92 PROBLEMS IN PHOTOSYNTHESIS 



At 20° C, k has been found to be 0.8. After 3 min illumination with the 

 measured hght, we have 



c = c,(l - ^-2-4) 



or 



c = 0.9 c, 



i.e., 90% of the steady value has been obtained. If /.: were 5.0, the same 

 percentage would have been found after only 0.48 min. According to equa- 

 tion 34, the value of c, would then be 6.25 times smaller than when k = 0.8. 

 Needless to say, the splitting effect would no longer be measurable. 



In these equations the quantum yield in the light reaction has been assumed 

 to be 1, the value of (1 — /) representing the quantum yield for the whole 

 cycle. When / = 0.624, the theoretical value of the quantum yield of the 

 cycle is 0.376. In the back reaction 0.624 X 112000 = 70000 cal/mole 

 are obtained, so that the quantum energy of 42000 cal/mole just makes up 

 the amount of energy required (112000 cal/mole). This is what happens 

 under physiological conditions. 



When/ = 1, the total Oo produced in the light reaction reacts back and the 

 quantum yield of the cycle is 0. When / > 1, the value for v is negative, 

 according to equation 33. Then, substance would not be gained in the light 

 reaction, but lost by photooxidation. It follows from this mathematical 

 analysis that when / > 0.624 energy is spoiled. Either more energy-rich 

 substance than necessary will be decomposed in the light reaction or too 

 much energy will be lost as heat when energy-rich substance is built up in 

 the back reaction. 



§ 39 Stoichiometric Oxygen Production 



Chlorella brought from dark into light immediately develops O2 at constant 

 velocity. However, when the alga is brought from its normal culture medium 

 into a modified, more acid medium, containing" only MgS04, KH2PO4 

 and NaCl without the usual trace elements, the production of Oj is much 

 higher in the first minutes of illumination than later in the steady state. This 

 initial O2 production is stoichiometrically related to the quantity of illu- 

 minated cells. Thus, the cells initially do not act as catalysers but merely 

 as reaction partners. Warburg (59) calls the stoichiometrically removed 

 O2 the O2 capacity of Chlorella. Depending on the chlorophyll content of the 

 cells, 1 50 to 250 /xl O2 can be removed from 1 ml Chlorella cells. 



The O2 capacity of Chlorella is of the same magnitude as that of erythrocytes, 

 i.e., about 300 /xl/ml. The difference between Chlorella and erythrocytes 

 is that O2 is firmly bound in Chlorella (42000 cal are required per mole O2) 

 but easily removed from oxyhemoglobin at low O2 partial pressures. 



To measure the O2 capacity, the cells are suspended in the culture medium 

 mentioned above. Initially, they are illuminated for 30 min with blue-green 

 light at low intensity so that the O2 capacity can be built up by means of the 



