260 C. p. WIIlTTINrrHAM 



7.3) containing 6% glucose and KCl and the rate determined by re- 

 duction of mcthemoglobin as described by Davenport, Hill, and 

 Whatley (1). The chlorophyll concentration of the two suspensions 

 was adjusted to be the same, i.e., 1 mg. chlorophyll per 5 ml. suspen- 

 sion with an equivalent optical path of 1.3 cm. The hemoglobin con- 

 centration was 7 X 10-^ M. At the highest light intensity the rate 

 of the chloroplast reaction was not increased by further addition of 

 methemoglobin or ''factor." The light source was a tungsten pro- 

 jection lamp; it was filtered of infrared radiation and adjusted by the 

 use of neutral filters (Chance glasses ON32,ON31). Measurements 

 were at 20°C. 



The maximal rate of Chlorella used here was Q02*'" 1600; with the 

 chloroplast preparation it varied between Qo^"^' 1600 and 2900. It 

 was found that despite differences in maximal rate and despite the 

 fact that the two systems had the same rate at low light intensities, 

 the chloroplast reaction always had a lower rate than photosynthesis 

 at intermediate light intensities (Fig. 1). 



Further information as to the nature of the photochemical reaction 

 in photosynthesis and in the chloroplast reaction has come from stud- 

 ies with intermittent illumination. These were briefly discussed 

 but are omitted here since they are referred to elsewhere in this book. 



It might be concluded that there is a fundamental difference in the 

 way in which water reacts with the photochemical system in the 

 chloroplast reaction (at least as at present reconstituted) and in 

 photosynthesis. For example, Franck has proposed that in the 

 quinone reaction chlorophyll may act as hydrogen donor subse- 

 quently recovering the hydrogen by reaction with water, whereas in 

 photosynthesis the chlorophyll is activated as a complex with the 

 photosynthetic reactants. 



REACTIONS OF CHLOROPLASTS OTHER THAN PHOTOLYSIS; THE 



ROLE OF CYTOCHROMES 



Vishniac and Ochoa (8) showed that chloroplasts could in the light 

 reduce DPN+ and TPN + ; since that time it has been clear that in 

 presence of a suitable enzyme system the reoxidation of this coenzyme 

 by oxygen could be accompanied by phosphorylation. This type of 

 oxidative phosphorylation, normally mediated by the cytochrome 

 system, is dependent on the presence of oxygen or oxidized cyto- 

 chrome, is inhibited by cyanide and uncoupled by DNP and methyl- 



