PROTOPLASM AND CELL 47 



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, 

 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. Cer- 

 tain of the single-celled blood parasites are about as small as any 

 cells known. They are barely seen with our highest magnifications. 

 At the other extreme of size we may refer to another parasitic 

 single-celled animal, Porospora gigantea, which lives in the intestine 

 of the lobster, and may reach from one-half to two-thirds of an 

 inch in length. Egg cells, including the yolk, may exceed this size. 

 Some of the nerve cells, though of less mass, may be several feet 

 in length. Muscle cells are relatively long also. 



The shape of the typical cell is spherical; but due to the effects 

 of mechanical pressure, specialized functions, and unequal growth 

 almost all cells are far from this shape. They vary greatly in shape 

 and include platelike, cubical, columnar, polygonal, and spindle- 

 shaped forms. The particular form of any cell is not a haphazard 

 matter but strictly controlled by morphological and functional 

 necessities. 



