THE STRUCTURE OF PROTOPLASM 257 



differ in their physicochemical properties. Kinds of polarity 

 may be classified as, (1) atomic, where there is a loss or gain of one 

 or more electrons by an atom (sometimes resulting in a 'polar 

 bond, as in primary valence) ; (2) chemical, where there are two 

 radicals, one acid and one basic, at opposite ends (as in the 

 amino acid molecule, NH2 — R — COOH); (3) electrical, where 

 there are two unlike poles, one positive and one negative; and 

 (4) magnetic, where there are also two unlike poles but purely 

 magnetic in character, one north and one south. (The classi- 

 fication is arbitrary and simply a matter of convenience.) 



The importance of polarity has long been recognized but is 

 only now beginning to be widely applied. In the living world, 

 it is manifest everywhere. The salt concentration, acidity, 

 electric potential, and metabolic activities of organisms as a 

 whole and of individual cells differ in different regions and 

 usually in such a way as to give a gradient. Votchal and Lund 

 have established electrical potential gradients in spruce and pine 

 trees. Child found metabolic gradients in very lowly animals 

 such as planarian worms and Amoeba. Tissues show electric 

 polarity, i.e., a drop in potential, between them. Cellular 

 polarity is described by F. Weber in that all cells of a tissue 

 plasmolyze at one and the same end. S. Prat of Prague tells 

 of polarity in the vacuoles of cells as determined by staining 

 reactions. The behavior of organisms is to a great extent 

 influenced by their polar properties. Our problem at present 

 has to do with the part that polar molecules play in determining 

 the structure of protoplasm. We can say so little that is direct 

 and definite in regard to protoplasmic structure that we must 

 approach our problem of the role of polarity in the structure of a 

 living system by first considering it in nonliving systems. The 

 part that polar molecules play in the stabilization of emulsions 

 by monomolecular films (Fig. 86, page 130) and in determining 

 the permeability of the plasma membrane and the iridescence 

 of soap films has already been discussed (page 129). 



Given long and polar molecules, i.e., molecules with ends 

 unlike as to electrical sign or as to acid and basic properties, it is 

 possible to picture their orientation in mass and to obtain certain 

 types of structure which presumably are typical of gels a,nd 

 at least have the virtue of giving a mechanical basis upon which 

 to interpret the behavior of gels and of protoplasm. 



