GENERAL STRUCTURE AND COMPOSITION 13 



Nucleo proteins, consisting of protein combined with nucleic acid. These_ 

 are of great interest physiologically since they form the chief constituents of 

 cell nuclei, to which structures are assigned the function of exercising special 

 control over the activities of living cells. 



Glycoproteins, consisting of protein combined with a carbohydrate (see 

 p. 15). Mucin, the substance which gives the secretions of the mouth, nose, 

 and throat their peculiar viscous character, is an example of this group. 



Phosphoproteins, consisting of protein combined with a phosphorus- 

 containing substance. The casein of milk, which forms the curd, is the most 

 familiar member of this group. 



Hemoglobins, compounds of protein with a pigment. These are of great 

 physiological importance on account of the property, common to all of them, 

 of acting as transporters of oxygen. The type member of the group, the 

 hemoglobin of Mammalian blood, is of interest chemically on account of the 

 great size of its molecules, which are estimated to contain not less than 2,300 

 atoms each and to have molecular weight exceeding 16,000. 



Derived Proteins. The members of this group are derived, as 

 their name indicates, from the simple proteins. In the process of 

 protein digestion, by which the protein portions of the food are 

 made available for the needs of the Body by being split into 

 simpler substances, the first steps in the digestive process give rise 

 to compounds which differ from the simple proteins by a slight 

 degree only. These are the derived proteins. The members of 

 the group which occur mOst commonly in the Body are the proteases 

 and peptones. These are present in the stomach during protein 

 digestion. They are characterized by greater solubility than 

 simple proteins possess. 



Nitrogenous Extractives. Under this head are grouped various 

 nitrogen-containing substances most of which represent materials 

 that have done their work in the Body and are about to be gotten 

 rid of. Nitrogen is present in the living tissues of the Body chiefly 

 as a part of their proteins. The vital activities of the tissues in- 

 volve the breaking down of these complex proteins into simpler 

 substances. Part of their carbon combines with oxygen and passes 

 out through the lungs as carbon dioxid; their hydrogen is similarly 

 in large part combined with oxygen and passed out as water; 

 while their nitrogen, with some carbon and hydrogen and oxygen, 

 is passed out in the form of crystalline extractives. 



Urea is the most important substance of this class; fully nine-tenths of all 

 the nitrogen that is eliminated from the Body is in this form. It is a diamide 



