94 S CONCENTRATION FACTORS CHAP. 27 



the deficiency of a "finishing" catalyst such as Eg. Whenever this is the 

 case, the rate of photoreduction cannot he increased by increasing the avail- 

 able quantity of the reductant or by adding a second reductant and thus put- 

 ting to work the (otherwise idle) photocomplexes specifically adapted to it. 



In kinetic work with purple bacteria, one has to keep in mind their 

 capacity to utilize organic materials — including intercellular ones — as re- 

 ductants. Competition rather than additivity seems to be the rule in this 

 case too. The "sigmoid" shape of the light ciu'ves of hydrogen consump- 

 tion, noted both by French, with Streptococcus varians, and by the Dutch 

 observers, with Chromatium (cf. figs. 28.8), can perhaps be interpreted as a 

 consequence of such competition: In weak light, intracellular reductants 

 (pei'haps sugars or their derivatives) supply all the hydrogen necessaiy for 

 the slowly i^roceeding ])hotoreduction of carbon dioxide, and, therefore, 

 only very little external hydrogen is used. In stionger light, the diffusion 

 supply of internal hydrogen donors proves insuffi(;ient, and the more 

 rapidly diffusing molecular hydrogen takes over as the main reductant. 



To minimize the role of internal reductants, and thus to obtain light 

 curves without a sigmoid initial section, Wassink (1942) has attempted to 

 starve the bacteria before the experiment; however, he found no significant 

 change in results. 



The experimental material on the "reductant curves" of j^hotoreduc- 

 tion is as yet rather limited and no attempts have been made to represent 

 these curves analytically. If one accepts the general scheme of photo- 

 synthesis given in scheme 7.1 (Vol. I, page 153), the kinetic role of the re- 

 ductants appears symmetrical to that of carbon dioxide. An analytical 

 treatment of the effect of reductants on the over-all rate would thus have to 

 deal with the same type of partial processes as were treated in the analysis 

 of the carbon dioxide factor, namely, supply by diffusion, preparatory 

 catalytic dark reactions (such as binding of hydrogen by an acceptor with 

 the help of the hydrogenase), lilieration of the acceptor from the primary 

 oxidation product, and "finishing" dark reactions (such as stabilization of 

 the primary oxidation product). According to Franck's concept, re- 

 peatedly mentioned before, the finishing dark reactions on the oxidation 

 side (which consist in the elimination of the primary oxidation products — ■ 

 "photoperoxides" — either by their conversion to molecular oxygen or by 

 their reduction with reductants such as hydrogen or hydrogen sulfide) 

 have the peculiar property that their failure to keep pace with the primary 

 photoprocess leads not merely to the loss of a large proportion of primary 

 products by back reactions but also to a reaction between them and oxidiz- 

 able metabolites. This side reaction produces, according to Franck, a 

 "narcotic," capable of enveloping chlorophyll and stopping the primary 

 photochemical process. 



