Photosynthesis 391 



The vessel contains Chlorella suspended in a carbonate-bicarbonate mixture that 

 maintains the CO2 pressure constant, so that pressure changes registered by the 

 manometer can only be changes in O2 pressure. The gas space contains argon and 

 a very little oxygen. 



The essence of our experimental arrangement is that we employ the cells them- 

 selves to attain the desired low O2 pressures. When we darken the cells the O2 

 pressure sinks at once on account of the respiration; and when we illuminate the 

 cells the O2 pressure rises at once on account of the photosynthesis. This cycle can 

 be repeated as often as desired without opening the vessel. The manometer shows 

 us at any time the prevailing O2 pressure, and the change in manometer fluid level 

 shows us for any time period of pressure change the respective respiration or 

 photosynthesis. We thus learn whether, and in what manner, respiration or photo- 

 synthesis changes as a function of O2 pressure. 



The result is shown graphically in Fig. 6, in which the changes of respiration 

 and photosynthesis are plotted against O2 pressure. As one sees, both respiration 

 and photosynthesis change with O2 pressure, and indeed identically. An O2 pres- 

 sure of 3 mm of water is the half-saturation value for both processes, and 20 mm 

 yields Virtual Saturation for both processes. Below an O2 pressure of 1 mm of 

 water, respiration and photosynthesis are both very small. 



The experiment shows much more than that oxygen gas is necessary for photo- 

 synthesis. It shows that not merely traces of oxygen are necessary, but definite and 

 easily measurable pressures of oxygen, and that these pressures are necessary 

 because they are necessary for the respiration. All is precisely as our equations 

 demand. Without respiration, no photosynthesis! 



Chemistry of Photosynthesis 



We now leave energetics and turn to the chemistry of photosynthesis. The Prob- 

 lems posed here are clearly given by the results of the energetics. What happens 

 chemically to carbonic acid in the dark reaction of photosynthesis ? Or, expressed 

 otherwise, what is the photolyte chemically ? The gates to this field were opened by 

 the following experiment 7 . 



The main compartment of a conical manometric vessel (Fig. 7) contains a Sus- 

 pension of Chlorella, the side arm contains fluoride, and the gas Space contains 

 argon free of CO2 and ö 2 . The pH of the Suspension and of the fluoride is 3.8. 

 Upon tipping the fluoride from the sidearm into the main compartment, a vig- 

 orous evolution of C0 2 from the cells takes place. From 100 mm 3 of Chlorella cells, 

 30 to 40 mm 3 of CO2 will be developed in a few minutes. The content of this labile 

 CO2 in Chlorella is thus very great — greater, for example, than the content of 

 oxyhemoglobin-0 2 in red blood cells. A trace of Cyanide diminishes the develop- 

 ment of the CO2, from which one must conclude that it is an enzymic reaction that 

 is activated by the fluoride. 



There are two facts of special interest about the fluoride reaction. First, if one 

 expels the CO2 with N 1000 fluoride anaerobically, and then passes O2 into the 



