200 



THE CELL AND PROTOPLASM 



no doubt connected with its hig'h dej^ree of 

 hydration and the relative absen"e of sul- 

 phur. The difficulty of explaining intra- 

 cellular structures in general has lain pri- 

 marily in this great degree of hydration. 



+ 



-h 



+ 



&■ 



-h 



+ 



Fig. 1A. Single ionic particles with ionic atmos- 

 phere (ions of opposite sign not shown) . 



It is difficult to explain the presence of any 

 structure in a cell containing 80 to 90 per 

 cent of water, and until very recently all 

 hypotheses as to intracellular structure, 

 fibers, foams, etc., could not receive any 

 physicochemical explanations, as the ac- 

 tual structures observed were in most cases 

 artefacts plainly due to the method of 

 preparation of the specimen. 



We now have, however, what may be the 

 first clue to the intercellular structures of 

 this intermediate degree of complexity. 

 Theoretical studies of colloids, particularly 

 those of Langmuir and Levine, combined 

 with the quantitative data provided by the 

 oriented sols of tobacco mosaic virus have 

 shown indisputably that interparticle 

 forces sufficient to maintain structures 

 against thermal agitation do exist and that 

 they are susceptible to quantitative theo- 

 retical explanation. These long-range 

 forces have long been suspected, but the 

 mechanism previously proposed for them, 

 polarization of water molecules (London 

 Van der Waals forces) were plainly physi- 

 cally unacceptable. The clue to the nature 

 of these forces was found in the ionic at- 

 mospheres which, according to the Debye- 

 Hiickel theory, nuist surround every 

 charged particle in an electrolyte. For a 

 large particle, such as a protein molecule, 

 this atmosphere extends to a distance of 

 the order of a few times the particle diam- 

 eter (Fig. 1). AVhen two such particles 

 ai^proach one another, the interaction of 

 the charges of each particle on the atmos- 

 phere of the other produces, at great dis- 

 tances, an attractive force ; at smaller dis- 

 tances the interpenetration of the atmos- 

 pheres makes this force repulsive. In gen- 

 eral the interaction between the two par- 

 ticles can be expressed by the familiar 

 potential energy curve (Fig. 1 D) which 



-f- 



+ 



-4- - 



r 



Fig. Hi. Two similar particles at a distance; attractive forces. 



