16 



enclosed by the meshes, the mixture does 

 not flow but is in a solid or semisolid gel 

 condition. Colloidal suspensions and the sol 

 and gel conditions are not confined to proto- 

 plasm; for example, jello forms a colloidal 

 suspension in water— when warm it is in a 

 fluid sol condition, but when cool it changes 

 to a solid or semisolid gel condition. As in 

 protoplasm, either condition may be 

 changed back into the other. Some other 

 colloidal substances are mayonnaise, cream, 

 butter, glue, and soap. 



Consideration of all the known details of 

 the fine structure of protoplasm goes beyond 



Figure 5. Electron microscope. It uses a beam of 

 electrons and magnetic fields, which take the place 

 of light and lenses. Magnifications of over 100,000 

 times (diameters) may be obtained. This microscope 

 is useful in studying the smallest living things such 

 as viruses and the submicroscopic structures of cells. 

 (Photo courtesy of George Jennings, Michigan De- 

 partment of Health.) 



COLLEGE ZOOLOGY 



the scope of this book. However, the elec- 

 tron microscope reveals that it is far, far 

 more complex than the studies made with 

 the light microscope led us to suspect. Life 

 now appears to result from the complex in- 

 terrelations of the microscopic and ultra- 

 microscopic components of protoplasm. 



Chemical composition 

 of protoplasm 



When protoplasm is studied chemically, 

 it is round to be built up of the same ele- 

 ments that occur in nonliving materials. 

 The 20 elements listed below appear to be 

 essential to protoplasm: 



These elements, with the exception of 

 oxygen, are generally combined to form 

 compounds. Compounds can be divided 

 into inorganic and organic. Organic com- 

 pounds occur in nature only in living plants 

 and animals, or in their products and re- 

 mains. Inorganic compounds are principally 

 water and salts, and organic compounds are 

 principally proteins, fats, and carbohydrates. 

 The percentages of these different com- 

 pounds in protoplasm are on the average as 

 follows: 



