40 ELEMENTS OF BIOLOGY 



ferences solely structural? (For in everyday life we make no mis- 

 takes in distinguishing animals and plants.) 



The answer to these questions must first be sought in the answer 

 to the question: What distinguishes animal cells from plant cells? 

 Before going farther, it may be stated that there are living forms 

 that partake of the characteristics of both plants and animals; in the 

 last analysis, there is no sharp line of distinction between the two 

 groups. But as regards typical plants and typical animals, the differ- 

 ences are clear. Some differences are to be found in cellular struc- 

 tures (Fig. 8). In general, typical plant cells possess a cell wall 

 composed of a substance peculiar to plants, called cellulose. It is 

 the chief component of manufactured papers and is a carbohydrate, 

 related to starches and sugars. Plant cells also frequently contain a 

 large cavity filled with a non-living fluid called cell sap; the cavity is 

 known as the sap vacuole. 



But the chief difference between typical plant and typical animal 

 cells is in their method of subsistence. In order to understand this 

 difference it is necessary first to discuss the main features of sub- 

 sistence in living objects in general. 



Metabolism. Previously the intricate nature of the physico- 

 chemical system, protoplasm, was described. It was also shown that 

 constant change is the constant feature of the system and that these 

 events are wrought in a watery colloidal medium. The collective 

 term that applies to this constant series of physico-chemical events 

 is metabolism. Some of the processes serve to build up new com- 

 pounds and new protoplasm; this phase of metabolism is termed 

 ANABOLisM. Somc of the metabolic processes are destructive. These 

 reactions are described as catabolism. Both are concerned in the 

 vital phenomena which we have called the tests of life. Both ana- 

 bolic and catabolic reactions are in reality transformations of energy, 

 and the vital phenomena are effected only by an expenditure of 

 energy. It is obvious, therefore, that protoplasm must have a source 

 of energy which must be available in a potential state so that it can 



