64 PROCESSES OUTSIDE THE LIVING CELL CHAP. 4 



synthesis of the intact leaf (if all three rates were related to the same 

 quantity of chlorophyll). 



One may attempt to explain Hill's results by a chlorophyll-sensitized 

 oxidation of a peroxide by ferric oxalate (cf. Eq. 4.1). As pointed out 

 by Kautsky (1938) ferric oxalate itself oxidizes hydrogen peroxide in 

 violet and ultraviolet light; this reaction could easily be sensitized by 

 chlorophyll. However, the total quantity of oxygen obtained in Hill's 

 experiments would require the presence in the chloroplasts of 0.1 mole 

 per liter of the peroxide, which is not plausible. Furthermore, only 

 one-half a gram atom of oxygen was produced for one gram atom of re- 

 duced ferric iron. For the oxidation of a peroxide, this ratio would be 1 : 1. 



light 



(4.1) Fe+++ + i H2O2 > Fe++ + H+ + ^ O2 



Thus, the substrate of oxidation must be an oxide {e. g. water) rather 

 than a 'peroxide: 



light 



(4.2) Fe+++ + \ H2O > Fe++ + H+ + i O2 



Equation (4.2) suggests that Hill's reaction is a chlorophyll-sensitized 

 reversal of the familiar oxidation of ferrous to ferric iron by oxygen, just 

 as photosynthesis is a reversal of the familiar process of combustion of 

 carbohydrates. 



In photosynthesis, oxygen can be liberated regardless of its partial 

 pressure in the atmosphere. In Hill's first experiments with isolated 

 chloroplasts, oxygen was evolved (in absence of hemoglobin) only if the 

 partial pressure of this gas was less than 1 mm. in experiments with 

 leaf extracts, and 4 mm. in experiments with ferric oxalate. Hill and 

 Scarisbrick (1940^) found, however, that the limitation was caused by 

 the reoxidation of ferrous oxalate by oxygen. If potassium ferricyanide 

 (which does not itself cause an evolution of oxygen by the chloroplasts 

 in light) was added to the mixture, it reoxidized ferrous oxalate more 

 rapidly than did oxygen, and thus allowed the latter to accumulate, 

 independently of its partial pressure, until its total quantity was equiva- 

 lent to the maximum quantity of oxyhemoglobin obtainable from the 

 same preparation. 



If no exhaustion of the oxidant (ferric oxalate) was allowed to occur, 

 the evolution of oxygen could be maintained for several hours; however, 

 it gradually became weaker, and sank to zero after five or six hours of 

 illumination. 



Hill and Scarisbrick (1940-) investigated the effects of different 

 external factors on the initial rate of liberation of oxygen and reduction 

 of ferric oxalate by chloroplasts. For the determination of ferrous 

 oxalate, they used the reduction of methemoglobin to hemoglobin (a 

 method which they considered more reliable than the complex formation 



