CHAPTER XXVI 

 NITROGEN METABOLISM 



Proteins and other related nitrogen-containing compounds are the prin- 

 cipal constituents of protoplasm and hence are directly or indirectly involved 

 in all of the physiological processes occurring in living cells. Proteins also 

 often occur in plant cells in the form of stored foods, especially in the seeds 

 of many species. Such "reserve" or storage proteins differ in their physical 

 and chemical properties from the protoplasmic proteins. The latter are more 

 complex than the former, they do not respond to the usual protein tests, and 

 they cannot be extracted from tissues by the same methods used in extracting 

 storage proteins. Our knowledge of the synthesis and properties of plant 

 proteins is based almost entirely upon studies of the storage proteins. The 

 structural organization of the protoplasmic proteins is believed, however, to 

 be essentially similar to that of the storage proteins, although in general they 

 seem to be more complex, many of them being of the type known as "con- 

 jugate proteins" (see later). In addition to the proteins a number of other 

 kinds of nitrogenous organic compounds occur in plants, some of which play 

 important parts in plant metabolism. 



The Proteins. — All proteins contain carbon (50-54 per cent), hydrogen 

 (about 7 per cent), nitrogen (16-18 per cent) and oxygen (20-25 per 

 cent). Although some animal proteins do not contain sulfur this element is 

 apparently present in all plant proteins. The percentage of sulfur in protein 

 molecules never exceeds 2 per cent, however. Phosphorus is also a constituent 

 of certain important types of plant proteins. 



The percentage composition of proteins gives no idea of the structure 

 of protein molecules nor of their size. The molecular dimensions of protein 

 molecules are enormous as compared with most other kinds of molecules. 

 Determinations made by the ultracentrifuge method indicate that proteins of 

 one group have molecular weights of 34,500, of another group 68,000, a 

 third 104,000, and a fourth 208,000 and that some have molecular weights 

 as great as 500,000 (Svedberg, 1930). While the molecular weight as de- 

 termined for a given protein by other methods does not always tally with 

 the value obtained by the ultracentrifuge method, all determinations agree 

 in indicating very high molecular weights for proteins. 



437 



