26 



INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



here is to distinguish between variations due to changing the composition 

 of the medium from variations due to dielectric constant. We have sought 

 to avoid this difficulty by using two substances which vary the viscosity 

 in the same direction but have opposite effects on the dielectric constant, 

 such as glycerol and methocel. 



By varying the viscosity, if one can identify the dielectric effects, one 

 can determine if the reaction is diffusion controlled. To study this one 

 needs to know the variation of the diffusion constant, D, with viscosity. 

 For larger molecules, the diffusion rate probably obeys the Einstein- 

 Smoluchowski relationship, i.e. D is inversely proportional to the coefficient 

 of viscosity, rj. For smaller molecules, this relationship cannot be used. 

 Very few values have been i)ublished for the variation of diffusion rates 



Fig. 1. Solid, liciuid and gas 

 molecular configurations. Note 

 the long-iange order and lat- 

 tice hole in the solid, the short- 

 range order and impurity (key) 

 in the liquid, and the complete 

 lack of order in the gas. 



with viscosity. This is true, in part, because of the difficulty of finding a 

 simple relationship between D and rj. We do not have values for HoOo , 

 but have used recent values for the diffusion of O2 in glycerol and sucrose 

 solutions (4) . The diffusion rates of all small molecules in a given solvent 

 are similar (5). 



The number of encounters per second between a catalase and a hydrogen 

 peroxide molecule is proportional to the diffusion constant. However, it 

 is naive to conclude from this that all reaction rates are diffusion con- 

 trolled. The cell theory of liquids developed by many scientists including 

 Leonard- Jones, Popple, Frenkel (5) and Eyring (2) makes it clear that 

 a molecule moves by taking small discrete jumps between quasi-equi- 

 librium positions. Figure 1 contrasts the quasi-equilibrium positions of a 

 liquid with the order of a solid and with the disorder and low density of a 

 gas. In a liquid, if one slows the diffusion rate, one may be slowing the 

 jump frequency. Then one reduces the number of encounters while in- 



