FUNCTIONS OF PROTOPLASM 



59 



plasm becomes stretched into a thin layer around the vacuole. 

 The vacuole is bounded by a delicate layer of cytoplasm, the 

 vacuolar membrane. Some animal cells possess vacuoles similar 

 to the food and water vacuoles of the protozoans, and contractile 

 vacuoles for the excretion of nitrogenous wastes. 



A chloroplast is a small mass of cytoplasm, colored green by 

 the chlorophyll pigment. The cell aided by this chlorophyll in the 

 presence of sunlight is able to manufacture raw materials into the 

 food necessary to build protoplasm. 



The centrosomes are the foci for the starlike formations ap- 

 pearing like fibers of cytoplasm. These seem to be of great 

 importance in the division of the animal cell. 



The sum total of all the functions of the cell is called metabolism. 

 It refers to the physiological activities of all living protoplasm. 

 There are two kinds of metabolic activity ; anabolism and katab- 

 olism. When the physiological activities tend to build up proto- 

 plasm, they are termed anabolic. Two examples of anabolic 



cell 

 TTiembra 



ccritroaoincl — ^— 



ilchnomottinr 



plasticL— 



Compare this animal cell with the plant cell on the previous page. 



activities are starch making and assimilation. When these pro- 

 cesses tend to break down protoplasm, they are katabolic. An 

 example of a katabolic process is the oxidation of food. Both types 



