102 PROBLEMS IN PHOTOSYNTHESIS 



In Figure 42 the changes in photosynthesis and respiration are plotted 

 against the O2 pressures. We see that respiration as well as photosynthesis 

 show identical behavior when the Oo pressure changes. For both processes 

 the half saturation value is reached at 3 mm H2O and the whole saturation 

 value at 20 mm HoO. Below an Oo pressure of 1 mm H2O, respiration and 

 photosynthesis are both very small. 



The light intensity is of particular importance in these experiments. With 

 weak illumination, i.e., when respiration is just compensated after saturation 

 with Oo, the Oo pressure, and therefore photosynthesis, cannot increase. 

 Thus, when photosynthesis is zero at very low Oo pressures, it remains zero 

 under these conditions no matter how long illumination lasts. However, 

 with strong illumination the saturation value of photosynthesis is so rapidly 

 reached that the effect of low O2 pressure can no longer be recorded. For 

 this reason, the intensity should be chosen in such a way that respiration is 

 1-fold overcompensated, as is the case in Figure 41. The auto catalytic 

 increase will then last about 30 min and it will be possible to measure the 

 increase of photosynthesis with increasing Oo pressure exactly. As photo- 

 synthesis is the sum of O2 production in the light and Oo consumption due to 

 respiration, we must write, at 1-fold over-compensation, 



Idp/dtXKo, 



In equation 43 factor 2 has been omitted. 



Various experiments have been performed to prove that Oo is not indis- 

 pensable to photosynthesis. Allen (1) kept Chlorella under strong anaerobic 

 conditions. His method is based upon the fact that traces of Oo decrease 

 the phosphorescence of many dyes. A stream of COo-containing nitrogen 

 is conducted over glowing copper, from there over water, then over a Chlorella 

 suspension, over liquid nitrogen, and finally over the dye acriflavin adsorbed 

 on silica gel. Photosynthesis was still detectable at an O2 pressure of lO"*' 

 mm Hg. As described above, manometry in closed vessels showed, according 

 to Warburg, that below 10~^ mm Hg photosynthesis in Chlorella is very small. 

 Allen's method necessitates empirical calibration, i.e., the O2 pressures in- 

 ducing a certain degree of phosphorescence have to be analytically deter- 

 mined. This means that in a rapid stream of gas O2 pressures of the order of 

 magnitude of lO"*' mm Hg have to be produced, maintained and measured. 

 Needless to say, this seems to be a problem which is rather difficult to solve 

 and Allen gives no information on the analytical determination of these 



traces of O2. 



This kind of experiment, however, does not give any information on the O2 

 pressure in the cells. As a matter of fact, it is not the Oo pressure in the gas 

 phase but the O2 pressure in the cells which is of primary importance. If 

 the O2 pressure outside the cells were zero, a sufficiently high O2 pressure must, 

 of necessity, be present in the cells. A rough calculation, with the aid of the 

 diffusion constant of O2 in the cells, makes it possible to estimate a value of 



