22 AN INTRODUCTION TO ZOOLOGY 



oxygen, hydrogen and nitrogen, and a small amount of sulphur. 

 In this respect they differ from the other two classes of organic 

 compounds which lack the nitrogen and sulphur. The proteid 

 molecules are extremely large, one often containing more than 

 a thousand atoms. They are not readily diffusible through 

 animal membranes, this being due in part to the size of the mole- 

 cules and also to the fact that such complex compounds do not 

 dissolve completely unless they break down into simple sub- 

 stances. Instead of dissolving, proteids absorb enormous 

 quantities of water, swelling up like a sponge ; they are called 

 colloids to distinguish them from crystalloids, like sugar, which 

 are easily soluble. Coagulation, or clotting, is another peculiarity 

 of proteids. 



Carbohydrates are compounds of carbon, hydrogen, and oxygen, 

 the last two always occurring in the same proportions in which 

 they are found in water (H 2 O). The most familiar examples 

 of this class of substances are the starches and sugars, which are 

 characteristic of plants rather than animals, though found in both. 

 Carbohydrates are comparatively simple compounds when 

 contrasted with the proteids, and are readily oxidizable, thus 

 producing energy. Some varieties of living substance apparently 

 contain no carbohydrates. 



Fats, likewise, are not invariable constituents of protoplasm. 

 Though of widespread occurrence, fatty compounds are particu- 

 larly characteristic of animals. They may be said to consist of 

 an alcohol (glycerin) which has lost some water and combined 

 with a fatty acid. They are all lighter than water, and do not 

 unite with it. 



Protoplasm consists, then, of complex and variable mixtures of 

 proteids, carbohydrates, and fats, together with water, salts, and 

 dissolved gases. No uniform chemical formula can be given for 

 it; in fact, one of its chief characteristics lies in the unceasing 

 changes that it undergoes. It has the properties of metabolic 

 activity, irritability, contractility, reproduction, and all others 

 peculiar to living organisms (p. 10). It seems strange that such 



