1904 



KINETICS OF PHOTOSYNTHESIS 



CHAP. 37d 



1.5 X 10~^ mole/1.). If, at the alkaline pH value, the concentration of 

 HCN (rather than that of total cyanide) is used as abscissa, the curves 

 indicate a consistent increase in inhibition with decreasing concentration of 

 free CO2 molecules (in agreement with Whittingham's findings) — ap- 

 parently, irrespective of the amount of simultaneously present HCOs" ions. 



pH7.0 





c a^ojjM 



<^HCO- ^°A^^ 



/o 



20 JO 40 



50 



30 AO 50 

 fjM KCN 



10 20 30 



AO 50 

 uM KCN 



Fig. 37D.7. Inhibition of photosynthesis of Scenedesmus quadricauda by cyanide 

 at various pH values and various KCN concentrations (Osterlind 1952). The broken 

 curve refers to the concentration of undissociated HCN. 



Osterlind therefore classified the inhibition as "competitive." (This does 

 not necessarily mean that HCN and CO2 compete for the same enzyme; 

 a similar kinetic relation can conceivably result also from mechanisms in 

 which CO2 shortage and HCN poisoning affect two different reaction 

 "bottlenecks.") 



As mentioned in section 1(a), Osterhnd hoped that experiments with cyanide could 

 help to decide whether at high pH values, when most of the carbon entering the cells of 

 Scenedesmus quadricauda is in the form of bicarbonate ions, these ions are used for photo- 

 synthesis as such, or first converted to carbon dioxide. One possibility of interpreting 



