16 PROTOPLASM AND THE CELL 



protoplasm are clear and transparent and homogeneous 

 in structure, while other kinds appear to have a foamy 

 structure, like soapsuds. Protoplasm may sometimes 

 appear opaque because it contains considerable quantities 

 of food. 



The Chemistry of Protoplasm. — Protoplasm is not 

 a simple chemical compound, nor indeed is it simply a 

 mixture of chemical compounds; it is, in reality, an 

 organized structure, composed of many different com- 

 pounds. The larger part of its bulk consists of water, 

 two-thirds or more, in fact. The solid portion of it con- 

 sists chiefly of the chemical substance known as protein, 

 with some fats, some sugar, and various mineral salts. 

 In addition to these substances which presumably con- 

 stitute a part of the living machine, there are usually 

 present foods or other materials formed by the proto- 

 plasm. 



The actual number of elements which enter into the 

 structure of protoplasm is small. Carbon, hydrogen, 

 oxygen, and nitrogen are always present in the proteins 

 and usually in addition sulphur or phosphorus, or both. 

 The fats and sugars contain carbon, hydrogen, and oxy- 

 gen ; the water contains hydrogen and oxygen ; the va- 

 rious salts present contain calcium, magnesium, potas- 

 sium, sulphur, phosphorus, and iron. It is difficult to 

 decifle as to which of the elements enumerated are abso- 

 lutely indispensable to the living substance. It will be 

 shown in a subsequent chapter that living organisms 

 cannot maintain life without all of the elements we have 

 named, but it does not follow from this that each and 

 all constitute an actual part of the living machine. It 

 seems certain that nothing can be alive without protein, 

 water, and certain sorts of foods. It is also certain that 

 if the living protoplasm be deprived of the presence of 

 some of the mineral salts mentioned it will presently 

 cease to exhibit the living functions. This might mean 

 that they are all a part of it; but more likely it means 



