CHAPTER 20 



METABOLISM OF FOODS AND DISPOSAL 



OF WASTES 



In the preceding discussion it was shown 

 how food and oxygen were delivered to the 

 individual cells in a complex animal. The 

 next step is to consider the processes by 

 which energy is derived from this food — 

 energy which is essential in bringing about 

 movement, growth, production of heat, as 

 well as all other living processes. This most 

 important series of processes, called metab- 

 olism, goes on within the cell, providing 

 energy to keep it active and material out of 

 which new protoplasm and new cells can 

 be constructed. The catabolic processes in- 



volve oxidation by which energy is released, 

 and the anabolic processes involve the 

 synthesis of new molecules for the construc- 

 tion of new protoplasm. Together, these de- 

 structive and constructive processes con- 

 stitute metabolism. 



METABOLISM 



Constructive metabolism 



Every cell of the body must rebuild or 

 replace itself from time to time. During the 



518 



