CONCENTRATION OF REDUCTANTS 947 



As with variable [CO2], the effect of variations in the concentration of 

 the reductants disappears in weak Hght when the supply of light quanta 

 becomes the rate-determining factor (c/. fig. 28.5B). 



Photoreduction with mixed reductants offers an interesting kinetic 

 problem. Wassink, Katz and Dorrestein (1942) conducted some experi- 

 ments in which Chromatium cells were supplied with both hydrogen and 

 thiosulfate (at pH 6.8). They used each reductant "in excess" (meaning 

 that the quantity of each alone would have sufficed to produce saturation) 

 and calculated (indirectly, from manometric measurements) the relative 

 amounts in which the two reductants were consumed. The average was 

 about two molecules of carbon dioxide reduced at the cost of thiosulfate 

 for one molecule of carbon dioxide reduced at the cost of hydrogen. If 

 the two acceptor systems (for H2 and H2S2O3) are separate, this result may 

 mean either that the cells contain twice as much of the thiosulfate acceptor 

 system as of the hydrogen acceptor system, or that the first acceptor re- 

 acts (in the hydrogenated state) twice as rapidly with the activated photo- 

 complex as the second one. On the other hand, if only one hydrogen ac- 

 ceptor is present, and the two donors compete in supplying it with hydro- 

 gen, then the result, as reported, is not very significant, since, in this case, 

 the relative utilization of hydrogen and thiosulfate would depend on rela- 

 tive concentrations (even if both reductants are present "in excess" — the 

 only information provided). 



In chapter 22, we mentioned the spectroscopic experiments of the same 

 investigators that made them think that the composition of the photocom- 

 plex X-Bchl-HZ (Bchl = bacteriochlorophyll) may itself be specific for 

 each reductant, i. e., that bacteria contain a multiplicity of complexes, 

 X- Bchl' -HZ', X- Bchl" -HZ" • • •, adapted to the utihzation of the several 

 reductants R'H, R"H, • • • • (This hypothesis was suggested to explain 

 the multiplicity and varying relative intensities of the absorption bands of 

 bacteriochlorophyll m vivo.) Wassink and co-workers now argued that, if 

 this were true, the photoreduction of carbon dioxide by a mixture of reduc- 

 tants would be additive, rather than competitive. Since rate measurements 

 indicated competition (the rate of total gas consumption in the presence of 

 both reductants always was lower than in pure hydrogen and larger than 

 in pure thiosulfate), the Dutch investigators concluded that their earlier 

 explanation of the spectroscopic phenomena w^as incompatible with the 

 results of kinetic studies. However, the argument would only be fully con- 

 clusive if it were definitely known that, under the conditions of the experi- 

 ment, the rate of photoreduction was limited by the amount of the reduc- 

 tant available for reaction with the photocomplex. It was mentioned above 

 that saturation of the over-all rate with respect to the reductant can often 

 be due to a limitation elsewhere in the photosynthetic apparatus, e. g., to 



