MECHANISM OF PHOTOSYNTHESIS I33 



Specific enzyme systems, which doubtless form part of it, 

 will eventually provide an explanation as to how the great 

 tendency for the reversal of the light-initiated reaction is 

 restrained. The discovery of a back reaction both in the 

 living cell and in chloroplast preparations which leads to 

 light emission was recently made by Strehler and Arnold 

 (Chapter 4). This seems to complete the biological picture 

 of photosynthesis in a remarkable way: so many reactions 

 in the living cell are reversible and this may now have been 

 shown to be true, in one sense, of the absorption of the 

 light itself. 



(b) hydrogen transfer in photosynthesis 



Historical 



Leaving now the consideration of the light absorbing 

 systems of the plant and taking the fact of the conversion 

 of light into chemical energy for granted, the problem of 

 photosynthesis may be considered in terms of the purely 

 chemical transformations which lead to the reduction of 

 carbon dioxide to carbohydrate. This approach can develop 

 only with the gradual discovery of all the components and 

 the definition of the course of the chemical reactions. Ever 

 since the development of biochemistry as a discipline of its 

 own the justification for the old theories based on the forma- 

 tion of formaldehyde began to look inadequate whilst the 

 purely kinetic theories became unsatisfying. Biochemical 

 fields had been widely explored and thus even though they 

 were faced by the lack of any active isolated systems from the 

 green plant, Kluyver and van Niel had by the early 1930's 

 been able to put the problem of photosynthesis in a bio- 

 chemical perspective (see Chapter 5). A few years later when 

 it was found that light could be converted into energy in a 

 chemical form by cell-free extracts from the green plant, i.e. 

 the chloroplast reaction, the direct biochemical approach to 

 photosynthesis was made possible. 



The in vitro production of oxygen 



The chloroplast system produces oxygen under the influ- 

 ence of light so long as it is supplied with a suitable hydrogen 



