56 PRINCIPLES OF ANIMAL BIOLOGY 



in every living organism. A large number of enzymes have already 

 been isolated and determined from plants and animals, and it is probable 

 that many more will be found. An enzyme is named according to the 

 substance which it acts upon or after the most important product of its 

 action. 



Important Enzjrmes and Their Action. — In both plants and animals 

 there arc three classes of digestive enzymes: the amylolytic, or carbo- 

 hydrate-splitting; the lipolytic or fat-splitting; and the proteolytic, or 

 protein-splitting enzymes. In plants the important amylolytic enzymes 

 are diastase which acts on starch producing maltose, maltase which splits 

 maltose into two molecules of glucose, and invertase which hydrolyzes^ 

 cane sugar into glucose and fructose. There are also other enzymes 

 capable of acting on other forms of carbohydrates. In animals ptyalin, 

 produced by the salivary glands and acting on starch, and amylopsin 

 which is contained in the pancreatic juice and changes starch to simpler 

 carbohydrates are the most important carbohydrate-splitting enzymes. 

 The lipolytic enzymes of both plants and animals break down the fats 

 into glycerol and fatty acids both of which are diffusible. Soluble 

 soaps are formed by combination of the fatty acids with bases, and these 

 and perhaps some unaltered fats are absorbed. The proteolytic enzymes 

 whether in plants or animals may be classed as pej^tic enzymes if they 

 require the presence of acids to make them chemically active, or tryptic 

 enzymes if they require an alkaliiie medium. The most important en- 

 zyme of the first group is pepsin which is produced by glands of the 

 stomach in animals. This enzyme splits proteins into peptones and 

 polypeptides. These substances are not diffusible and are further broken 

 down by trypsin, the most important of the second group of proteolytic 

 enzymes. Trypsin completes the splitting of the proteins into amino- 

 acids and other compounds which are readily diffusible. 



Digestion in Cells. — In unicellular animals digestion of necessity 

 occurs within the cell. Food having been ingested comes to lie in 

 vacuoles. Along with the food there is usually engulfed a small quantity 

 of water. Into this water digestive solutions of acid or alkaline reaction 

 are secreted by the surrounding protoplasm. These solutions contain 

 enzymes, the nature of which may be inferred from the reaction of the 

 solution. Obviously it would be quite impossible to secure a sufficient 

 quantity of the digestive solutions of these minute animals to permit of 

 analysis in the ordinary manner, but by making use of very dilute 

 watery solutions of the dye neutral red, it is possible to determine the 

 reaction of the digestive solutions in the living animals. The dye 

 penetrates without killing the protoplasm, and the color changes may be 

 observed in the vacuoles of the living animal by means of the microscope. 

 Neutral nsd does not directly distinguish the enzyme which is present. 

 Since, however, it is known that enzymes of the pepsin class operate 



