A common procedure is to extract groups (a) and (b), using a 10 per cent salt solution as the solvent, and then to separate the albumins, globulins, etc., from this solution by suitable precipitants; then to treat the material with 80 per cent alcohol, to extract the prolamines; and finally with dilute alkali, to extract the glutelins. The dissolved proteins in each extract can be subsequently purified by dialysis, precipitation, etc. The insoluble proteins can be studied only after removing the other materials associated with them in the tissue, by suitable mechanical or chemical means.
THE SYNTHESIS OF PROTEINS IN PLANTS
The synthesis of proteins in plants is not a process of photosynthesis, as it can take place in the dark and in the absence of chlorophyll, or any other energy-absorbing pigment. However, protein-formation normally takes place in conjunction with carbohydrate-formation. The carbon, hydrogen, and oxygen necessary for protein synthesis are undoubtedly obtained from carbohydrates. 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 sulfates 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 amino-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 amino-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 protoplasm, 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 rôle of proteins, 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.
They are, moreover, the source for the material which goes into some of the secretion groups of organic compounds; as they are easily broken down by various agents of decomposition into nitrogen-free alcohols, aldehydes, and acids, which produce the essential oils, pigments, etc.
Much, if not all, of their physiological activity is due to their colloidal nature, the importance and effects of which will be more apparent after the chapters dealing with the colloidal condition of matter and with the physical chemistry of protoplasm have been studied.