56 



PROBLEMS IN PHOTOSYNTHESIS 



b) Yellow light (5780 A) 



E = 0.245 cal 



xoj = 25.1 m1 



U = 25.1 X 4.5 X 10-8 = 113.0 X 10-« mole 



■q = U/E = 113.0 X 10-8/0.245 = 4.62 X 10-^ mole/cal 



c) Blue light (4360 A) 



E = 0.280 cal 



xo., = 17.8 m1 



U= 17.8 X 4.5 X 10-8 = 80.1 X 10"" mole 



77 = U/E = 80.1 X 10-8/0.280 = 2.86 X lO-e mole/cal 



The dependence of tj upon the wave-length is easily explained by the 

 quantum theory. In § 6 we saw that the quantum yield is 



If the radiation energy absorbed is equal to E, then 



E = qhv 

 i.e., the energy £" consists of ^ quanta of /?i^. Since 



77 = UE = n'E 



we have 



(3) 



■n = 



qhv 



and therefore 



(f = rjhv 



If 7] is expressed in mole/cal, the photochemical equivalent Nhv must be 

 used instead of quanta, N being Avogadro's number. T hus 



if = rjX/iv (26) 



The values of -q in the three examples mentioned, together with the values 

 of if and \/<p, are indicated in Table 4. From these experiments we can see 



TABLE 4 

 Calculation of quantum yields and requirements 



that to reduce a CO2 molecule about 4 quanta are needed in red and yellow 

 light and about 5 quanta in blue light. Numerous comparative experiments 



