in chapter 5 (Vol. I, page 106) how, when exter- 

 nally supplied organic compounds are utilized by photosynthesizing purple 

 bacteria, the proportion of "coassimulated" carbon dioxide can vary widely 

 (or carbon dioxide may even be liberated), depending on whether the or- 

 ganic compound is utilized mainly or exclusively as hydrogen donor (as in 

 Foster's experiments with secondary alcohols), or serves also as the source of 

 carbon. Wassink, Katz and Dorrestein suggested that the same applies to 

 photochemical utilization of intracellular organic materials; here, too, the 

 consumption of external carbon dioxide may be more or less completely 

 suppressed by the utilization of the carbon (in the form of freshly formed 

 carbon dioxide, or of oxidation intermediates) produced by dehydrogenation 

 of the organic reductant. 



These two considerations provide a plausible explanation of sigmoid 

 gas exchange curves, but do not fully justify calculation of the maximum 

 quantum yield of photoreduction from the slope of the steepest section of 

 the light curve. In the first place, the utilization of internal reductants is 

 at present merely a hypothesis. In the second place, assuming this hypo- 

 thesis is correct, it is still possible — and, indeed, likely — that, as light in- 

 tensity increases, photoreduction replaces (and not merely supplements) 

 the photochemical transformation of intercellular substrates. This can 

 occur either because of exhaustion of the intracellular material, or because 

 of changes in the enzymatic system (as in the "de-adaptation" of hydrogen- 

 adapted green algae; cf. Vol. I, chapter 6). More precise measurements 

 of the photosynthetic ratio A [CO2]/ A [reductant], at ditTerent light intensi- 



