EFFECT OF CATALYST POISONS 1905 



fig. 37D.7 is to postulate that the reaction proceeds entirely through CO2 molecules; 

 however, a possible alternative is to assume that HCOs" ions also enter photosynthesis, 

 but their participation is inhibited by cyanide in a "noncompetitive" way {i. e., in the 

 same proportion at all [HCOs"] concentration). 



Gaffron (1953) also found that the effect of cyanide on the photo- 

 synthesis of Chlorella is dependent on CO2 concentration. The curves in 

 fig. 37D.8 show that adding cyanide has the same influence as removing a 

 certain fraction of carbon dioxide. 



Brilliant and Krupnikova (1952) confirmed, on several species of 

 algae and higher aquatic plants, an observation which Emerson and Arnold 

 first made on Chlorella — that the effect of cyanide on the rate of photo- 

 synthesis in flashing light declines as the intervals between the flashes are 

 prolonged. (These experiments have been described in chapter 34, section 

 B3.) With some algae, addition of cyanide was observed to cause not only 

 complete inhibition of respiration, but also oxygen liberation in the dark 

 subsequent to a period of intense illumination — an observation which, if 

 confirmed, may be significant for the identification of the cyanide "block" 

 in photosynthesis. 



Whittingham's observations (table 37D.IV) added another instance of 

 cyanide affecting photosynthesis below the compensation point — as pre- 

 viously found by van der Paauw with Hormidium, but not by Warburg with 

 Chlorella or by van der Paauw with Stichococcus* According to Franck, 

 the extent to which respiration intermediates can be utiHzed by the photo- 

 synthetic apparatus, depends on growth conditions and pretreatment of the 

 cells. This offers a possible explanation of why different species (or even 

 different cultures of Chlorella) have shown such a different response to 

 cyanide below the compensation point: cells capable of using respiration 



* It was suggested on p. 309 that the cells which show no cj^anide-resistant residual 

 photosynthesis are the ones in which respiration is as sensitive (or more sensitive) to 

 cyanide than photosynthesis. Van der Paauw (personal communication) pointed out 

 that one cannot quote, in this context, his data on Hormidium together with Gaffron's 

 observations on Scenedesmns, since only one among his many experiments with Hormi- 

 dium had shown an inhibition of respiration equal to that of photosynthesis; in all 

 others, respiration was much less affected. 



Concerning the stimulation of photosynthesis of Stichococcus in weak light by cj^- 

 anide, whose reaUty was questioned on p. 309, van der Paauw himself had said that it 

 "could not have been real." Since the enhanced rate of photosynthesis was calculated 

 by using as correction the — strongly cyanide-stimulated — rate of respiration, he sug- 

 gested that respiratory stimulation may disappear in light. One could hypothesize 

 that in the presence of cyanide, respiration intermediates become more easily available 

 for re-absorption into the cycle of photosynthesis (as assumed in Franck's interpretation 

 of Burk and Warburg's experiments, cf. section 4 below), and that in this way respira- 

 tion can be compensated with a quantum requirement much lower than that of true 

 photosynthesis. 



