THE PROTOPLASMIC SYSTEM 



fatty acid plus lipase = lipin — depending upon the amount 

 of water present.^ 



The chemical make-up of the three organic constituents 

 of protoplasm renders easy their conversion into different 

 compounds; the chains are broken up and when the links 

 are reformed they show a different relation to each other. 

 The proteins have the greatest lability in this respect. 

 From the 21 amino acids which compose them it is theoreti- 

 cally possible to derive millions of different kinds of proteins . 



This is one reason why the proteins are regarded as 

 responsible for specificity; all species now in the world can 

 doubtless be distinguished from each other on the basis of 

 the chemical constitution of their proteins. But it should 

 be borne in mind that protoplasm never exists entirely 

 free of carbohydrates and lipins; that from sugars plants 

 make protein; that sugar as a pentose or as hexose is an 

 essential constituent of nucleic acid, that lipins are the 

 greatest source of energy and are the great binding sub- 

 stance; that lipins if not present as such in the nucleus 

 probably give rise to the phosphatized portion of the 

 chemical substance, nucleo-protein, found in the nucleus; 

 and finally, that much of the protein in cells does not exist 

 as simple protein but as a conjugant with lipins or lipin- 

 derived phosphate. Protoplasm is not a compound but a 

 complex of compounds of these three organic constituents 

 together with other constituents, water, gases, salts. 



Water is the most abundant compound in protoplasm. 

 Roughly, two-thirds of an animal's body is water; in some 

 cases it may amount to more than 90 per cent, of the body 

 weight. Even water-poor cells, as the enamel of the teeth 

 and spermatozoa, still possess a great deal of water. Active 

 protoplasm both liberates and utilizes COo and Oo. Dis- 



^ See Bradley and others. 



41 



