THE NATURE OF PROTOPLASM 75 



cells, specialize in the production of enzymes, which are passed 

 out to catalyze external reactions, as in digestion. 



The inorganic salts of the protoplasmic system include the 

 ones found in sea water, chiefly sodium and calcium chlo- 

 ride. Water constitutes 60 to 90 per cent of protoplasm and 

 holds the inorganic salts in crystalline solution. This watery 

 solution is the dispersion medium for the very finely divided 

 masses of carbohydrates, fats, lipins, and proteins, which are 

 the coexisting dispersed phases. The ionization of the dissolved 

 salts is responsible for the electrical phenomena that protoplasm 

 exhibits. 



It cannot be too strongly emphasized that protoplasm is not a 

 living compound but a highly organized colloidal system. Under 

 the conditions of enormous exposure of surface of the dispersed 

 phases and the intimate associations of these ultramicroscopic 

 masses, are made possible the multitudinous autocatalyzed 

 chemical reactions and the physical phenomena which are char- 

 acteristic of living organisms. These chemical reactions and 

 physical processes are intordependent, and in normal protoplasm 

 a very delicately balanced equilibrium between these processes 

 gives rise to the visible manifestations that we call life. Life is not 

 a property of any single constituent of protoplasm, or of all of 

 them taken together, or of their manner of organization, but is the 

 result of the reactions made possible under the physico-chemical 

 conditions which have just been explained. As Hopkins puts it, 

 the life of a coll, and, therefore, of organisms, "is the expression of a 

 particular dynamic equilibrium which obtains in a polyphasic 

 system," known as protoplasm. 



Distinguishing Capacities of Living Organisms. — Living organ- 

 isms differ from lifeless things in three fundamental respects. 

 These distinguishing capacities are metabolism, irritability, and 

 reproduction. Under metabolism, as a distinguishing character- 

 istic of living organisms, are included all the chemical reactions 

 that occur in protoplasm. These reactions group themselves 

 under the headings of assimilation and dissimilation. Assimilation 

 includes those reactions by which suitable materials are taken in 

 by protoplasm and built up into the constituent parts necessary 

 for its normal functioning. Dissimilation is, on the other hand, 

 the sum of the disintegrative changes, chiefly oxidations, which 

 occur in protoplasm, resulting in the transformation of energy, with 



