PHYSIOLOGY OF CELLS 51 



acidic properties, and one or more amino radicals (NH2) which give 

 them basic properties. A simple example of an amino-acid is alanine, 

 whose structural formula is 



CH3 - CH - COOH 



I 

 NH2 



About twenty different amino-acids have been derived from proteins, 

 all capable, because of their NH2 and COOH groups, of acting either as 

 bases or as acids or as both at once. The amino-acids readily combine 

 with one another, the amino radical of one and the carboxyl group of 

 another serving as the point of combination, thus: 



- CO(OH H)NH- 



A molecule of water (H2O) is lost, and the bonds which formerly held the 

 — OH and H— of the resultant water to their respective molecules of 

 acid now hold these two molecules together, the point of union of the 

 two molecules being represented as 



-CO - NH— 



The two molecules thus joined may be of the same amino-acid, or of 

 two different acids. Thus, from twenty amino-acids, a considerable 

 number of new substances could be produced, even if the acids were 

 combined only two at a time. However, the variety of combinations is 

 not thus limited, for several or many molecules may be joined, and each 

 molecule may be of a different acid, or the various acids may be repre- 

 sented in different proportions or the arrangement may vary. Further- 

 more, other substances than amino-acids may be joined to the combined 

 product at various points, so that the variety of substances that can 

 conceivably be created wholly or chiefly of amino-acids is almost un- 

 limited. Add to this the fact that the arrangement of the molecules of 

 the acids in the combined product may be different in differcj(t cases, 

 owing to the fact that unions may often be made at any one of several points 

 in the molecule, and the diversity of the possible aggregations is greatly 

 multiplied. Other circumstances could be named which w'ould lend still 

 further emphasis to the fact that from a comparatively small number 

 of amino-acids an enormous number of compounds may be produced, 

 but the diversity of possible amino compounds has already been 

 sufficiently indicated. 



From such combinations of amino-acids and other substances all the 

 proteins of plants and animals are constructed. This statement is not 

 purely hypothesis, for the construction of polypeptides, as the compound 

 substances described above are called, out of amino-acids, and the careful 

 decomposition of proteins, have been carried to a point at which they 



