Life: Its Nature and Origin 35 



Three omino acids 



H -N - C„ -H /H)-N- C,- H 



Lose 2H2O 



R? H 



II I I 



NH,^ H Cb\ /Cc\ /Nx H 



a 



I 



R, H Rj 



' OH 



^1 



And form a peptide clioin 



Fig. 14. The formation of peptide chains. The carbon atoms have been 

 designated individually from o to / to aid comparison between the amino 

 acids and the peptide chain. 



NH2 NH2 



Phe Vol Asp Gl'u His Leu Cy Gly Ser His Leu Vol Glu Ala Leu Tyr Leu ValCy-Gly Glu Arq Gly PhePhe TyrThrProLysAlo 



I I 



S S 



I I 



NH, S NH? NH2 S NH2 



I I I I I I 



Gly Leu Vol Glu Glu Cy CyAlo SerVol Cy Ser LeuTyrGlu Leu- AspTyrCyAsp 



Ls S-J 



Fig. 15. The arrangement of amino acid residues in the molecule of the 

 protein insulin. The abbreviations indicate the identity of the amino acid 

 residues. (After Sanger.) 



Proteins are not the only groups of highly complex organic com- 

 pounds found in the living cell. A group of complex alcohols called 

 steroids comprise some of the important hormones and vitamins 

 necessary for cell functioning (Fieser, 1955). Complex sugars, cer- 

 tain phosphorus compounds active in respiration, a wide assort- 

 ment of pigments, and many other molecules may contain nearly 

 a hundred or more atoms per molecule. Among the largest molecules 

 in the cell are the nucleic acids, which are long chains of sugar 

 molecules with other groups attached to them. 



These compounds and many more must be doubled between each 



