CHAPTER FIVE 



THE SURFACE OF VIRUSES 



Viruses are perhaps the smallest functioning objects in 

 nature. They are in the realm of colloids, and yet they have an 

 internal structure and are able to use this internal structure 

 to multiply when in a host. Their small size greatly exaggerates 

 the relative importance of the virus surface. In a human being, 

 the ratio of surface to mass is about 0.3 cm-/gm, whereas in 

 southern bean mosaic virus it is about 3-million cm^/gm, or 

 10-million times larger. A human being functions considerably 

 by reason of his surface, and so a virus must do so much more. 



So great is this surface-to-mass ratio that it is impossible 

 to conceive of a virus maintaining any kind of metabolism out- 

 side a host. If the remarkable relation between surface area 

 and internal metabolism [as, for exam])le, measured by respira- 

 tion studies (see Brody, 1945)] is extended down to viruses, the 

 respiratory rate is such that a gram of virus would require an 

 energy turnover 20 times that of a normal human being. This is 

 much too high to be going on continuously, and so this very 

 simple consideration indicates that outside the host a virus can 

 be treated as nonliving. Inside the host it is quite different. 

 This is a striking illustration of the importance of environment. 



The small size of a virus also influences the vapor pressure 

 of water at its surface. If p' is the vapor pressure at a surface of 

 radius r, and p is the ambient vapor pressure, then, for a liquid 

 of density p, molecular weight J/, and surface tension 7, the re- 

 lation holding is (Adam, 1941, p. 14) 



RTln^ ^ 

 P 

 122 



27J/ 



rp 



(5.1) 



