MORPHOLOGY OF THE CELL 39 



meat, eggs, beans, and similar foods. Proteins are compounds of what 

 are known as amino-acids, substances in which the NH2 group and the 

 COOH radical are always present. Thus the proteins contain the four 

 elements carbon, hydrogen, oxygen and nitrogen, which are often named 

 as the only elements necessarily present in protoplasm. In fact, the 

 occurrence of all of these elements in every amino-acid, and hence in 

 every protein, is the reason for naming these four elements as the minimum 

 content of protoplasm. No chemical analysis of actual protoplasm 

 ever yields these elements alone. 



The amino-acids are combined with each other in various ways, 

 with elimination of water, to form proteins. Other substances not in 

 the amino-acid class may also enter into the composition of proteins. 

 Since there are many kinds of amino-acids, and many kinds of other sub- 

 stances which may combine with them, the variety of possible proteins 

 is numerous. Nearly all the proteins have huge molecules, composed 

 of hundreds or even thousands of atoms, and the arrangement of these 

 atoms is always complex. It has been impossible, up to the present 

 time, to determine this arrangement for any but the simplest proteins. 

 Structural formulas, by which the chemist aims to portray, not only the 

 number of each kind of atom in the molecule, but also the position of 

 these atoms relative to each other, are commonly used for inorganic 

 and simple organic compounds. These structural formulas are derived 

 from a knowledge of the behavior of the compound in reactions, and 

 are designed to express that behavior. The absence of structural formu- 

 las for most of the proteins means, not that they have no molecular 

 structure, but that the structure has not been discovered. Ignorance 

 of their structural formulas is mute testimony to the complexity of those 

 compounds. 



Certain proteins, owing to their importance in vital processes, may 

 be mentioned more particularly. The chromatin of the nucleus, which, 

 as stated above, serves as a sort of cell-governor, is composed of nucleo- 

 proteins. These are compounds of nucleic acid and some other sub- 

 stances, and are characterized by an abundance of phosphorus. In view 

 of the importance of chromatin in life processes, some one has said, 

 figuratively, that we are what we are because of the phosphorus that is 

 in us. The composition of the chromatin can be studied in quantity, 

 because it resists peptic digestion; so that, when the cell bodies are 

 dissolved away by the pepsin, the nuclei may be collected as a residue. 



The complexity of protoplasm, which is largely a mixture of proteins, 

 is necessarily much greater than that of any one protein. Protoplasm 

 cannot, of course, have a structural formula, since it is not a substance 

 in the chemical sense. But the uncertainty with regard to the nature of 

 any one protein is small when compared with the ignorance which sur- 

 rounds the nature of the protoplasm in its entirety. It will not occasion 



