14 ELEMENTS OF BIOLOGY 



gether and, of course, many side bonds that may in turn be attached 

 to complex arrangements of carbon and associated atoms. Their 

 structure and the fact that the carbon atom has four bonds to be 

 satisfied make for the almost infinite variety of compounds that 

 form the protein group. Some are known to be made up of many 

 thousands of atoms; the weight of a single protein molecule may 

 equal that of more than sixty thousand hydrogen atoms. 



Protein molecules are in reality made up of various combinations 

 of molecules of distinctly simpler nature, known as amino-acids. 

 Nineteen different amino-acids are known to exist. They all contain 

 one or more radicals, known as carboxyls, with the structure 

 COOH, and one or more nitrogen-containing radicals, NH2. The 

 COOH gives them an acid property, since the H will ionize when 

 in water, and the NH2 gives them their alkaline property because 

 in water it tends to behave like ammonia, NH4OH. Proteins are 

 formed by the combination of various amino-acids, the COOH radi- 

 cal of one being combined with the NH2 radical of another, and 

 so on. The amino-acid molecules usually contain more than one 

 COOH radical and more than one NH2, so they are capable of be- 

 ing combined in many ways to form many varieties of protein. For 

 purposes of clarifying the role played by amino-acids in the forma- 

 tion of proteins and the intermediate compounds that represent 

 steps in the reaction, we may compare the relation of the various 

 substances with the use of type in forming words, sentences, and 

 paragraphs. Let the letters represent the amino-acids. Words then 

 represent various combinations of letters, as intermediate compounds 

 represent various combinations of amino-acids. Sentences are formed 

 of combinations of words; and the combination of intermediate 

 compounds are steps in the formation of proteins. Finally, sen- 

 tences form paragraphs that have definite meanings; and intermedi- 

 ate compounds form proteins of definite character. As the number 

 of words, sentences, and paragraphs that may be formed by the 

 letters of the alphabet are almost infinitely great, so the number of 



