THE ENERGETICS OF PHOTOSYNTHESIS 



87 



and 



xco, = h'C - hD = -23 X 13.21 + 5.5 X 22.81 = -178 m1 

 The photosynthetic quotient is 



-178 



7 = 



+ 149 



= -1.19 



In 9 min the number of quanta absorbed is 



9 X 0.48 X 44 = 190 /xl 

 so that the quantum requirement is 



'^ 149 



As can be seen in Table 16, the quantum requirement increases as the fre- 

 quency of intermittency decreases. With increasing frequency, it decreases 

 to a value of about 1 . 



TABLE 16 

 Time of intermittency and quantum requirement 



§ 36 Quantum Requirement and Photosynthetic Quotient 



Burk (9, 12) pointed out that the quantum requirement is low when the 

 photosynthetic quotient is high. This fact has been confirmed by Warburg 

 (44, 57). The quantum yield is good when 7 is —1.3 but poor when 7 is 

 — 0.8. It may be assumed that CO2 is reduced in two steps: firstly, reductive 

 carboxylation to the stage of formic acid 



COo + 2H + RCHO -> RCHOH-COOH 



and, secondly, reduction of the carboxylic group to the stage of a carbohydrate 



R-CHOHCOOH + 2H -> RCHOHCHO + HoO 



The photosynthetic quotients are 



for the first stage reduction 7 = —2.0 

 for the second stage reduction 7 = 

 for the two reactions together 7 = —1.0 



