8 LIGHT AND LIFE 



phores or chloroplasts. As far as we know, from studies of photo- 

 synthetic bacteria, the simplest P-O-P producing system consists of 

 very few components. The excited electron passes in it from chloro- 

 phyll to a quinone (Vitamin K or its equivalent) , from here to a 

 cytochrome and then back again to chlorophyll to replace its lost 

 electron. All that is left from the electrons circling around a cyclic 

 pathway are the terminal phosphates on ATP. This is a "closed 

 J circle" from which nothing leaks out but energy invested into ~P's, 

 (high-energy phosphate bonds) . 



The next discovery of Nature was that of pyridine nucleotides, 

 TPN, which can take over the excited electrons from chlorophyll, 

 and then send them back to it over riboflavin and cytochromes. This 

 new, somewhat more sophisticated circle is also a "closed" one and 

 may involve oxygen as a catalyst. The energy liberated by the electron 

 dropping down to its ground level again produces ATP only. 



The next discovery involved water, the mater (mother) and matrix 

 of life. The electron, taken over by TPN can be stabilized by bind- 

 ing protons. This means the decomposition of water, the details of 

 which still need clarification. The simplest assumption is to suppose 

 that the chlorophyll regains its lost electrons over a cytochrome from 

 the OH- ions, left unbalanced after the H+ was linked to pyridine. 

 The remaining (OH) radicals can then be dismutated into water 

 and Oj. 



By introducing ATP and TPN the storage problem was not yet 

 completely solved, for neither of these two substances can be stored 

 by the cell in quantity. The solution was achieved, and the way to 

 higher forms of life was opened, by involving COo. Once the cell 

 had ATP and reduced TPN, it could reduce with them CO^ to carbo- 

 hydrate. The carbohydrate could further be transformed into fat, 

 substances which could be stored in almost unlimited quantity. To 

 mobilize their energy the process of energy storage had simply to be 

 reversed, the H from carbohydrates and fats being now transferred 

 back onto TPN or DPN. The "closed" circle of photosynthesis was 

 thus opened up by the introduction of the two extraneous substances, 

 HgO and CO^,. These remarks made no pretence of originality. They 

 are based chiefly on the work emanating from the laboratory of 

 D. Arnon. 



Until recently, the classical objects of study in photosynthesis were 

 green leaves or chloroplasts belonging to the highest levels of organi- 

 zation. Only lately were the bacterial chromatophores introduced 

 as material for study. With this, research went back on the phylogen- 



