172 CHEMISTRY OF PLANT LIFE 



Physical Properties. (1) The proteins are all colloidal in 

 character, that is, they form solutions in water, out of which they 

 cannot be dialyzed through parchment, or other similar mem- 

 branes. (2) All natural proteins, when in colloidal solution, may be 

 coagulated, forming a semi-solid gel, which cannot again be ren- 

 dered soluble except by decomposition. The most familiar exam- 

 ple of this type of coagulation is that of egg-albumin, when eggs 

 are cooked. This coagulation may be produced by heat, by the 

 action of certain enzymes, or by the addition of alcohol to the 

 solution. (3) All solutions of plant proteins are optically active, 

 rotating the plane of polarized light to the left, in every case. 

 (4) Proteins are precipitated out of their solutions, without change 

 in the composition of the protein, by saturating the solution with 

 various neutral salts of the alkali, or alkaline earth, metals, such 

 as sodium chloride, ammonium sulfate, magnesium sulfate, 

 etc. This is only another way of saying that the proteins are 

 insoluble in strong salt solutions. Separation from solution by 

 the addition of salts is different from coagulation by heat, etc., as 

 in this case simple dilution of the salt solution will cause the pro- 

 tein to redissolve, whereas a coagulated protein cannot be redis- 

 solved without some change in its composition. 



Chemical Properties. (1) Precipitation reactions. The pro- 

 teins have both acid and basic properties (due to the presence in 

 their molecules of both free NH2 groups and free COOH groups). 

 Bodies of this kind are known as " amphoteric electrolytes," since 

 they yield both positive and negative ions, if dissociated. The 

 proteins readily form salts, which are generally insoluble in water, 

 with strong acids. For this reason, they are generally precipitated 

 out of solution by the addition of the common mineral acids. They 

 are also precipitated by many of the " alkaloidal reagents," tio 

 tion of matter has been studied. A more logical arrangement so far as the 

 systematic study of these properties is concerned would be to take up chapter 

 XV before undertaking the study of the proteins (this order is actually fol- 

 lowed in some texts on Physiological Chemistry). But from the standpoint 

 of the consideration of the various groups of organic components of plants, 

 it seems a better arrangement to consider these groups in sequence, and then 

 to discuss the various physical-chemical phenomena which govern their activ- 

 ity. However, it is recommended that the student refer at once to Chapter 

 XV for an explanation of any terms used here, which may not be familiar to 

 him; and that after the study of Chapter XV, he return to this chapter dealing 

 with the proteins for an illustrative study of the applications of the principles 

 presented there. 



