48 PRINCIPLES OF ANIMAL BIOLOGY 



pounds or the simple elements, they must get them from plants. Plants 

 store any excess of carbohydrates above their immediate needs, in some 

 insoluble form, usually starch or some similar substance. Animals, from 

 the simplest one-celled ones up to the most complicated, use these stores 

 of plant starch for food. Out of these plant carbohydrates the charac- 

 teristic components of animal protoplasm are made. Glucose is to be 

 had by merely breaking down the starch. Glucose can be converted, 

 mostly by rearrangement, into glycerol and fatty acids; from these, fats 

 may be formed. 



For one of the essential parts of animal protoplasm, however, the 

 plant starches will not suffice; that is the highly important class of 

 proteins. Animals in general cannot make proteins out of inorganics 

 substances. Only a few can make proteins out of carbohydrates. There 

 is something lacking in the physiology of most animals which prevents 

 them from making this particular synthesis. The missing thing is 

 probably an enzyme or a set of enzymes. Animals must therefore get 

 their proteins, as well as their carbohydrates, either directly or indirectly 

 from plants. They may obtain these proteins from other animals, as 

 the carnivorous animals almost exclusively do, l^ut these other animals 

 must get the proteins ultimately from plants. • 



Conversion of Food. — Very little of the food which animals take can 

 be utilized at once for its ultimate object, unless water and oxygen be 

 considered food. Most of the food has to be worked over in some way. 

 Glucose and other equally simple sugars are ready to use, but these 

 constitute only a very small fraction of the food of animals. One of the 

 chief reasons why other foods cannot be used at once is that they are 

 not soluble. The starches, lipids, and proteins must all be converted 

 into some form that will diffuse through protoplasm. This conversion 

 is effected in the process of digestion. 



Digestion is essentially the same process everywhere but will be con- 

 sidered here chiefly as it occurs within cells rather than in the cavities of 

 large organs like the stomach. Unicellular animals take in small organ- 

 isms and surround them with a droplet of water containing one or more 

 enzymes, thus forming a food vacuole. All such animals can produce 

 enzymes that will digest proteins, many can digest starches, most of 

 them can digest fats. Proteins are dismembered to yield their amino 

 acids; fats are split up into glycerol and fatty acids; starches are con- 

 verted into simple sugai's. The final products named in each case are 

 all soluble in water and can diffuse through protoplasm. 



In this soluble form they pass to every part of the cell, or from cell 

 to cell. Oxidation of them may occur if energy is needed. The deriva- 

 tion of energy from oxidation of glucose is represented by an equation 



