66 PROBLEMS IN PHOTOSYNTHESIS 



The general formula for the quantum requirement is 



/ _ ii^ol^ quanta 

 ^'"^ ^ ^ mole O2 



It is customary to use the /^l as unit, so that 



]A quanta 



or 



intensity absorbed in /xl quanta in t min 

 '^ O2 produced in ;ul in / min 



The value of the numerator is determined by means of the bolometer and the 

 Ulbricht sphere. The value of the denominator is calculated from the 

 product of the manometrically measured pressure change and the vessel con- 

 stant or, using the two-vessel method, from equation 17. 



§ 27 The Significance of Compensation 



The intensity of iUumination can be chosen in such a way that the photo- 

 synthetic production of Oo is equal to the consumption of Oo in respiration. 

 This intensity is called the compensating intensity. If the photosynthetic 

 quotient in this compensation point is equal to the respiratory quotient, we 

 also have compensation with respect to CO.,. However, the two quotients 

 do not have exactly the same value, so that the compensating intensity for 

 O2 is not necessarily the same as that for COo. Here the term is used with 

 respect to O2 only. It is further assumed that the compensating intensity 

 exists throughout the whole cell suspension so that in every part of the suspen- 

 sion uptake of Oo will be equal to removal of O2. It looks as if neither uptake 

 nor removal of O2 takes place. At the compensation point respiration seems 

 to be inhibited because apparently no O, is used. In reality, respiration goes 

 on but the O2 used is quantitatively replaced by O2 developed photosyntheti- 

 cally. However, many years ago the question was raised whether illumination 

 itself inhibited respiration. It had been imagined that illumination may 

 cause inactivation of respiratory enzymes or that certain intermediary prod- 

 ucts formed during respiration were reduced. Warburg and Burk (44, 45, 46) 

 pointed out that such hypotheses are incorrect when light intensities giving 

 maximum quantum requirements are used. It is not the light itself but the 

 O2 production due to photosynthesis which is at the origin of the mechanism 

 of compensation by light. Compensation does not eliminate respiration 

 chemically but only manometrically (44). 



We can express the process of O2 production during illumination and the 

 gain in Oo obtained by the following equation 



gain = photosynthesis — light respiration 



