PROTEINS 167 



in that the proteins are unchanged and do not combine with the 

 salt. It also differs from coagulation by heat in that here mere 

 dilution of the salt solution will bring the proteins back into solu- 

 tion again; but when coagulated by heat the protein must be 

 decomposed before it will redissolve. 



The precipitation by salts is important physiologically because 

 of its connection with cold injury to plants. When water gets 

 down to the freezing point the proteins of the protoplasm are not 

 injured. As the temperature falls still farther the water in the 

 cell sap may crystallize out, leaving the salt in solution and ulti- 

 mately resulting in the production of a saturated salt solution 

 within the cell. As this condition approaches, the proteins in solu- 

 tion may be precipitated, with the result that the cell dies if this 

 condition persists for a long time. If the precipitated condition 

 has not lasted too long, raising the temperature will restore the 

 proteins to their original state and the cell protoplasm to its 

 normal function. 



In Begonia the soluble proteins are precipitated when the temper- 

 ature reaches -3° C. while in the pine, according to Gorke (1906), 

 a temperature of —40° is required to obtain a similar result. This 

 is probably due to the difference in the amount of soluble salts 

 and sugars in the two plants. Plants which are subject to periodic 

 drought like many conifers are said to possess only small amounts 

 of solutes in their sap. 



Classification of Proteins. — Proteins were originally classified 

 upon the basis of their solubility, coagulability, and precipitability. 

 Since more has been learned of their chemical characteristics, 

 these are used in addition to the physical characters for classifi- 

 cation but, in general, the consideration of chemical characters 

 has not required many important changes in the original classifi- 

 cation based upon physical characters. The following is the scheme 

 generally adopted in America, but it differs in some particulars 

 from that used by English biochemists: 



I. Simple proteins. II. Conjugated proteins. III. Derived proteins. 



1. albumins. 1. chromoproteins. A. Primary derivatives. 



2. globulins. 2. nucleoproteins. 1. metaproteins. 



3. glutelins. 3. glucoproteins. 2. coagulated pro- 



4. prolamins or 4. phosphoproteins. teins. 



gliadins. B. Secondary derivatives. 



5. histones. 1. proteoses or 



6. protamines. albumoses. 



7. albuminoids or 2. peptones. 



scleroproteins. 3. peptids. 



