PROTEINS 179 



bohydrate-formation. The carbon, hydrogen, and oxygen neces- 

 sary for protein synthesis are undoubtedly obtained from carbo- 

 hydrates. The nitrogen and sulfur come from the salts absorbed 

 from the soil through the roots and brought to the active cells in 

 the sap. Atmospheric nitrogen cannot be used by plants for this 

 purpose, except in the case of certain bacteria and other low plants, 

 notably the bacteria which live in symbiosis with the legumes in 

 the nodules on the roots of the host plants. In general, the sulfur 

 must come in the form of sulf ates and the nitrogen in the form of 

 nitrates; although many plants can make use of ammonia for 

 protein-formation. Presumably, the nitrate nitrogen must be 

 reduced in the plant to nitrites, and then to ammonia form, in 

 order to enter the amino-arrangement required for the greater 

 proportion of the protein nitrogen. 



The mechanism by which ammonia nitrogen becomes amino- 

 acids in the plant is not understood. Artificial syntheses of 

 armno-acids, by the action of ammonia upon glyoxylic acid and 

 sorbic acid, both of which occur in plants and may be obtained 

 by the oxidation of simple sugars, have been accomplished, and it 

 seems probable that similar reactions in the plant protoplasm 

 may give rise to the various ammo-acids which unite together to 

 form proteins. Nothing is known, however, of the process by 

 which the more complicated closed-ring amino-acid compounds, 

 such as proline, histidine, or tryptophane, are synthetized. 



The condensation of amino-acids into proteins, or the reverse 

 decomposition, is very readily accomplished in all living proto- 

 plasm, under the influence of special protein-attacking enzymes, 

 which are almost universally present in the cytoplasm. These 

 reactions in connection with the proteins are similar to the easy 

 transformation of sugars to starches, and vice versa, under the 

 action of the corresponding carbohydrate-attacking enzymes. 



PHYSIOLOGICAL USES OF PROTEINS 



There can be no doubt that the all-important role of pro- 

 teins, in either plant or animal tissue, is to furnish the colloidal 

 protoplasmic material in which the vital phenomena take place. 

 Their occurrence in seeds, and other storage organs, is, of course, 

 in order to provide the protoplasm-forming material for the young 

 seedling plant. 



