CHAPTER 8 

 THE MECHANISM OF PHOTOSYNTHESIS 



(a) the light reaction 

 Conversion of light into chemical energy 



A DESCRIPTION of the mechanism of the conversion of Hght 

 energy into a chemical form by the green plant presents a 

 problem which lies on the borderline of physics and 

 chemistry. This may be true of all the cases of energy con- 

 version from one form to another when they are associated 

 with living organisms. The most complete understanding of 

 such processes can best be reached in terms of the experi- 

 mentally observed properties of the isolated components in 

 relation to one another, provided that these relationships 

 can be proved to hold in the living cell. 



The brilliant researches of Willstatter, the insight of 

 Kiister, and the chemical studies of H. Fischer demon- 

 strated how the pigment chlorophyll may be isolated, its 

 molecular structure determined and its properties studied. 

 The wealth of chemical and of physical information now 

 available cannot however wholly be applied to the pigment 

 as it exists in the cell. This is because once the pigment has 

 been isolated in pure form it has not been found possible 

 to have it in the same state as it is in the living cell. As we 

 discussed in Chapter 2 chlorophyll is present, apparently, 

 in combination with some specific cellular constituent, very 

 possibly a protein. This complex has not yet been isolated 

 in an unmodified form from the plastids, in contrast with 

 the haem protein compound haemoglobin which has been 

 isolated from the red blood corpuscle and for which it has 

 been possible to relate the properties of the pure substance 

 to its physiological function. It follows that the properties 

 of chlorophyll alone can give but a partial indication of the 

 mechanism of its function in the cell. Nevertheless the 



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