38 Introduction 



cultivated. In the A cells, which give the best quantum yields in light, Saturation 

 with photolyte is reached when the carbon dioxide pressure equals 10 to 20° of an 

 atmosphere; the concentration of photolyte then usually equals the concentration 

 of Chlorophyll. 



The photolyte arises only under aerobic conditions, when the cells are respiring ; 

 it decomposes, giving off carbon dioxide, when the cells are deprived of oxygen. 



The formation of the photolyte is inhibited when the cells are suspended in 

 arsenate instead of phosphate, but the amount of photolyte formed reverts to its 

 normal value when phosphate is added to the arsenate Solution. 



The photolyte content is closely related to the glutamic acid content. If glutamic 

 acid in living Chlorella is permitted to decompose into y-aminobutyric acid and 

 carbon dioxide, the photolyte will also decompose and thereby give off carbon 

 dioxide; if glutamic acid is resynthesized in living Chlorella, the photolyte is 

 formed once more. In A cells, which give the best quantum yields, the photolyte 

 concentration is usually found to equal the glutamic acid concentration. 



All the above-mentioned reactions of the formation and degradation of the pho- 

 tolyte are very rapid; they are partial reactions of the mechanism of photosyn- 

 thesis and have nothing in common with the slow reactions of protein synthesis, 

 transaminations, etc. For example, the half-life period of the formation of photolyte 

 during a change from anaerobic to aerobic conditions is 5 minutes at 20° C. 



The first reaction in the formation of photolyte may be the reversible phos- 

 phorylation of carbon dioxide by ATP (adenosine triphosphate) 



H2CO3 + ATP ;= HCO3H2PO3 + ADP 



a reaction that explains the dependence on carbon dioxide pressure and on respi- 

 ration. The carboxy-phosphate formed then enters into a reversible reaction with 

 glutamic acid, the resulting glutamic acid Compound reacts reversibly with Chloro- 

 phyll, (cf. Seite 567 und 627) 



7. Light-induced respiration of grana 



The following properties of grana are here discussed : 



1. Grana from leaves or from Chlorella have no dark respiration. 



2. Grana although they develop oxygen in light cannot fix carbon. 



3. Oxygen development by grana in light is not inhibited by hydrocyanic acid. 



It may be added that respiration in Chlorella can easily be destroyed, for instance 

 by very small amounts of quinone, and that Chlorella by such treatment acquires 

 the properties of grana inasmuch that it can no more fix carbon and that oxygen 

 development in light is no more inhibited by hydrocyanic acid. 



It has been one of the main problems of the grana reactions how it is possible 

 energetically, that grana develop oxygen from carbon dioxide, although they have 

 no respiration at their disposal, the energy of which could transform carbon dioxide 

 into the photolyte. The answer is that grana have no dark respiration but that they 

 have light-induced respiration. 



