CHAPTER IV 

 COLLOIDAL SYSTEMS 



Most biological problems lead ultimately to a consideration of one phase 

 or another of the more fundamental problems of the structure, constitution, 

 and physico-chemical properties of protoplasm. Certain characteristic physico- 

 chemical properties of protoplasm have been recognized and studied. Proto- 

 plasm is more or less elastic. It may vary in viscosity from values not much 

 greater than that of pure water to those of jellies. It possesses a pronounced 

 capacity for the imbibition of v^^ater. Protoplasm is usually high in water 

 content, but nevertheless often behaves as if immiscible with water. Me- 

 chanical disturbances may markedly influence its physical state. When ex- 

 posed to very high or very low temperatures, to high concentrations of salts, 

 or to certain other factors, protoplasm may be irreversibly coagulated. Under 

 certain conditions it may lose its fluidity and assume a jelly-like condition, a 

 change in physical state which is usually reversible. 



Protoplasm is predominantly composed of substances in the colloidal state, 

 and it is to these colloidal systems that it owes most of its characteristic 

 physico-chemical properties, some of the better known of which have been 

 listed above. The world of living organisms has, in fact, been largely molded 

 on a colloidal pattern. Most physiological processes, reduced to their ultimate 

 tangible mechanism, either occur in a colloidal matrix, or under such condi- 

 tions as to be strongly influenced by the colloidal organization of the cells in 

 which they take place. Many physiological processes occur only under the 

 influence of the organic catalysts known as enzymes, which are supposed by 

 most authorities to be in the colloidal condition and to owe many of their 

 properties to that fact. It is impossible, therefore, to obtain any adequate 

 comprehension of the properties of protoplasm without a background of facts 

 and principles regarding colloidal systems. The more fundamental properties 

 of such systems will be discussed in this and the following chapter. 



Some of the most important components of the material environment of 

 plants and animals are also essentially colloidal. Most soils contain a con- 

 siderable proportion of matter in the colloidal or near-colloidal condition, and 



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