CHAPTER 4 



STUDIES WITH THE LIVING PLANT: THE 

 PHYSIOLOGY OF PHOTOSYNTHESIS 



The early work in photosynthesis briefly described in Chap- 

 ter I was due largely to chemists and contributed to the early 

 development of the chemistry of gases. The next advances 

 in the study of photosynthesis were the result of work in 

 plant physiology. Plant physiology may be defined as the 

 study of the response of living plants or parts of living plants 

 to variable external and internal agencies. The approach is 

 causative; the responses being related in this sense to speci- 

 fied physical or chemical changes in the internal or external 

 environment. The single cell is often taken as the unit of 

 activity but ultimately it becomes necessary to consider the 

 subcellular organization of the cell, especially when we wish 

 to analyse a particular process in chemical terms. Thus the 

 study of fermentation was rapidly advanced by the discovery 

 that fermentation could be made to occur in cell-free ex- 

 tracts. This w^as the origin both of the name and the concept 

 of an enzyme, i.e. a catalyst which would function apart 

 from the living cell as a whole. From this point in the study 

 of fermentation the biochemical and the physiological 

 aspects seemed to diverge, the latter studying responses of 

 whole living cells whilst the former investigated individual 

 reactions in a cell-free extract obtained in as pure a form 

 ^s possible. Ultimately however it is clear that the two 

 approaches are complementary and that both are necessary 

 in order to relate the response of the living cell to specified 

 isolated reactions. It must be emphasized that the two ap- 

 proaches differ only in technique and in nothing else. Until 

 very recently it was not possible to obtain any reactions in 

 cell-free extracts which could be brought to bear on the 

 physiology of photosynthesis. Thus up to the middle nine- 

 teen-thirties the very large literature of photosynthesis was 



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