52 PRINCIPLES OF ANIMAL litOLOCV 



meet. The brilliant work of Fischer and others in the synthesis of 

 polypeptides has resulted in substances which are identical with substances 

 that may be derived by the hydrolysis of natural proteins. Whether 

 synthesis may ever be carried so far as to produce substances identical 

 with the natural proteins themselves cannot be foretold ; but already the 

 synthetic work has yielded a large portion of the secret of protein structure. 

 The importance of this revelation will become apparent in a later chapter 

 when the digestion of the proteins, is discussed. For in the process of 

 digestion, by a process the reverse of Fischer's synthesis, that is, by hydroly- 

 sis or the addition of water, the protein molecules are dissociated into their 

 amino-acid components. 



The sources of the amino-acids from which the proteins of organisms 

 may be constructed remain to be mentioned. Animals are almost wholly 

 dependent upon other animals or upon plants, whose proteins they digest 

 (hydrolyze) into amino-acids. Plants, on the contrary, are capable of 

 synthesizing amino-acids from soluble carbohydrates and nitrates. The 

 substitution of amino radicals (NH2) for OH radicals of the carbohydrate is 

 suggested as a step in the process. Fatty acids derived from fats may also 

 be employed for similar amino substitutions. 



Fat Formation. — Fats are compounds of a basic substance glycerol 



(glycerine) and various of the higher fatty acids. Glycerol has the 



formula 



CH2-OH 



1 

 CH-OH 



I 

 CH2-OH 



and all of the OH radicals are readily replaceable by fatty acids. The 

 principal fatty acids in animals are palmitic, stearic, and oleic. The 

 formula of palmitic acid, for example, ignoring the structure in part, is 



CH3 - (CH2)i4 - COOH 



The H of the carboxyl group at the right is readily replaceable by the 

 glycerol. If all three of the OH radicals of glycerol are substituted by 

 molecules of palmitic acid, three molecules of water are lost and a sub- 

 stance called tripalmitin is produced, with the formula 



CH3 - (CH2)i4 - COO - CH2 



I 

 CH3 - (CH2),4 - coo - CH 



I 

 CH3 - (CH2),4 - coo - CH2 



The substitutions need not all be made with the same acid, but may 

 involve two or three different acids. Thus a variety of fats is possible 



