46 ESSENTIALS OF ZOOLOGY 



teins have a slow rate of diffusion, high resistance to electric current, 

 and usually coagulate upon heating or upon addition of acids, alco- 

 hol, or salts to form a clot. Egg albumen, gelatin, and lean meat 

 are common examples of proteins. They are split into numerous 

 amino acids which serve as the building stones of the stable portions 

 of protoplasm. 



Enzymes are substances whose exact chemical nature is not yet 

 known, but whose importance to protoplasm is probably unequaled. 

 Chemically and physically they seem to be more like proteins than 

 anything else. These substances are not only found in the cells, 

 but they are also secreted by cells into the digestive tract and into 

 the blood stream, where they act as organic catalysts. The general 

 function of the catalyzer or catalytic agent is that of facilitating 

 and speeding up certain chemical exchanges without the agent itself 

 entering into the reaction. The well-known example of catalysis 

 is the effect of a small amount of platinum in increasing the rate 

 at which hydrogen and oxygen combine to form water. A particu- 

 lar enzyme is usually specific for one kind of reaction, but not for 

 the species of animal in which it will function. Enzymes taken 

 from one species will usually facilitate the same kind of specific 

 reaction in other species. The digestive enzymes may be thought 

 of as an example. Of these, pepsin will bring about the same gen- 

 eral reaction, whether it is in the stomach of a frog or of a man, 

 under favorable conditions. Since many enzymes influence only one 

 specific type of chemical reaction and since there are numerous 

 types of reactions going forward in active protoplasm, it is seen 

 that there must be numerous enzymes present in the cells of every 

 organism. 



Water constitutes 60 to 90 per cent of protoplasm and maintains 

 many substances in solution. Water is not only a very efficient 

 solvent; but it is important to protoplasm because of its compara- 

 tively high surface tension, because its presence gives the proto- 

 plasm a consistency compatible with the range of variation neces- 

 sary for metabolism, and because of its high specific heat. This 

 latter point is important in maintaining protection against sudden 

 and extreme temperature changes in the living organism. Young 

 cells contain more water than old ones, young organisms likewise 

 contain more than old ones. The relative amounts of water in rela- 

 tion to other materials of the protoplasm vary in different cells and 

 in different species. 



