U^ Animal Cells 



and Their Nutrition 



NUTRITION includes all processes that 

 have to do with obtaining and utilizing 

 food — and obviously these matters are very 

 important in every living thing. In animals, 

 food furnishes the only source of energy; and 

 in all organisms, food is the only source of 

 matter for sustaining and increasing the liv- 

 ing structure. Without food no plant or ani- 

 mal can maintain life, except during periods 

 of virtual dormancy. 



A food is any substance that an organism 

 obtains from its environment and utilizes, 

 directly or indirectly, in its metabolism. 

 Some foods serve primarily as protoplasmic 

 fuels whereas others serve as raw materials 

 for the synthesis of essential protoplasmic 

 compounds; but every food in one manner 

 or another participates in the metabolism of 

 the organism. Sometimes the food substance 

 may pass without change directly into the 

 cells, but often it must undergo chemical al- 

 teration (that is, digestion) before the deriva- 

 tives may be absorbed. Moreover, it is neces- 

 sary to realize that when we speak of a "food" 



126 



in common language, we usually are refer- 

 ring to a mixture of many food substances. 

 Steak, for example, consists of a high propor- 

 tion of water, a variety of inorganic salts, 

 considerable amounts of protein, fat, and 

 carbohydrate, and a number of other indi- 

 vidually distinct food substances. Only a few 

 of the foods of man are highly purified sin- 

 gle compounds, such as table salt and sugar. 

 Moreover, what is a food for one organism is 

 not necessarily a food for another (Table 7-1). 



MODES OF NUTRITION; THE ANIMAL vs. THE 

 PLANT 



Typical animals and plants are distin- 

 guished by very fundamental differences in 

 their nutrition. Green plants require only 

 inorganic foods, but animals must obtain at 

 least a minium of organic foods, in addition 

 to their inorganic requirements. Water, car- 

 bon dioxide, an assortment of the inorganic 

 salts, and sometimes oxygen are all that the 

 green plant needs as raw materials for the 



