30 GENERA/. CONCEPTS 



purine or pyrimidine molecule, one ribose or desoxyribose molecule and 

 one phosphoric acid molecule. 1 he nucleotides differ in the particular 

 knid ot purine or pyrmiichne present and the nucleic acids differ in the 

 proportions and sequences ot their constituent nucleotides. Ribonucleic 

 acids are iound, linked to proteins, in all parts oi protoplasm— nucleus, 

 mitochondria, microsomes and in the liquid ground substance. 



9. Physical Characteristics of Protoplasm 



The properties of protoplasm depend not only on the kinds and 

 quantities of substances present, but on their physical state as well. A 

 mixture of a substance with water, or other liquid, may result in a true 

 solution, a suspension or a colloidal solution, differentiated by the size 

 of the dispersed particles. In a true solution, the ions or molecules of 

 the dissolved substance (called the solute) are of extremely small size, 

 less than 0.0001 micron in diameter. The solute particles are either ions 

 or small molecules dispersed among the molecules of the dissolving 

 liquid (called the solvent). A true solution is transparent and has a 

 higher boiling point and a lower freezing point than pure water. Most 

 acids, bases, salts and some nonelectrolytes, such as sugars and amino 

 acids, form true solutions in water. 



The dispersed particles in a suspension, in contrast, are much 

 larger (greater than 0.1 micron) and are composed of aggregations of 

 many molecules. They tend to settle out if the suspension is allowed to 

 stand. Muddy water, for example, contains particles of clay in suspension. 

 Suspensions are opaque rather than transparent, and have the same 

 boiling and freezing points as pure water. 



A colloidal solution contains particles intermediate in size between 

 those of a true solution and a suspension, particles from 0.0001 to 0.1 

 micron in diameter. A colloidal solution, or colloid, is transparent or 

 translucent, has about the same boiling and freezing points as pure 

 water, and is stable; it does not tend to separate into its constituent parts 

 on standing. The particles of a colloidal solution may have a positive 

 or a negative charge, but usually they all have the same charge and 

 tend to repel each other. The presence of the charge is a factor which 

 tends to keep the particles dispersed. A colloid solution has the unique 

 property of changing from a liquid state, or sol, to a solid or semisolid 

 state or gel (Fig. 2.7). A familiar example of the change from sol to gel 

 occurs when a package of gelatin is dissolved in hot water. The particles 

 of gelatin (a protein) are dispersed through the water and a liquid 

 colloidal solution, a sol, results. As the gelatin cools, the gelatin par- 

 ticles aggregate and become the continuous phase, the water particles 

 become dispersed as small droplets in the gelatin and a semisolid gel 

 results. The gel can be converted back to a sol by reheating. The 

 gelatin-water mixture is a liquid sol when it consists of particles of 

 gelatin dispersed in water and a solid gel when the droplets of water 

 are dispersed in gelatin. The sol-gel change may be effected by changing 

 the temperature, the pH or the salt concentration or by mechanical 

 agitation (whipping cream, for example). The change is reversible, but 



