Chapter 19 

 PHOTOCHEMISTRY OF PIGMENTS IN VIVO 



A. Photautoxidations in vivo* 



1. Photosynthesis, Photautoxidation, and Photorespiration 



In chapter 18, we found that most reactions sensitized by chlorophyll 

 in vitro are autoxidations, and that only a few chlorophyll-sensitized 

 oxidation-reductions have been studied. The most important reaction 

 sensitized by chlorophyll in vivo — photosynthesis — is an oxidation- 

 reduction; but chlorophyll in the cell can sensitize autoxidations as well. 

 Their substrates are either cellular reserve materials, or externally 

 supplied oxidizable compounds. 



The maximum rate of chlorophyll-sensitized photautoxidation in 

 vivo is a whole order of magnitude lower than that of photosynthesis in 

 strong light. It is therefore difficult to say whether a slow photautoxi- 

 dation may not sometimes take place simultaneously with photosynthesis 

 even while the latter proceeds at its normal steady rate. Whenever an 

 attempt is made to enhance photoxidation (e. g., by an increase in the 

 partial pressure of oxygen), it soon causes a partial or complete inhibition 

 of photosynthesis. 



There are thus two methods of studying the photautoxidation in vivo, 

 unhampered by photosynthesis. One is to inhibit photosynthesis before- 

 hand, e. g., by narcotization or by the removal of all carbon dioxide 

 from the medium; the other is to stimulate autoxidation (e. g., by in- 

 creasing the concentration of oxygen or by stepping up the intensity of 

 illumination), until photosynthesis is inhibited "autocatalytically" 

 (probably by photoxidative deactivation of some of its enzymes). In 

 both cases, oxygen liberation in light yields place to oxygen consumption, 

 whose maximum rate is three or four times higher than the rate of 

 respiration in the dark. This oxygen consumption may proceed at a 

 steady rate for several hours, without causing damage to the plant. 

 After this, photoxidation slows down — obviously in consequence of 

 exhaustion of cellular oxidation substrates — and, at the same time, a 

 bleaching of the pigments becomes apparent. Thus, contrary to the 

 earlier views of Noack (1925, 1926), the pigments are not the first sub- 



* Bibliography, page 558. 



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