1128 THE LIGHT FACTOR. II. QUANTUM YIELD CHAP. 29 



holism, which makes the exact evaluation of the quantum yield difficvilt. 

 The largest 7 values ever observed by Eymers and Wassink were about 

 0.11. 



In a subsequent investigation from the same laboratory (Wassink, Katz 

 and Dorrestein 1942), a summary of additional 7 determinations for the 

 same species {Chromatium D) was given, which included values obtained 

 at two different pH values and with hydrogen as well as with thiosulfate 

 as reductant. They are shown in Table 29. IX. The figures are described 



Table 29. IX 



Quantum Yields of Chromalium under Different Conditions 

 (after Wassink, Katz and Dorrestein 1942) 



Quantum yields, \/y 



as having been calculated from ACO2, and (ACO2 -|- AH2) values of the 

 order of 50 or 100 mm.Vhr. According to the figures in text of the paper, 

 this means light intensities of the order of 1000-3000 erg/cm.^ sec. 



Sapozhnikov (1937) concluded, from his own not further described measui'ements, 

 that the quantum yield of CO2 reduction by Thiorhodaceae is 1.0. Tlie scepticism one is 

 bound to feel about an experimental finding so in variance with all other observations in 

 the field is not reduced by the thermodynamic treatment the author uses to make his 

 results plausible. He argues that the light energy required to reduce carbon dioxide de- 

 pends on the oxidation-reduction potential of the medium in which the reduction takes 

 place. Experimentally, he found this potential (in the medium in which bacteria have 

 lived for a while) to be positive enough for the reduction of carbon dioxide to be possible 

 with only 40 kcal/mole extra free energy. He suggested that, for the same reason, the 

 photosynthesis of green plants could also require only one quantum per molecule of 

 carbon dioxide. Sapozhnikov's argument ignores two facts: (a) that free energy is 

 also needed to establish and maintain the high positive redox potential, which he sug- 

 gests does exist in photosynthesizing cells; and (6) that photosynthesis is not merely re- 

 duction (of carbon dioxide) but also oxidation (of water, or of the other reductants used 

 by bact^eria), and that whatever one can gain in energy required for reduction only means 

 that correspondingly more energy is required for oxidation. 



Rieke (1949) determined the quantum yield of the green alga Scenedes- 

 mus, which was adapted (by anaerobic incubation) to the use of molecular 

 hydrogen as reductant {cf. Vol. I, chapter 6). He found, both in 4% CO2 

 and in 0.025 M KIICOs, (luantum yields of between 7 = 0.05 and 0.12, 

 i. 6'., close to those determined for ordinary photosynthesis in the same 



