Animal Cells and Their Nutrition - 1 39 



from the glycogen reserves of the cell, and in 

 the splitting of stored fat into glycerol and 

 fatty acids, as a prelude to the oxidation of 

 these products. Proteins, once formed in a 

 cell, are seldom hydrolyzed; that is, animal 

 cells normally sacrifice only excesses of ab- 

 sorbed amino acids to meet their energy re- 

 quirements. But during starvation, when all 

 reserves of carbohydrate and fat have been 

 exhausted, cells may hydrolyze their struc- 

 tural proteins and use the resulting amino 

 acids for energy. This, however, is only a 

 stopgap procedure. If it continues very long 

 the living structure will be destroyed beyond 

 repair, and the cell is bound to die. 



SUMMARY 



The enzyme system evolved by typical ani- 

 mals is not capable of synthesizing proteins, 

 carbohydrates, and the other organic compo- 

 nents of protoplasm solely from inorganic 

 substances. Therefore, animals must obtain 

 a constant supply of preformed organic foods, 

 particularly the essential amino acids, vita- 

 mins, and adequate amounts of organic fuel 

 substances if they are to grow — or even if 

 they are to maintain normal structure and 

 activity. Such organic substances are abun- 

 dant only in the bodies and remnants of 

 other organisms, and therefore animals must 

 resort to a predatory sort of life. An animal 

 must possess well-developed sensory, nervous, 

 and locomotor faculties, to find and appre- 

 hend its foods. And since the bulk of avail- 

 able organic foods exists in colloidal form 

 (proteins, complex carbohydrates, and fats) 

 the animal must have a digestive system in 

 which to prepare the food compounds for 

 absorption. 



The ultimate source of all energy in the 

 animal body is the potential energy of or- 

 ganic compounds, which are absorbed as such 

 by the protoplasm. If destructive metabolism 

 is suspended, all the more obvious move- 



ments and activities of the cells come to a 

 halt, and all constructive metabolism also 

 ceases. The various synthetic reactions in the 

 animal cell, being all endothermic in nature, 

 depend upon catabolic reactions for the en- 

 ergy that makes them proceed. As a whole, 

 therefore, the metabolism of the animal is 

 destructive in character. Inevitably animal 

 metabolism results in a diminution of the 

 quantity of organic matter available in the 

 environment. Even the growth of an animal 

 sacrifices part of the existing supply of or- 

 ganic matter. Preceding or accompanying 

 each upward step there must be a slightly 

 larger downward step. Part of the original 

 quantity of organic matter must be degraded 

 to the inorganic level in order to supply 

 energy needed for raising the remaining part 

 to a higher level of structural complexity. 

 The few pounds or ounces gained by an 

 animal in growing to maturity are obtained 

 at the expense of a much greater quantity of 

 organic matter. The smaller part of this 

 material provides matter and energy for the 

 synthesis of new components of the proto- 

 plasm as the body becomes larger; but the 

 greater part is sacrificed in providing energy 

 for finding, ingesting, digesting, absorbing, 

 and distributing the foods, and for disposing 

 of the waste products after they are formed 

 within the cells. 



The following chapters will show that the 

 metabolism of animals is complementary in 

 many ways to the metabolism of plants. As a 

 whole the metabolism of plants is construc- 

 tive, so that it replenishes the organic matter 

 of the earth. This unique characteristic of 

 plants depends upon their ability to draw 

 upon the energy of sunlight in supporting 

 their constructive metabolism. In short, it is 

 the possession of a different set of enzymes, 

 among which the green pigment chlorophyll 

 is the keystone, that enables plants to base 

 their metabolism entirely upon inorganic 

 forms of matter. 



