194 PLANT BIOCHEMISTRY 



proteins are not nearly so extensive as those devoted to the seed pro- 

 teins for the simple reason that the subject is complicated by (1) the 

 instability of leaf proteins and (2) the large number of components 

 involved. Seed globulins are not easily denatured, whereas the leaf 

 proteins are very readily altered by relatively mild treatment. 



In discussing cellular constituents, mention was made of centrifugal 

 separation of cellular components. By this means it is possible to in- 

 vestigate separately the protein components of the nucleus, chloroplasts, 

 mitochondria, microsomes, and cytoplasmic lluid. Although the bio- 

 logical activities of these cellular fractions has been studied in some 

 detail (pages 178 to 180), information on the complete spectrum of 

 proteins involved is lacking. Cytoplasmic proteins have been sep- 

 arated into a number of fractions by the use of classical methods. 

 From the known enzymatic reactions of the soluble components it is 

 obvious that each fraction must contain a large number of different 

 enzymes. 



The nuclear proteins have also been subjected to extensive investiga- 

 tion. It was once thought that the nucleoproteins were confined to 

 the nucleus, therefore the name. It is now known that, whereas 

 ribonucleoproteins are to be found primarily in the cytoplasm, some 

 occur in the nucleoli. Deoxyribonucleoproteins are the main con- 

 stituents of chromosomes and are presumably directors of the heredi- 

 tary process. Ribonucleoproteins are constituents of microsomes and 

 mitochondria. The microsomes are thought to be involved in protein 

 synthesis and thus would serve as the connecting link between the 

 controlling nucleus, the heredity, and the metabolically active cyto- 

 plasm. 



Metabolic Cof actors 



Proteins reqixire the assistance of large numbers of cofactors in the 

 performance of their roles as catalysts. Included in this class are 

 the requisite inorganic elements (page 141), vitamins (page LS9), 

 nucleotides (page 120), and the plant growth regulators (page 280). 

 Strictly speaking, the term vitamin does not apply to plants since 

 these cofactors are synthesized by the plant; however, the term will be 

 used to avoid confusion. 



In general, the vitamin distribution in plants follows the same 

 pattern as in animals. The greatest concentrations are found in the 

 regions of most active vegetative growth or in the centers of reproduc- 

 tion. For example, many of the B vitamins are associated closely with 

 the mitochondria (page 179). The fat-soluble vitamins E and K are 

 closely associated with the lipide and phospholipide portions of the 



