46 GENERAL BIOCHEMISTRY 



Colloids in Biology 



Many giant molecules known as macromolecules function bio- 

 logically. They include blood proteins, cellular enzymes, starch, gly- 

 cogen, viruses, nucleic acids. All are necessary and all fall in the 

 colloidal size range. In addition some of the fats may be transported 

 in body fluids as aggregates of or near colloidal size. Cells contain 

 many suspended particulates, some of colloidal dimensions, that are 

 believed to be aggregates active in cellular reactions. In addition, 

 blood colloids, for example, adsorb small solutes, and the complexes 

 circulate, serving as reservoirs of the adsorbed materials. 



Also there are non-crystalline solid-like materials having some col- 

 loidal properties. Gels like gelatin and jelly possess some sort of inter- 

 locking structure that includes a great deal of water and are really 

 neither liquid nor solid. The protoplasm of living cells is viscous like 

 the gels and resembles them. So does muscle tissue. Some hard mate- 

 rials like horn, finger nail, and glue appear to be concentrated gels 

 of high tensile strength. A nimiber of these biological colloids will be 

 considered in greater detail later. 



COARSE DISPERSIONS 



Interfacial Tension 



The coarse dispersions include all systems of two or more phases 

 with the particles exceeding about 200 millimicrons in diameter. At 

 each surface separating the various phases of the system there is a force 

 resembling the surface tension of the boundary surface between a 

 liquid and air. In fact the ordinary surface tension of liquids is but a 

 special case of this general type of force called interfacial tension. 



There are five different classes of phase boundaries, gas-liquid, 

 liquid-liquid, gas-solid, liquid-solid, and solid-solid. All represent 

 zones of special force with the consequent possibilities for interactions 

 with solutes and resulting adsorption. The first case has already been 

 discussed. Of the others, the last three are thought to exist but no 

 methods of measurement have ever been devised. Presumably the 

 forces involved lead to adsorption, but little more can be said. 



However, liquid-liquid interfaces definitely possess tensions that can 

 be measured by using modifications of some of the techniques devel- 

 oped for gas-liquid interfaces. The measured interfacial tensions have 

 correlated with adsorption and play major roles in studies of emul- 



