14 



PROBLEMS IN PHOTOSYNTHESIS 



Expressing wave-lengths in A instead of in cm, we find 



2.86 X 10« 



E = 



cal/mole 



(2) 



The energy values of the photochemical equivalents of various radiations in 

 the visible spectrum are given in Table 3. 



TABLE 3 

 Photochemical equivalents of some kinds of radiation used in photosynthesis experiments 



The translational kinetic energy of a molecule can be calculated from the 

 equation 



?>kT 



u = 



2 



The Boltzmann constant k has the value 1.381 X lO-i*^ erg/degree. At 20 °C 

 the energy u amounts to 6 X lO^^"* erg per molecule. When illuminated 

 with red light, one mole absorbs about 40000 cal, i.e., 280 X 10"^^ erg per 

 molecule. This simple calculation shows that the energy of the irradiated 

 molecule is 50 times higher than before. The new energy level corresponds 

 to a temperature of about 1 4000 ° C. This example gives an idea of the rather 

 important difference in energy between the ground state and the excited 

 state of a molecule. 



§ 6 Examples of Photochemical Reactions 



When n molecules are excited by the absorption of q quanta, the quotient 

 n/q is called the quantum yield 



<f 



(3) 



The quantum requirement is the reciprocal value of the quantum yield 



Wf 



n 



(4) 



