58 TEXTBOOK OF ZOOLOGY 



types of reactions going forward in active protoplasm, it is seen 

 that there must be numerous enzymes present in the cells of every 

 organism. 



Water constitutes 60 to 90 per cent of protoplasm and maintains 

 many substances in solution. Water is not only a very efficient 

 solvent; but it is important to protoplasm because of its compara- 

 tively high surface tension, because its presence gives the proto- 

 plasm a consistency compatible with the range of variation neces- 

 sary for metabolism, and because of its high specific heat. This 

 latter point is important in maintaining protection against sudden 

 and extreme temperature changes in the living organism. Young 

 cells contain more water than old ones, young organisms likewise 

 contain more than old ones. The relative amounts of water in rela- 

 tion to other materials of the protoplasm vary in different cells and 

 in different species. 



The inorganic salts are present in considerable numbers but in 

 relatively small amount. They are electrolytes, and therefore split 

 up in aqueous solution into ions, which are able to combine with 

 all the other substances in protoplasm. The chlorides, phosphates, 

 iodides, carbonates, and sulphates of sodium, potassium, calcium, 

 magnesium, and iron are important salts of living cells. The relative 

 proportion of these salts is kept at a fairly constant level, and slight 

 changes in this balance have regulatory effects on metabolism. 



From the chemical standpoint, living protoplasm is considered 

 the most complex of all systems of compounds. Even the proteins, 

 as a part of protoplasm, are more complex than any other sub- 

 stances. In a sense, protoplasm is quite unstable in that it changes 

 its composition in response to every change in the environment, and 

 when active it is not the same for any two consecutive moments. 

 The exceeding variability of protoplasm chemically, makes possible 

 all of the necessary adjustments of living matter to its environment. 

 On account of the extreme complexity of protoplasm it is not sur- 

 prising that the chemistry of all of its activities is not yet com- 

 pletely understood. 



Structure of a Typical Animal Cell 



The quantity of protoplasm comprising a single cell varies within 

 wide limits ; therefore cells vary greatly in size. The majority of cells, 

 but not all of them, require considerable magnification to be seen. Cer- 



