CYANIDE INHIBITION OF PHOTOSYNTHESIS 



301 



A. Catalyst Poisons* 

 1. Cyanide Inhibition of Photosynthesis 



Experiments on cyanide poisoning have usually been carried out 

 with algal suspensions in aqueous solutions (although it is also possible 

 to poison land plants by gaseous hydrocyanic acid). The poisonous 

 molecular species is the nondissociated acid, HCN, and many measure- 

 ments — as those of Warburg and Emerson — have been referred to its 

 concentration rather than to the total concentration of the cyanide. At 

 a given pH, the concentrations of HCN molecules in a solution whose 

 total cyanide concentration is [Cy] can be calculated from the equation: 



(12.1) 



[HCN] = 



[H+][Cy] 

 K + [H+] 



where K (the dissociation constant of hydrocyanic acid) is approximately 

 5 X 10-1" (at 20° C). Consequently, [HCN] is almost equal to [Cy] 

 for pH < 8.5; the difference becomes marked only in more alkaline 

 solutions, as in carbonate-bicarbonate buffers (c/. page 178). The 

 assumption that only the neutral molecules, HCN, are poisonous, 

 reminds one of the postulate that only the neutral molecules, CO2, take 

 an active part in photosynthesis (cf. page 195), and can be explained in 

 a similar way, that is, by selective cell penetration by neutral molecules. 

 The sensitivity of photosynthesis to cyanide was discovered by 

 Warburg in 1919. (The poisonous effect of cyanide on the respiration 

 of plants was known before.) Warburg found that 3.8 X 10"^ mole per 

 liter HCN decreases the rate of photosynthesis in Chlorella (in strong 

 light and in the presence of abundant carbon dioxide) by almost 50%. 

 Equally strong is the cyanide effect on the respiration of nonchlorophyllous 

 plants {e. g., yeast); but the respiration of Chlorella, instead of being 

 poisoned by 3.8 X 10"* m./l. HCN, was stimulated by about 57%. The 

 lowest cyanide concentration which produced inhibition (by — 18%) 

 was 10-2 m./l. Even in 0.1 molar hydrocyanic acid, the respiration of 

 Chlorella still proceeded at 40% of its normal rate. Thus, the presence of 



* Bibhography,' page 324. 



