The Chemical and Physical Structure of Protoplasm - 93 



Suspensions and Emulsions. In a suspen- 

 sion or emulsion, the scattered particles con- 

 sist not of single molecules, but of molecular 

 aggregates of the dispersed substance. In a 

 suspension the dispersed particles are con- 

 sidered to be solid; in an emulsion they are 

 liquid; but in practice this distinction tends 

 to be quite arbitrary. When the dispersed 

 particles are very small, as in colloidal sus- 

 pensions and emulsions, it is difficult to deter- 

 mine whether the individual particles are in 

 a solid or a liquid state. 



The size of the dispersed particles in dif- 

 ferent emulsions and suspensions ranges 

 from colloidal to microscopic dimensions. In 

 fact, colloidal suspensions and emulsions are 

 sometimes so finely subdivided that they 

 scarcely can be distinguished from colloidal 

 solutions. Moreover, substances emulsified 

 or suspended in water may even be hydrated 

 and ionized, just as in true solutions. 



Diphasic and Multiphasic Dispersions. In 

 a true solution it is customary to regard the 

 intermingled molecules and ions of the solute 

 and solvent as all belonging to the same 

 phase of the system. But in suspensions and 

 emulsions it is necessary to distinguish be- 

 tween the discontinuous and the continuous 

 phases. The discontinuous phase consists of 

 all the larger dispersed particles or droplets, 

 taken collectively; whereas the continuous 

 phase is constituted by the dispersion me- 

 dium itself, considered as a homogeneous 

 mass. Milk, for example, is an emulsion in 

 which the discontinuous phase is comprised 

 by the microscopically visible globules of 

 butter fat; these globules float in the con- 

 tinuous phase, which is an aqueous solu- 

 tion containing milk sugar, various inorganic 

 salts, and proteins. 



A complex system such as protoplasm dis- 

 plays more than one discontinuous phase, 

 each represented by a different kind of dis- 

 persed particle or droplet. Also in proto- 

 plasm one may find more than one continu- 

 ous phase (see gel structure, below). 



Emulsifying Agents; Adsorption. If olive 

 oil and water are shaken together in a test 



tube, an unstable emulsion is formed that 

 begins to break down as soon as the shaking 

 is stopped. The agitation scatters a multitude 

 of oil droplets throughout the water, but 

 when the shaking ceases, these droplets be- 

 gin to coalesce into larger and larger drops, 

 and finally all the oil comes together as a 

 separate layer, which floats on top of the 

 water. If, however, a trace of soluble protein, 

 or a small amount of soap solution, is added 

 before shaking, a stable emulsion is obtained, 

 and such an emulsion keeps its structure 

 more or less indefinitely. 



The foregoing example indicates that pro- 

 tein (or soap) can act as an emulsifying 

 agent. In the stable emulsion most of the 

 added protein accumulates at the surfaces of 

 contact between the two phases, that is, at the 

 interfaces between the oil droplets and the 

 water. Such an accumulation coats each drop- 

 let with a protective film that prevents it 

 from coalescing with other droplets. The 

 tendency for certain dissolved substances to 

 become concentrated at the interfaces of an 

 emulsion or suspension is called adsorption. 

 Many colloidal substances, particularly pro- 

 teins and phospholipids, tend to be adsorbed 

 heavily at the various protoplasmic inter- 

 faces. This stabilizes the finely emulsified 

 droplets and delicately suspended particles 

 that are so characteristic of living matter. 



Phase Reversal. An emulsion such as is 

 formed by shaking oil with water may take 

 either of two stable forms: that of an oil-in- 

 water emulsion (Fig. 4-22A), or that of a 

 water-in-oil emidsion (Fig. 4-22C). The in- 

 termediate form (Fig. 4-22B) is unstable. It 

 tends to pass into either of the two stable 

 forms; or it tends to "break" the emulsion 

 structure, with a complete separation of the 

 phases. These differences in the form of an 

 emulsion depend chiefly upon the chemical 

 nature of the emulsifying agent. Any chemi- 

 cal change affecting the constitution of the 

 emulsifying agent may therefore affect the 

 stability of the emulsion and the relation of 

 its phases. Without doubt, some of the 

 changes in the appearance and consistency 



