54 



Cellular Structure and Activity 



Fankuchen, '41). Negative tactoids may also 

 be formed, in which case the dispersed phase 

 is low in protein content. Tactoids may also 

 be formed by smectic systems such as dis- 

 persions of certain types of lipids, soaps and 

 certain inorganic materials such as iron oxide 

 and vanadium pentoxide. 



The forces involved in the formation and 

 behavior of tactoids have received consider- 

 able attention theoretically and experimen- 

 tally (for recent reviews see Verwey and 

 Overbeek, '48; Oster, '50). The behavior of 

 the system is determined importantly by 

 the shape, size and concentration of the 

 particles (rodlets or platelets), their charge 

 density, the concentration and valence of the 

 ions in solution and, of course, the pH. De- 

 terminative is the type and extent of the 

 ion atmosphere about the particles, since 

 the interaction between the particles is 

 primarily electrostatic in nature. Except at 

 the isoelectric point the particles will bear a 

 predominantly negative or positive charge. 

 Since the reaction of most protoplasmic sys- 

 tems is near to neutrality and since most of 

 the proteins have isoelectric points in the 

 acid range, the charge will, in general, be 

 negative (basic proteins such as histones and 

 protamines are of course positive). Assembled 

 about the particles are the ions of opposite 

 sign (counter ions). The thickness of the 

 diffuse double layers, hence the interaction 

 between the particles, is inversely propor- 

 tional to the square root of the salt con- 

 centration. It has been shown that colloidal 

 particles of this sort will, in the presence 

 of their counter ions, repel each other, the 

 equilibrium distance of separation varying 

 with the concentration of ions, the repulsive 

 force between the particles and the potential 

 of the particles. Verwey and Overbeek sug- 

 gested that van der Waal's attraction between 

 particles of this sort will, in the presence 

 of their counter ions, repel each other, the 

 atmospheres. Van der Waal's forces are very 

 short-range (inversely proportional to the 

 sixth power of the distance in the case of 

 atoms) but, being additive, become important 

 in the case of large particles such as virus 

 particles, fibrous proteins or smectic lipid 

 systems. The intensity of the van der Waal's 

 attraction and of electrostatic repulsion de- 

 pends upon the conditions obtaining in each 

 particular system and it would be pointless 

 for our purposes to discuss the theoretical 

 aspects further. Suffice it to say that the 

 evidence that long-range attractive forces 

 need be invoked to explain the behavior of 



most colloidal systems which have thus far 

 been carefully studied is highly questionable. 

 Long-range forces have been supposed by 

 some to be important in determining the 

 structure of systems composed of long chain 

 molecules containing alternate single and 

 double bonds. London ('42) has shown that 

 such conjugated double bond chains may at- 

 tract each other over distances comparable 

 to the lengths of the chains owing to the 

 fact that they behave like oscillators. Such 

 forces may be quite specific. Rothen ('47, 

 '50, '52) has invoked them to explain his 

 experiments in which he finds that films of 

 antigen and antibody may interact with 

 each other even when separated by inert 

 films several hundred Angstrom units thick. 

 Enzymes were also thought to attack sub- 

 strate molecules separated by similar dis- 

 tances. Although this finding has given rise 

 to much speculation on the role of long-range 

 forces in protoplasmic systems [even includ- 

 ing the attractive forces during the somatic 

 pairing of dipteran chromosomes (Cooper, 

 '48)], Rothen's experiments have been criti- 

 cized on technical grounds (Iball, '49; Karush 

 and Siegel, '48; Singer, '50) and on theoreti- 

 cal grounds (Pauling, '48; Winter, '52). At 

 the present writing it seems that Rothen's re- 

 sults are susceptible of explanation without 

 reference to long-range forces. However, as 

 pointed out above, each system must be con- 

 sidered as a special case in order effectively 

 to analyze the possible role of long-range 

 forces. Bernal ('49) is convinced that "they 

 must play a very large part in the inner 

 organization of the cell" and that the prop- 

 erties of the mitotic spindle are explicable in 

 terms of a tactoidal organization. The latter 

 view has been strongly criticized (Schrader, 

 '44; Hughes, '52). 



SOL-GEL TRANSFORMATIONS, 

 CONTRACTILITY AND THE CELL CORTEX 



In a sol the asymmetric colloidal or macro- 

 molecular particles have a relatively large 

 average interparticle separation, depending 

 on the concentration of particles, charge on 

 the particles, pH, ionic strength and type of 

 ionic environment, as discussed in the pre- 

 ceding section. When these environmental 

 factors are altered the particle interaction 

 may be greatly increased, causing the sol 

 to be transformed into a gel. 



Thus a sol containing elongate macro- 

 molecules of nucleic acid dissolved in salt 

 solution may be transformed into a gel by 



