Living Cells and What They Are Made Of 17 



which does not contain protein in every part. We are justified in re- 

 garding proteins as the characteristic typical substances of life. 



The constitution of proteins, therefore, became a matter of great 

 interest. It was found that they are composed of simpler substances, 

 known as amino acids. The characteristic feature of these compounds 

 is that they contain both 'acidic' and 'basic' parts.^ Because of this they 

 are capable of uniting with each other, the basic group of one amino 

 acid combining with the acidic group of another. (Fig. 1.) The 



Five 



different 

 amino 

 acids 



A 



peptide 



chain 



FIG. 1. A diagrammatic representation of the molecules of five different amino 

 acids, showing how they combine together (with the elimination of water, 

 HOH) to form a peptide chain. This is the basic structure of proteins. The 

 different characteristic groups of the amino acids are represented by con- 

 ventional signs. For their chemical formulae see Appendix 1 



amino acids have, as it were, two hands : an acidic hand and a basic 

 hand. Suppose all the acidic hands wear red gloves and all the basic 

 hands wear black gloves; then the protein can be pictured as like a 

 row of people in which every hand v/ith a red glove clasps his neigh- 

 bour's black glove. 



In this way long chains of amino acid groups are formed. As there 

 are found in proteins about twenty separate kinds of amino acids, 

 which can be joined together in this way, it is easy to see that the num- 

 ber of ways in which they can be arranged is almost endless. It is thus 

 possible to have a very large number of different 'chains' of amino 



^ The acid group is CO. OH and the basic group H:>N 

 R 



