328 



RADIATION BIOLOGY 



2 0- 



state of the cell, the reducing effect of the light is dominated by oxidative 

 tendencies. 



The work of Aronoff (1946a, b), who found that the reduction of 

 quinones by chloroplast suspensions (see also Chap. 7) under certain 

 conditions was related to the redox potential of the quinones (benzo-, 

 naphtho-, and anthraquinone), will be briefly mentioned. To a certain 

 extent this is the reverse experiment of that by Wassink (1947, 1949), 

 who found that metabolism creates a definite shift in redox potential. 

 Aronoff's experiments tend to show that a redox potential imposed from 

 outside influences at least the rate of reaction. Some observations by 

 Aronoff (1946a), however, suggest that this view may be too simple. 



Gerritsen (1949-1950, 1951) made 

 rather extensive measurements of 

 redox potential and pH shifts in 

 chloroplast preparations of Avena 

 leaves upon illumination under var- 

 ious conditions. A strong shift of 

 the potential to the oxidized side 

 was observed upon illumination, as 

 well as consumption of ascorbic 

 acid. It seems conceivable that 

 photooxidations play a more im- 

 portant role in the observed phe- 

 nomena than do partial processes 

 of photosynthesis. When a small 

 amount of carbon dioxide is intro- 

 duced, the suspension becomes 

 more alkaline upon illumination, 

 whereas the redox potential de- 

 creases considerably. From this 

 Gerritsen (1951) concludes that 





■z 

 o 



o 



> 



I 5 



00 



50 



^ 3 xio 



INCIDENT INTENSITY, e ins/ cm ^ • SeC 



Fig. 5-17. Rate of photosynthesis vs. 

 incident intensity in diatoms {Nitzschia 

 dissipata) with repeated exposures of 

 5 sec/min (A) and under continuous 

 light (O) (Warburg buffer No. 9, 25°C). 

 (From Wassink and Kersten, 1943-1945.) 



carbon dioxide participates actively 

 in photochemical processes in anaerobic crude cell-free chloroplast (or 

 grana) suspensions. He claims that in anaerobic suspensions carbon 

 dioxide is hydrogenated photochemically by cell constituents of the 

 type RH2. 



4-6. ON THE CONNECTION BETWEEN PHOSPHATE METABOLISM 



AND PHOTOSYNTHESIS 



Calvin et al. showed that the action of the "reducing agent" upon 

 the carbon dioxide complex (see Sect. 2) first leads to the formation 

 of 2- and 3-phosphoglyceric acid (see Sect. 4-3). Thus this first detecta- 

 ble compound of carbon dioxide assimilation contains phosphate. Also 

 the carbon dioxide acceptor will therefore contain phosphate; it has been 



