Animal Cells and Their Nutrition - 1 37 



Table 7-2— Essential Amino Acids 



These cannot be synthesized by animal cells and, 

 therefore, must be included as preformed units in the 

 protein foods of the animal. 



H 



I 

 R—C— COOH 



I 

 NH, 



Amino acid 



R 



Threonine 

 Phenylalanine 



Lysine 

 Tryptophan 



Valine 

 Methionine 

 Leucine 

 Isoleucine 



CH^CHOH- 

 -CH, 



NH 2 — (CH 2 ) 3 — CH 2 - 

 -CH,— 



(CH :J ) 2 — CH— 

 CH S — S— CH 2 CH 2 — 

 (CH,) 2 — CH— CH 2 — 

 CH 8 — CHo— CH(CH 3 



city for organic synthesis, the synthetic capa- 

 bility of the animal is by no means negligi- 

 ble. The purine and pyrimidine bases, 

 needed for the manufacture of nucleic acids, 

 can be formed from catabolic fragments of 

 the glucose molecule (Fig. 8-5) together with 

 ammonia (or ammonium salts). Some pre- 

 formed organic bases, particularly the pyri- 

 midines, may also be utilized, if they are 

 present in the food of the animal. Inorganic 

 phosphate may be used in the synthesis of 

 nucleic acids and phospholipids. Moreover, 

 animal cells generally possess efficient enzyme 

 systems for the manufacture of its various 

 essential sterol compounds (p. 77). 



Complex carbohydrates, particularly gly- 

 cogen, which is a very common type of fuel 

 stored in animal cells, can be derived by de- 

 hydration synthesis from absorbed glucose, of 

 course, or from glucose generated from cer- 

 tain amino acids, subsequent to their deami- 

 nation (Fig. 8-10). In man and other verte- 

 brate animals, large reserves of glycogen may- 

 be found in the muscles and liver, deposited 



in the form of microscopic granules, and 

 these reserves may be replenished if the ani- 

 mal eats adequate quantities of either carbo- 

 hydrate or protein foods. 



True fats can be synthesized directly from 

 glycerol and fatty acids, absorbed from the 

 digestive cavity. Or they may be manufac- 

 tured indirectly, in which case the animal 

 obtains the glycerol and fatty acids from 

 metabolic fragments derived from glucose or 

 from certain amino acids, subsequent to 

 deamination (Fig. 8-10). Thus it is common 

 knowledge that an animal can "get fat" by 

 overeating not only fatty foods, but carbo- 

 hydrate and protein foods as well. 



Starting with organic precursors, animals 

 generally can synthesize various other sub- 

 stances, including hormones (Chap. 22), 

 which may be important in particular cases. 

 But no animal can synthesize its own vitamin 

 requirements. Consequently the vitamin com- 

 pounds must be present, as such, in the daily 

 diet (Chap. 18). The initial synthesis of vita- 

 mins occurs in plants, and this is another 

 instance of the dependence of animals upon 

 the synthetic capacities of plants. 



The growth of protoplasm implies more 

 than the synthesis or direct acquisition of es- 

 sential component substances. The substances 

 must be organized and oriented in relation to 

 the protoplasmic structure — a process that is 

 vaguely referred to as assimilation. Also there 

 are trace elements, particularly copper, man- 

 ganese, and cobalt, which must be present, 

 even though in exceedingly small amounts. 

 Apparently these metals serve as specific ac- 

 tivators in certain of the enzyme systems of 

 the cell. 



Destructive Metabolism. The principal or- 

 ganic substances absorbed by animal cells are 

 simple sugars, glycerol, fatty acids, and the 

 amino acids. All these are high-potential 

 compounds, capable of liberating consider- 

 able energy (p. 81). The decomposition of 

 organic compounds in the protoplasm in- 

 volves a variety of chemical processes as, step 

 by step, energy is liberated for the use of the 

 cell. Finally, when the catabolism of the 



