In principle, protoplasm may be considered to be primarily protein- 

 aceous, and many of its properties are similar to those of colloidal 

 dispersions of complex proteins. A colloid may be defined as the disper- 

 sion of one substance in another, with chemical properties intermediate 

 between those of true solutions and suspensions. Most of their reactions 

 are surface rather than molecular and depend primarily on size, shape, 

 and type of dispersion as well as electrical charge. The most important 

 type of colloid in living cells is apparently the coacervate, which may 

 be somewhat oversimply defined as a semiflocculated colloid in which 

 the dispersed particles tend to occur in aggregates. Most biologically im- 

 portant coacervates appear to be those which result from the interaction 

 of two hydrophilic (water-loving) colloids of opposite charge, as, for 

 example, histones and nucleic acids. It would be a mistake to assume, 

 however, that the properties of protoplasm in general could be dupli- 

 cated by any test-tube colloidal complex. While certain kinds of colloidal 

 systems may be used as models to demonstrate different protoplasmic 

 properties especially with respect to flow phenomena and sol-gel trans- 

 formations, protoplasm as already noted has structural qualities and 

 organization not matched by any of the known synthetic colloids. 



TABLE 2-3. Relative Abundance of Wafer and Profein Molecules 

 in Culfured Carrot Cells.* 



* The number of protein molecules per cell was determined from the total pro- 

 tein nitrogen per cell. The molecular weight of protein was estimated to be 64,000. 

 Courtesy of Dr. F. C. Steward, Cornell University. 



Water is an absolutely essential constituent of living matter. Without 

 it the cell could not function or even exist. It may exist "free," that is, 

 as water of solution, or "bound," usually to ionized groups of proteins. 

 The number of water molecules bound to protein in the cell may be 

 appreciable, as can be seen from examination of Table 2-3. Free water 

 is the major solvent of the cell and serves as a medium for a variety of 

 metabolic reactions. Both free and bound water are intimately associated 

 with maintenance of the colloidal state of protoplasm, though the precise 

 details of the association are poorly understood (Frey-Wyssling, 1953). 



GENERAL MORPHOLOGY AND CHEMISTRY OF THE CELL / 13 



