1902 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



2. Other Chemical and Physical Factors 

 (Addendum to Chapters 1,3, 12 and 13) 



(a) Catalyst Poisons 



Cyanide. The inhibition of photosynthesis by cyanide was discussed in 

 chapter 12 (section Al). It has again been studied by Tamiya and Huzi- 

 sige, Gaffron, Fager and Rosenberg, Calvin et. al, Osterhnd, Whittingham, 

 and by BrilHant and Krupnikova. 



Tamiya and Huzisige (1949) investigated the cyanide inhibition in 

 connection with a study of the inhibition of photosynthesis by excess oxy- 

 gen {cf. section (6) below) ; they found that the effect of oxygen is weaker in 

 the presence of cyanide (and vice versa) . They suggested that cyanide does 

 not affect the first carboxylation step (which they believed to be com- 

 petitively inhibited by oxygen) , but slows down a subsequent transforma- 

 tion (reduction?) of the carboxylation product. In support of this hy- 

 pothesis, they referred to the observation of Ruben and co-workers (p. 203) 

 that the cyanide inhibition of the dark carbon dioxide uptake requires 

 as much as 10 "^ mole/1. HCN; we will see, however, that a higher sensi- 

 tivity was found by more recent observers. Furthermore, Tamiya and 

 Huzisige believed that the inhibition of photosynthesis by cyanide does not 

 depend on [CO2], and saw in this another indication that the carboxylation 

 proper is not the cyanide-sensitive step. However, studies to be described 

 below proved that cyanide inhibition is stronger the lower the CO2 concen- 

 tration. 



Gaffron, Fager and Rosenberg (1951) measured the cyanide inhibition of the fixa- 

 tion of C*02 by plants in the dark. They found, in 2 X 10 -^ M KCN, an inhibition of 

 86% if KCN was added in light, one minute before the darkening and admission of 

 C*02; an inhibition of 97% if KCN was added in light 2 mmutes before the darkening 

 and admission of C*02; and an inhibition of 79% if HCN was added simultaneously 

 with C*02 in the dark. The uptake of C* in the water-soluble fraction was particularly 

 strongly affected by cyanide. 



According to Calvin et al. (1951), cyanide (3 X 10-^ M), added to a 

 lively photosynthesizing Scenedesmus suspension two minutes before the 

 introduction of C( 14)0-2 (an addition that inhibits photosynthesis as a 

 whole by 95%), reduces the C (14) -fixation in phosphorylated sugars much 

 more strongly than its fixation in PGA, malic acid, and alanine — as if the 

 most strongly cyanide-sensitive step in photosynthesis were not the primary 

 carboxylation, but a subsequent reaction leading to sugar synthesis. 



Whittingham (1952) measured the cyanide inhibition of photosynthesis 

 in Chlorella -pyrenoidosa at different concentrations of free carbon dioxide; 

 the results are summarized in table 37D.IV. The pairs of values listed 

 under [CO2] = 7.87 X IQ-^ mole/1, and [CO2] = 0.98 X 10-^ mole/1. 



