PHOTOSYNTHESIS 



of inorganic or organic hydrogen donors (purple bacteria, 

 unicellular algae). 



Unique to the green plants is the coupling of photochemical re- 

 duction with an evolution of free oxygen. Such an evolution of oxygen 

 can be obtained by illuminating isolated chloroplasts in the presence 

 of oxidizing substances like ferric salts or jf?-benzoquinone (5,9,21). 

 The existence of several more or less autonomous enzyme systems 

 working together can best be demonstrated in unicellular algae be- 

 longing to the Scenedesmaceae. In these algae it is possible to inter- 

 change at will all three known types of carbon dioxide reduction; a 

 chemoreduction promoted in the dark by the burning of hydrogen 

 with oxygen to water; a photoreduction in which two hydrogen 

 molecules disappear together with a molecule of carbon dioxide; and 

 a complete photosynthesis with the liberation of an equivalent amount 

 of oxygen (7). 



A rough summary of the way in which the problems of carbon 

 dioxide reduction in an alga like Scenedesmus can be subdivided accord- 

 ing to present knowledge is given in the scheme below. More complete 

 and complicated schemes can be found in a recent review (7) and in 

 Rabinowitch's new book (14). 



COj 



fixation of 

 carbon dioxide 



/ 



carboxyl group 



carbohydrate 



i 

 (CHjO), 





Light absorbed by plant pigments 

 (chlorophylls, xanthophylls, phycocyanins) 



V . . t . 



photochemical reaction dismutation to 



(chlorophylls, water, enzymes) oxygen and water 



^ . ^.- ^ 



intermediate intermediate 



hydrogen a hydrogen 



donors > .2 < acceptors 



\ 



intermediate 

 hydrogen 

 donors 

 \ 



u 



u 



\ 



hydrogen 



acceptors 



Oj 



oxidoreduction 



(different from 



respiration) 



intermediate 

 hydrogen 

 acceptors 



/ ^ 



hydrogen 

 donors 



H2 



reduction to water 



i 

 H,0 



35 



