PROTOPLASM AND THE CELL 55 



Physical Nature of Protoplasm 



Protoplasm is a semifluid material which is heavier than water 

 and somewhat more refractive to light. Its physical constitution 

 is similar to glue or gelatin, rather than to crystalloids, such as 

 sugar or ordinary table salt (sodium chloride). Instead of being 

 in the form of a true solution like salt in water, it consists of sus- 

 pensions of relatively large molecular aggregations varying roughly 

 between 0.0001 and 0.000001 of a millimeter in diameter. These 

 particles keep up an expression of energy in that they move against 

 each other as though they were dancing in a limited space. This 

 activity can be seen only with a special optical arrangement known 

 as the ultramicroscope and the phenomenon is known as Brownian 

 movement (characteristic of colloidal substances). Protoplasm dif- 

 fuses slowly or not at all through animal membranes. It changes 

 from a fluid or sol state to a more solid or gel state and may return 

 in the other direction. Ordinarily the viscosity of the continuous 

 phase or supporting liquid is only three or four times that of water, 

 while with the dispersed particles included it is only eight or ten 

 times that of water. The viscosity of the nuclear fluid is only twice 

 that of water. Since glycerin has a viscosity about a thousand times 

 as great as water, it will be realized that most protoplasm is quite 

 fluid in its active state. Changes in viscosity accompany and are 

 essential to the activity and functioning of it. 



Protoplasm is not a single compound; it is a colloidal system of 

 a number of chemical compounds existing together. Colloidal systems 

 are known as disperse systems of the emulsoid type. The more 

 watery or continuous part of the system is loiown as the dispersion 

 medium, while the particles or molecular aggregations constitute 

 the dispersed phase. An important consequence of the colloidal 

 systems in protoplasm is the enormous surface of particles exposed 

 to the continuous phase. If a sphere of material has a radius of 

 one centimeter its total surface will be 12.6 square centimeters. 

 Now, if the same volume of material is in colloidal particles of the 

 average size given above, the total surface of these will be approxi- 

 mately 7,000 square meters. This increase in surface is one of the 

 significant effects of colloidal organization of substances, because 

 many important reactions occur at these surfaces. By the presence 

 of salt ions in the continuous phase and these becoming adsorbed 



