PROTOPLASM 9 



compound with a high potential energy content, is vaporized and 

 mixed with air containing oxygen. An electric spark is used to 

 upset the chemical equilibrium : a new alignment of the chemical 

 components takes place with explosive violence by which a con- 

 siderable proportion of the stored-up energy is released. In part 

 this active, or kinetic, energy is utilized in moving the car, some is 

 dissipated as heat, while at the same time various simpler com- 

 pounds with greatly reduced energy content are formed which may 

 be regarded as the excretory products. 



It is, of course, apparent that the katabolic processes could not 

 long continue in the cell protoplasm in the absence of the con- 

 structive anabolic processes which function in the replacement of 

 the materials destroyed to obtain energy. Accordingly we find 

 that certain foodstuffs, rich in potential energy, are taken into the 

 cell and, as the result of the anabolic activities, are actually built up 

 into the living protoplasm. 



Three great classes of foods are recognized, namely, carbohy- 

 drates, fats, and proteins. The proteins are essential for cell 

 nutrition because they always contain nitrogen and various other 

 elements, all of which are necessary constituents of protoplasm, 

 linked up with carbon, hydrogen, and oxygen. The carbohydrates 

 and fats contain only carbon, hydrogen, and oxygen — in varying 

 proportions — and furnish a source of energy which may be at once 

 utilized or stored away for a later emergency, but, since they lack 

 nitrogen and other elements, they cannot be used to repair proto- 

 plasmic waste. Also the cells of a living organism must have water, 

 free oxygen, various inorganic salts, and minute but constant 

 quantities of certain very complex organic compounds, known as 

 vitamines, the exact nature of which, as well as the roles they play 

 in cell metabolism, being largely unknown at the present time. 

 (W. pp. 21-25.) 



2. Photosynthesis 



The metabolic processes described above are believed to be 

 essentially the same in plant and animal cells. However, in certain 

 cells of green plants, an additional and all-important life process 

 takes place. This process, which is known as photosynthesis, is 

 superimposed upon the underlying metabolic processes, and it is 

 based upon the presence of the complex, greenish-colored com- 

 pound, chlorophyll. This unique substance makes it possi- 



