42 



LIFE: ITS BEGINNINGS AND NATURE 



As in the case of starch, the reverse ac- 

 tion, hydrolysis, takes place during diges- 

 tion when water is added, splitting the fat 

 into fatty acids and glycerol. 



There are other lipids called phospho- 

 lipids and steroids, which have elements 

 such as phosphorus in combination with 

 one of the fatty acids. They resemble fats 

 in many ways, but have other characteris- 

 tics that tend to set them off in a group by 

 themselves. Some of them play their role 

 in the plasma membrane where they are 

 responsible for the selective action of this 

 delicate structure. They are also impor- 

 tant in some of the intricate chemistry of 

 the animal body which we shall touch on 

 later. 



Proteins. In addition to C, H, and O, 

 proteins contain the elements nitrogen ( N ) 

 and sulfur (S); usually phosphorus (P) is 

 also included. These are all combined into 

 huge molecules, some composed of thou- 

 sands of atoms. Like starch and fats, pro- 

 tein molecules can be hydrolized to simpler 

 components, called, in this case, amino 

 acids. There may be hundreds of amino 

 acid molecules in a single protein molecule 

 but, when broken down, it yields only a 

 few different kinds of amino acids. There 

 are about 25 amino acids known, all of 

 which are not usually found in any one kind 

 of protein. The proportion of the different 

 amino acids will depend on the nature of 

 the original protein molecules. 



The atoms of the amino acids are ar- 

 ranged in a definite manner so as to pro- 

 duce two distinctly specific groups by 

 which they can always be identified. They 

 have the general formula 



R 

 I 

 H— C— NH2 



COOH 



where R represents the main portion of the 

 molecule. The remainder is found in vir- 

 tually all amino acids. It will be noted that 



there is one group containing NHo, which is 

 spoken of as the amino group; the other 

 contains COOH, the carboxyl group, with 

 which we are already familiar (p. 41). 

 These two groups are responsible for the 

 behavior of amino acids. It is obvious that 

 the presence of the carboxyl group gives 

 the molecule acidic properties, just as is 

 true of any organic acid. Strangely enough, 

 an amino acid can also act like a base due 

 to the presence of the NH2 group. It re- 

 sponds like a base by removing hydrogen 

 ions, not by delivering hydroxyl ions. This 

 is demonstrated by the following equa- 

 tion: 



R • NH, -f H+ + CI- ^ R • NH3 + Cl- 

 in the presence of an acid, then, an amino 

 acid acts like a base by absorbing or remov- 

 ing the hydrogen ions. On the other hand, 

 in the presence of a base it acts like an acid 

 by delivering hydrogen ions from its car- 

 boxyl groups, thus: 



R • COO- + H+ + Na+ -f OH" 

 -» R- COO--H Na+ + HoO 



A substance that responds in this fashion is 

 said to possess amphoteric properties. It 

 therefore always has a tendency to bring 

 a solution to neutrality, that is, to balance 

 the number of hydrogen and hydroxyl ions. 

 For this reason, amino acids tend to prevent 

 too much fluctuation in the number of 

 hydrogen and hydroxyl ions in a solution. 

 Such a substance is spoken of as a buffer; 

 amino acids are good buffers. 



Amino acids unite to form proteins in 

 the plant leaf by the same process that 

 starch and fats were produced, namely, by 

 the loss of water. Likewise, in every cell 

 of the animal body proteins must be assem- 

 bled from the amino acids that come to it 

 through the blood stream. Just how this is 

 done has puzzled chemists for a long time, 

 but it is thought to occur through the so- 

 called peptide linkage. The following equa- 

 tion will illustrate how this occurs: 



