88 ORGANIC AGRICULTURAL CHEMISTRY 



and use of any foodstuff. All this will be considered again at 

 greater length in the later part of our study. 



Now, while proteins are like fats and carbohydrates in con- 

 taining carbon, hydrogen and oxygen, they differ from them 

 in containing also the element nitrogen. Protein, as the name 

 signifies, is the primary or fundamental substance of living 

 matter. It is one of the chemical constituents of the biological 

 substance protoplasm, which is the essential of all living cells. 

 As we shall find later, animals are unable to use the element 

 nitrogen for the purpose of forming new protein unless that 

 nitrogen is in the form of protein itself or of compounds very 

 closely related to protein. That is, to form animal protein, 

 food protein is essential. We thus see how important a place 

 the substances we call proteins occupy in connection with plant 

 and animal life. What then are these proteins in their chemi- 

 cal nature ? 



While in most cases proteins contain only the four elements 

 carbon, hydrogen, oxygen and nitrogen, they are among the 

 most complex compounds in organic chemistry. In addition 

 to being very complex, their physical and chemical properties 

 are such that they are among the most difficult of all compounds 

 to study. From the examples of proteins previously given, 

 viz. egg albumin or white of an egg, cheese curd or casein and 

 wheat gluten, the tough elastic constituent of wheat left when 

 the starch is washed away, we see that these substances are 

 of a somewhat different nature from that of ordinary chemical 

 compounds. In fact, we have hardly any evidence that any 

 one of the substances we know as a protein is a single individual 

 compound. Some proteins are definitely known to be complex 

 mixtures of several individuals, and probably no one of them 

 can be definitely considered as an individual. 



Physical Properties of Proteins 



The physical properties of proteins are in general those of 

 non-crystallizable (though some have been crystallized) colloidal 

 compounds. They do not have definite melting points or boil- 



