REACTIONS OF CARBON DIOXIDE 93 



COOH COOH 



The activity of both systems depends on the presence of 

 divalent metal ion: of these Mn++ is found at its optimum 

 concentration (about io~^M.) to be the most effective. Mg 

 gives a lower activity and has for its optimum effect to be 

 present in higher concentration. In this connexion it should 

 be noted that extreme manganese deficiency in plants results 

 in chlorosis of the leaves; in Chlorella the addition of Mn+"^ 

 causes an almost immediate increase in rates of photo- 

 synthesis and respiration. 



In connexion with the role of coenzyme in these reactions 

 it is of interest to note that Whatley has shown that the con- 

 centration of coenzyme II (TPN) is higher in leaves than 

 in the non-green parts of many of the plant species he 

 examined, and Vennesland has shown that DPN is present 

 in green leaves in about the same amount. 



The equilibrium constant for reaction II is given as 



(^-isocitrate3-)(TPN+) 



(a-ketoglutarate2-)(C02)(TPNH) 



= 1-3 litres mole~i (22° C. ; pH y-o) 



where (CO2) represents the total concentration in solution 

 including H2CO3. Thus when the concentrations of oxidized 

 and reduced coenzyme are maintained equal with a con- 

 centration of 5% carbon dioxide (i.e. 2-23 x iq-^M.) in the 

 gas phase the molar concentration of ^-isocitrate would be 

 about 0-29% that of the ketoglutarate. Thus the equilibrium 

 is in the direction of decarboxylation for all ordinary pres- 

 sures of carbon dioxide. The same is also true for the 'malic 

 enzyme' system. Carboxylation is observed when the ratio 



