CELLULAR TOPOCHEMISTRY 275 



The complex macromolecule represented by the network of protein 

 fibres in the cytoplasm is maintained intact by a series of junctions between 

 polypeptide chains (see p. 99). Certain of these linkages are homopolar in 

 nature, for example the disulphide bonds. Others are joined by heteropolar 

 bonds, or salt linkages. In addition there are cohesive bonds between non- 

 polar groups (for example attraction between CHg groups) and polar groups 

 (for example attraction between groups of a dipole nature). 



These various linkages are shown in Fig. 83. 



According to the theory of Frey-Wyssling, the four types of bond occur- 

 ring between the peptide chains can explain the behaviour of cytoplasm. 



1 . Cohesive bo?ids between non-polar groups {binding by Van der Waals forces) 



These are the same forces as are responsible for the cohesion of a crystal 

 of a paraffin. The attraction between non-polar hydrocarbon groupings, 

 feeble as it is, cannot be accounted for by an electric field for such a field is 

 practically non-existent. (In substances containing lipophilic groups it is 

 difficult to explain the attraction of these groups since their electric field 

 of force is negligible when compared with that between polar molecules.) 

 The cohesion between methyl groups, for example, is very small and 

 temperature-sensitive. The change in viscosity of cytoplasm with tempera- 

 ture must be attributed to the presence of these bonds. 



2. Cohesive bonds between polar groups 



These are bonds due to residual or secondary valencies and are caused 

 by dipole interaction. They are semi-chemical in character and they are 

 commonly called hydrogen bonds (Pauling) (see p. 99). When a polar group 

 carr}dng a peripheral hydrogen atom ( — OH, — NHg, etc.) is present, it can 

 be attracted electrostatically by the negative charges located on the polar 

 groups of neighbouring molecules thus uniting them through a hydrogen 

 atom. If the polar groups of the two adjacent molecules cannot approach 

 each other sufficiently, the electric field between them will attract water 

 molecules. In this case the bridge is made up of attracted water molecules 

 and the linkage is sensitive to any swelling of the cell, an action depending 

 on the presence of inorganic ions. Small ions like Li+ or Na+ have a 

 hydration shell thicker than that surrounding larger ions like K"*" or Cs^. 

 As the size of an ion increases with its atomic weight the hydration shell 

 decreases correspondingly. Hence it can be seen that the introduction of 

 various ions can modify the hydration of such structures as cytoplasm. 



3. Heteropolar valencies (salt linkages) 



If the positive and negative ends of two side-chains are in proximity 

 they can enter into a salt linkage, their charges neutralizing each other and 

 the degree of hydration decreasing. To break such linkages it is necessary 

 to change the pH. 



