EFFECT OF CATALYST POISONS 1903 



correspond to different pH values; their identity confirms that only the 

 concentration of free HCN molecules is significant for inhibition (as was 

 suggested on p. 301). 



Table 37D.IV 



Inhibition of Photosynthesis of Chlorella pyrenoidosa, in Saturating Light, by 



3.8 X 10~* Mole per Liter HCN (only Undissociated Molecules are Counted!), 



AT Different [CO2] (after Whittingham 1952) 



[CO2], m./]. X 10-5.... 179 



pH 4.6 



% inhibition 7.2 



Table 37D.IV shows the inhibition to increase with decreasing carbon 

 dioxide concentration. 



In varying the concentration of HCN, Whittingham obtained the re- 

 sults shown in table 37D.V. 



Table 37D.V 



Effect of [HCN] on Rate of Photosynthesis of Chlorella in Saturating Light and 

 Saturating [CO2] (7.87 X 10 "^ Mole per Liter) (after Whittingham 1952) 



[HCN],m./l.. 



R 1.5; 1.4 



P(net) 21.7; 20^ 



Half-inhibition was reached, in Whittingham's experiments, at about 

 3 X 10"^ mole/1. HCN (in satisfactory agreement with the data in table 

 12. V). 



Hill and Whittingham (1953) — similarly to Tamija, and to Calvin — 

 suggested that cyanide inhibits, in photosynthesis, not the carboxylation 

 reaction itself, but a reaction coupled with it, which, they suggested may be 

 an oxidation. According to some recent evidence, (c/. chapter 36, section 

 00), the most likely mechanism of carbon dioxide uptake in photosynthesis 

 is a reaction with pentose diphosphate, leading to two molecules of phos- 

 phoglyceric acid. Although the net reaction involves, in this case, no oxi- 

 dation or reduction, it seems to require the presence of pyridine nucleotides 

 (and thus probably involves a reversible oxidation-reduction). 



Osterhnd (1952) made similar experiments with Scenedesmus quadri- 

 cauda. The relative inhibition, (presumably in saturating light) was 

 measured as a function of [KCN] at different pH and [CO2] values (and 

 correspondingly varying [C02]/[HC03~] ratios). Fig. 37D.7 shows the 

 effect between pH 6.0([CO2] = 5.6 X 10-^ mole/1.) and pR 9.9 ([CO2] = 



