306 



CATALYST POISONS AND NARCOTICS 



CHAP. 12 



without injury to photosynthesis — a reversal of conditions prevaiHng in 

 Chlorella and Stichococciis. The addition of 2 X 10~^ m./h potassium 

 cyanide will leave photosynthesis in Gaffron's Scenedesmus Dl unaffected, 

 but will suppress respiration almost completely. The "compensation 

 point" of Scenedesmus Dl is shifted by cyanide towards lower light 

 intensities, so that, in weak light, consumption of oxygen may be replaced, 

 upon the addition of cyanide, by an evolution of this gas. The effect 

 of higher concentrations of cyanide on the photosynthesis of Scenedesmus 

 Dl is shown by table 12.VI. Low concentrations of HON (e. g., 5 X 10~^ 



Table 12.VI 



Inhibition of Photosynthesis by HCN in Scenedesmtts Species Dl 



(after Gaffron) 



m./l.), which have no effect on photosynthesis in Scenedesmus Dl at 

 5000 or even 10,000 lux, will cause an inhibition if the light intensity is 

 increased still further, approaching the "saturating" value (which lies 

 at about 30,000 lux). 



Nakamura (1938), who believed that catalase plays an important 

 part in photosynthesis (cf. page 284), refused to admit the existence of 

 plants whose photosynthesis is resistant to cyanide (since catalase is 

 invariably inhibited by this poison). He suggested that the results of 

 Gaffron on Scenedesmus Dl (and implicitly — also those of van der Paauw 

 on Hormidium) should be attributed to experimental errors (for example, 

 a rapid decomposition of the cyanide). In support of this criticism, he 

 quoted his own results with Scenedesmus nanus. However, table 12.V 

 reveals such wide variations in the cyanide sensitivity of different algae> 

 that conclusions by analogy, not only from species to species, but even 

 from strain to strain, are unconvincing, and we see no reason to doubt 

 Gaffron's results more than any others. As a matter of fact, we have 

 even used these results in chapter 11 (page 286) as a decisive argument 

 against the hypothesis that hydrogen peroxide is an intermediate in 

 photosynthesis. 



Differences in absolute and relative sensitivity to cyanide of photo- 

 synthesis and respiration in different plants can be caused either by 

 qualitative factors — as variations in the chemical structure of the relevant 

 enzymes — or by differences in the available quantities of otherwise iden- 

 tical enzymes. Although the second explanation seems to be more plau- 



