100 PROBLEMS IN PHOTOSYNTHESIS 



Certain investigators rejected the existence of Oo capacity and induced 

 respiration by stating that the observed manometric effects could be due to 

 thermic influences. Figure 39 and Figure 40 clearly show the irrelevance of 

 such statements. In spite of illumination or darkness no pressure changes 

 occur when the chemical reactions in Chlorella are inhibited (62). 



Table 18 shows the influence of HCN and phenanthroline on some func- 

 tions and reactions oi Chlorella. 



§ 42 Photosynthesis and Respiration 



Since the studies of Boussingault (6), it has been assumed — but not proved — 

 that O2 is indispensable to photosynthesis. Willstatter and Stoll (69) drew 

 attention to the fact that plants do not exhibit assimilation after long anaero- 

 biosis. According to the one-quantum theory of photosynthesis, O2 

 is used in the induced respiration to liberate the energy necessary for the 

 change of CO2 into the photolyte COo*. For this reason, photosynthesis can- 

 not proceed without O2. The use of manometry and Chlorella has made it 

 possible to establish conclusive proof. Hill and Whittingham (27) were the 

 first to employ the new methods to solve this problem. Hemoglobin was 

 added to a Chlorella suspension, and the O2 produced upon illumination was 

 determined bv optical measurement of the oxyhemoglobin formed. They 

 found that at a Oo pressure of 2 mm Hg photosynthesis already began to 

 decrease. Later, Whittingham (68) corrected this statement, indicating a 

 pressure of less than 10~^ mm Hg. 



The following experiments carried out by Warburg el al. (43, 53, 64) 

 provide definite proof. In the manometer vessel 100 jA Chlorella suspended 

 in carbonate buffer (pH 8.8) are in equilibrium with 2 vol % COo and argon 

 in the gas phase. According to § 20, pressure changes registered by the 

 manometer can only be changes in Oo pressure. The gas phase contains very 

 little Oo. The cells themselves are used to produce the desired low O2 

 pressures in the vessel. If the Oo pressure has to be decreased, the cells are 

 allowed to use Oo in the dark. If the O2 pressure has to be increased, the 

 cells are illuminated to produce O2. Initially, the cells are allowed to respire 

 in the dark until all the O2 is used up and respiration is zero. Afterwards, 

 the cells are illuminated : O2 pressure increases until photosynthesis attains a 

 constant end value. The cells are then saturated with O2 so that photosyn- 

 thesis reaches its saturation value. After removal of the light, the Oo pressure 

 due to respiration decreases to about zero. Figure 41 shows two curves ob- 

 tained with vessels of different size. Tangents can be drawn at different 

 O2 pressures p, indicating photosynthesis + dp/dt on the ascending branches 

 and respiration —dp/dt on the descending branches for the various O2 pres- 

 sures. The experiment — utilizing intermittent periods of light and dark — 

 can be repeated at will without opening the manometer vessels. 



The increase of photosynthesis with time is dependent only upon the O2 

 pressure. This can be proved if equal cell amounts are illuminated under 



