METABOLISM OF FOODS AND DISPOSAL OF WASTES 



growing stage of the whole animal, and in 

 some cases during adulthood, cells also du- 

 plicate themselves. This means that protein 

 must be synthesized in each cell, and this 

 is done by the utilization of the amino acids 

 that come to the cell from the blood stream. 

 The proper amino acids are selected and 

 put together, by means of specific enzymes, 

 into the protoplasm of the particular ani- 

 mal. The amino acids are synthesized into 

 proteins by dehydration, that is, by the loss 

 of water, just the reverse of the process of 

 hydrolysis that occurs in digestion. This is a 

 simple process that passes easily from one 

 stage to the other in the presence of proper 

 enzymes, though it is not so easy outside 

 of the cell. 



Each cell builds its own specific proteins 

 from the amino acids that are available to 

 it. The proteins of every species of animal 

 differ from those of every other. The en- 

 zymes within the cell determine the propor- 

 tions of different amino acids and how they 

 are to be fitted together to produce spe- 

 cific proteins. The source of any particular 

 amino acid, such as glycine, does not mat- 

 ter, since it is the same whether it be from 

 a cow, oyster, or plant. As it combines with 

 others, however, the resultant protein is 

 highly specific. This relationship may be 

 compared to bricks in a house. The red 

 bricks are all alike no matter from which 

 brick yard they came, and they only be- 

 come distinctive when they form a part of 

 a particular house. They are then part of a 

 pattern, which in this case is Jones's house, 

 not Johnson's or Stoopnagle's. It follows 

 that all of the necessary amino acids must 

 be present in the blood if this constructive 

 work is to go forward. For some reason 

 while animals, man included, can build cer- 

 tain amino acids within their own cells, they 

 cannot construct others. The amino acids 

 they build are called non-essential amino 

 acids, and are not needed in the diet al- 

 though they can be utilized if available. 

 The amino acids they are unable to produce 

 must be provided in their diet. These are 



519 



the essential amino acids referred to earlier. 

 For example, if a person tried to live on 

 gelatin alone he would starve, because sev- 

 eral of the essential amino acids are not 

 present in this protein. By a combination of 

 foods, which is usual in the diet of most 

 animals, all of the essential amino acids are 

 made available. The ultimate source of the 

 essential amino acids must be plants, be- 

 cause they alone are able to build all of 

 them. 



The complex carbohydrate, glycogen, is 

 synthesized in the liver, muscles, and other 

 tissues of the body from glucose. Much of 

 the glucose that comes to the liver through 

 the hepatic portal vein is converted into 

 glycogen by the loss of water (dehydra- 

 tion), a process not gready unlike the one 

 which forms proteins from amino acids. As 

 was pointed out in an earlier chapter, gly- 

 cogen is stored and used as it is needed by 

 the tissues. Glycogen can also be formed 

 from proteins and fats by a process that is 

 not completely understood at present. 



Fats are produced in much the same way 

 as glycogen and protein, that is, by the 

 union of fatty acids and glycerol with the 

 loss of water (dehydration). Proteins and 

 carbohydrates can be converted to fats also, 

 but here again the process is not too well 

 understood. Proof of this lies in the fact that 

 carnivores do produce fat even though they 

 eat very little of it. Likewise, to the sorrow 

 of many people, fat can be produced and 

 stored from carbohydrates such as sugar. 



Hormones are also synthesized by animal 

 cells. Vitamins, on the contrary, must be de- 

 rived from the diet, because there seems to 

 be no means of producing them within the 

 animal cell itself. This is another instance 

 where animals depend entirely on the plant 

 world for an essential food substance. 



There is thus less synthesis of organic 

 materials in animals than in plants. By far 

 the most complex metabolic activities of 

 animals are the destructive forces, those 

 which release the energy that is necessary 

 to sustain life in all of its complexities. 



