460 FURTHER EVOLUTION 



Thus it is probable that ' active hydrogen ' enters the cycle 

 of the assimilation of CO2 as reduced forms of pyridine nucleo- 

 tides which take part in the reaction whereby CO2 is reduced 

 as far as carbohydrate (system 3). 



The elaboration in the chloroplasts (in system 5) of 

 reduced pyridine nucleotides, using the hydrogen which was 

 formed in the initial photochemical reaction, has also been 

 demonstrated in model experiments. In particular, as early 

 as 1949 A. A. Krasnovsk.il and his colleagues^^' succeeded 

 in this way in showing that chlorophyll sensitises the transfer 

 of hydrogen to pyridine nucleotides and to flavines, the 

 energy of light being accumulated in the products of the 

 reactions. These authors put forward the hypothesis that it 

 is just this reaction which links the light-induced stage with 

 the process of reduction of cOg."" 



Somewhat later W. Vishniac and S. Ochoa^*^ shov/ed that, 

 in fact, isolated chloroplasts, together with homogenates, can 

 reduce photochemically the pyridine nucleotides which are 

 associated with the enzymic stages of the assimilation of 

 CO2. This is confirmed by the finding of various dehydrogen- 

 ases in chloroplasts.^*^ 



The methods of formation of high-energy phosphorus 

 compounds (system 6) in the process of photosynthesis have 

 still been only very poorly studied, as was rightly remarked 

 by R. Hill.^*^ It is evident that the cell of the green plant 

 has a number of mechanisms for carrying out this task, which 

 it borrows from the metabolic arsenal of enzymes of the 

 heterotrophs and the chemoautotrophs. Associated with these 

 mechanisms are the specific pathways of photochemical phos- 

 phorylation. According to Hill, the energy needed for the 

 esterification of inorganic phosphate is obtained, in green 

 plants, by the oxidation of whatever compounds have been 

 reduced under the influence of light. The energy obtained 

 by such oxidation is accumulated in high-energy phosphorus 

 compounds and in this form it enters into the photosynthetic 

 cycle of reduction of fixed CO2. 



The recent work of D. I. Arnon and colleagues^** has 

 shown that when isolated chloroplasts are illuminated they 

 can form ATP from ADP and inorganic phosphate ; this 



