94 - The Cell 



A. OIL IN WATER 



B. TRANSITIONAL 



C. WATER IN OIL 



Fig. 4-22. Emulsion formed from olive oil and water, as seen with the microscope; the oil contains a dye to 

 make it darker. The changes in form of an emulsion are brought about by the addition of small amounts of a 

 substance that changes the chemical constitution of the emulsifying agent. A, water the continuous phase, oil 

 the discontinuous (dispersed) phase. B, intermediate form, the emulsion "breaking"; this state is very unstable, 

 the two phases being in continual rapid motion and rearrangement. C, oil the continuous phase, water the dis- 

 persed phase. 



of the protoplasm are due to such phase re- 

 versals, which result from chemical changes 

 wrought by the metabolism of the cell, or 

 from chemical changes in the nearby envi- 

 ronment. 



Gelation and Solation. Protoplasm fre- 



The capacity to gelate is found in many 

 colloidal systems, especially when the dis- 

 persed particles are elongate, like protein or 

 polysaccharide molecules. In the sol condi- 

 tion such elongate particles behave more or 

 less as separate units (Figs. 4-23A). But as 



quently alters its consistency by undergoing gelation occurs, attractive forces come into 



gelation or solation. Like a gelatin solution, 

 the fluid protoplasm of a cell may become 

 set at a certain moment into a semisolid 

 elastic mass; and then later it may revert to 

 a more fluid consistency. Recently the func- 

 tion of these changes has become clearer. 

 The evidence indicates that the contractility 

 of protoplasm depends upon its capacity to 

 undergo gelation; and that many cell move- 

 ments, such as amoeboid movement, cannot 

 occur if the sol-gel changes in the protoplasm 

 are inhibited. 



SOL 



GEL 



play between the colloid particles, so that the 

 dispersed particles of the system become in- 

 terlinked, forming a colloidal network that 

 extends throughout the gel (Fig. 4-23B). 

 The water and other crystalloidal compo- 

 nents are enmeshed by the gel framework 

 and consequently the whole mass develops 

 rigidity, elasticity, and contractility — prop- 

 erties that ordinarily are found only in solid 

 systems. 



The precise manner in which the elongate 

 particles of a sol become linked together to 



Fig. 4-23. Ultramicroscopic structure 

 of a sol and a gel (diagrammatic). A, 

 a sol, in which the elongate colloidal 

 molecules or particles are folded and 

 separate. B, same system after gelation 

 has occurred, in which the elongate 

 particles are unfolded and interlinked, 

 forming a colloidal network. Gels tend 

 to be elastic and contractile. The zig- 

 zag lines represent elongate colloidal 

 particles (e.g., protein molecules); the 

 dots represent water and other crystal- 

 loidal molecules. 



