1738 



CHLOROPLASTS, CHROMOPLASTS AND CHROMATOPLASM CHAP. 37A 



showed that suspensions of chloroplast fragments (consisting predomi- 

 nantly of grana), suspended in a silver nitrate solution, liberate silver in 

 light. Whereas, according to Metzner, the photochemical reduction of 

 AgNOs in "living" chloroplasts is best observed at pH 3, the same reac- 

 tion in "grana suspensions" was fastest in neutral solution. The rate in- 

 creased with light intensity slower than linearly, but without definite indi- 

 cation of saturation, which would indicate the participation of an enzyme — 



200 



ISO 



100- 



50- 



• Christiansen-Weichert 



o -• ■■ *lcm6°/oCuS0t 



m ■■ ■*R62*B620 



a - ■ - • -fRGS 



4 •- ■■*RG5 



a •' .+RGe 



750 



700 



650 



600 



550 



500 



400 



450 mfj 



Fig. 37A.19. Three sections of the action spectrum of Molisch reaction (Ag 

 precipitation from AgNOs in acid solution) in a suspension of chloroplasts from 

 Hibiscus rosa sinensis, obtained with different sets of filters (after Thomas, Post 

 and Vertregt 1954). Abscissae: relative rates, measured by increase of opacity 

 with time (rate in Na light = 100). Light filters as indicated in figure. 



in agreement with Metzner's observations that it can occur also in cells 

 killed by silver nitrate; Thomas et al. found that 10 minutes boiling of the 

 suspension also did not stop the reaction. 



Experiments with colloidal suspensions of pure chlorophyll in silver 

 nitrate solution indicated that chlorophyll in itself has the capacity to re- 

 duce AgNOs in light, without the aid of other cellular components; the 

 chlorophyll-free alga, Prototheca zopfii, showed no increase in the rate 

 of AgNOs reduction in light. These observations suggest that, even while 

 ascorbic acid appears essential for the silver nitrate reduction by chloro- 

 plasts in the dark, it may not be recjuired for the photochemical reduction. 

 The latter may occur at the cost of chlorophyll itself (although it will then 

 have to stop soon), or at the cost of other cellular hydrogen donors. Re- 

 duction at the cost of water (Hill reaction with Ag+ ion as oxidant) seems 

 unlikely because of the apparently non-enzymatic kinetics, and also be- 



