CHAPTER XIII 

 PROTEINS 



THE proteins are the most important group of organic com- 

 ponents of plants. They constitute the active material of pro- 

 toplasm, in which all of the chemical changes which go to make up 

 the vital phenomena take place. Combined with the nucleic 

 acids, they comprise the nucleus of the cell, which is the seat of 

 the power of cell-division and, hence, of the growth of the organism. 

 Germ-cells are composed almost exclusively of protein material. 

 Hence, it is not an over-statement to say that proteins furnish the 

 material in which the vital powers of growth and repair and of 

 reproduction are located. A recognition of their importance is 

 reflected in the use of the name " protein/' which comes from 

 a Greek word meaning "pre-eminence," or "of first importance." 



In addition to the proteins which constitute the active proto- 

 plasm, plants also contain large amounts of reserve, or stored, 

 proteins, especially in the seeds. In the early stages of growth, 

 the proteins are present in largest proportions in the vegetative 

 portions of the plant; but as maturity approaches, a considerable 

 proportion of the protein material is transferred to the seeds. 



GENERAL COMPOSITION OF PROTEINS 



The plant proteins are fairly uniform in their percentage 

 composition. The analyses of some sixteen different plant pro- 

 teins show the following maximum limits of percentages of the 

 different chemical elements which they contain: Carbon, 50.72- 

 54.29; hydrogen, 6.80-7.03; nitrogen, 15.84-19.03; oxygen, 

 20.86-24.29; sulfur, 0.17-1.09. Animal proteins vary more 

 widely, both in percentage composition and in properties, than 

 do those of plant origin. 



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