92 



OXIDATION-REDUCTION POTENTIALS 



OXYGEN EVOLUTION BY CHLOROPLASTS 



Isolated chloroplasts in the presence of light and an appropriate hydrogen 

 acceptor evolve oxygen (Hill, Warburg, French and others). The energy of light is 

 utiUsed to produce oxygen and at the same time to effect reductions. Experiments 

 with water enriched with the 0^^ isotope indicate that the oxygen arises from the 

 photochemical decomposition of water. 



2H2O -> 4H + O2 



As hydrogen acceptors, ferricyanide, chromate, quinones and a number of 

 oxidation-reduction potential dyes have found to be effective. Aronoff found a 

 relationship between the normal oxidation-reduction potential of quinoid substances 

 and the rate of oxygen evolution in their presence. The following table is given by 

 Holt and French (1948). 



TABLE 21 

 Deooloueisation of Dyes by iLLXXMrNATED Chloroplasts at pH 6-6 



Thionine showed rather exceptional behaviour in its rate and extent of decolour- 

 isation and some other effect may have interfered. It is necessary to use dilute 

 solutions of dyes since highly coloured solutions absorb so much Hght that the 

 photochemical reaction cannot occur. By using dilute dyes it may be possible to 

 investigate isolated constituents of the system. It appears possible that a combina- 

 tion of chlorophyll and protein behaves as an enzyme system, but the mechanism of 

 the reactions require further study. 



PLANT RESPIRATION 



It is of interest to consider in what respects the metabolic cycles of plants resemble 

 those of higher animals and bacteria. A fundamental difference, the utilisation by 

 plants of the energy of light in effecting photosynthesis has already been mentioned 

 briefly in a previous section. 



The phosphorylation of starch as an initial process in glycolysis is closely similar 

 to glycogen phosphorylation. Plants, however, make considerable use of sucrose, 

 whereas animals have to invert the sucrose before it is broken down further, and many 

 bacteria are unable to utilise sucrose. It appears likely that hexose diphosphates 

 and monophosphates, triosephosphates, phosphoglycerate, pyruvate, acetaldehyde 



