132 



FUNDAMENTALS OF STRUCTURE AND FUNCTION 



may be further subdivided into inorganic and organic compounds. 

 The former comprise most of the non-Hving compounds such as soil 

 and rocks and their decomposition products. However, in proto- 

 plasm, inorganic compounds are usually present as water, salts, or 

 gases. Water is important not only because it comprises 70-98 per 

 cent of protoplasm by weight, but also because it dissolves so many 

 different substances. Furthermore, water is an important factor in 

 promoting the dissociation of many salts into their constituent ions. 

 The inorganic salts which occur in marine organi.sms, for example, 

 are usually those commonly found in sea water. Some, such as ni- 

 trates and nitrites, occur chiefly in plants, while compounds con- 

 taining sodium and chlorides are characteristic of animal tissues. 

 Only three gases are found in varying amounts in the living cell, — 

 free oxygen, carbon dioxide, and ammonia. 



Protoplasm a Colloidal Mixture 



Matter exists in three states, gaseous, liquid, and solid. Frequently 

 it passes from one state to another, as when ice melts under the in- 

 fluence of heat, turn- 

 ing to steam as the 

 water boils away. 

 That protoplasm at 

 different times and 

 under different con- 

 ditions varies in ap- 

 pearance is probably 

 due to the fact that 

 A B c it is a colloid and as 



Colloidal constitutions. The continuous phase in a such can change from 

 being fluid; in a jell (C) solid; while in the "sol " or liouid tO 



sol (A 



intermediate phase (B) the solid forms a net through 



which the fluid is continuous. 



a "gel," or solid state 

 and then, under cer- 

 tain conditions, back again. The scientist examines protoplasm 

 under the ultramicroscope and finds tiny dancing particles which 

 are invisible under the ordinary illumination of the microscopic field 

 (Brownian movement). This condition is known as a dispersion, 

 the dispersed particles being carried in the dispersion medium, 

 in this case water. A fog composed of tiny droplets of water is 

 an example of dispersion in nature. If the particles in a disper- 

 sion are small, the substance is called a crystalloid, when large it 



