PHYSICOCHEMICAL ORGANIZATION OF THE PLANT 3 



of the protein molecule. The amino acids constituting the pro- 

 teins and the type of their interconnections will be further 

 described in the discussion of the metabolism of cells. 



The indispensable companions of proteins in the living pro- 

 toplasm, the lipoids, are of very complicated chemical compo- 

 sition and high molecular weight. Most of them contain 

 phosphorus and are termed ^^phosphatides." Chemically they 

 are closely allied to fats, which are esters of glycerin and fatty 

 acids. Lipoids contain in addition phosphoric acid and nitrogen- 

 containing organic bases. Like fats, lipoids do not form true 

 solutions in water but are soluble in organic solvents such as 

 esters and benzene. 



Solutions of substances having large molecules always possess 

 the properties of colloids. The study of the structural peculi- 

 arities of protoplasm is therefore closely connected with a study 

 of the colloidal state. 



2. General Concept of the Colloidal State and Its Significance 

 for Vital Phenomena. Fundamental Properties of Colloids. — 

 The colloidal state, in terms of physical chemistry, is one of 

 very fine division of the substance as a phase separate from the 

 medium in which the minutely divided particles are dispersed. 

 These dispersed systems may be divided into three types: 



(1) coarsely dispersed systems with particles of the dispersed phase 

 not less than O.l^i (1 micron = 1 thousandth of a millimeter); 



(2) colloidal systems in which the particles of the dispersed 

 phase range from 0.1^ to O.OOV; and (3) molecularly dispersed 

 systems, in which the division in the dispersed phase is so great 

 that it is represented by separate molecules or ions that have 

 lost all connection with one another. The coarsely dispersed 

 systems are mechanical mixtures whose heterogeneity may be 

 readily observed under the microscope. Such systems are 

 usually unstable. Their dispersed phase is either precipitated 

 or rises to the surface on standing. With sufficient subdivision 

 of particles, however, a greater stability is obtained. Examples 

 of such stable. mixtures are fine suspensions of clay, which do not 

 settle for many days, or common India ink, a suspension of 

 minute carbon particles in water. 



Similar to suspensions are the emulsions, or mixtures of two 

 immiscible fluids, e.g., water and oil. Ordinary milk is such an 

 emulsion, in which particles of fat in water may be seen under 



