48 THE CHEMISTRY OF LIFE 



cells and a necessary part of the environment of cells. Water serves as 

 the solvent for the dissolved materials in the cell and as a medium in 

 which the larger particles are dispersed. 



Inorganic Minerals. Within the protoplasm there will be a number 

 of different inorganic salts dissolved and broken down into the ionic 

 state. It is interesting to note that these salts are present in about the 

 same proportions as in sea water. (The theory of a marine origin of 

 life might explain this coincidence.) The most common of these is 

 common table salt (sodium chloride), but there are also salts of 

 calcium and potassium. We know that these salts must be maintained 

 in a proper balance if the protoplasm is to function properly. A frog's 

 heart, when removed and placed in water containing a little sodium 

 chloride, will soon stop beating; but when the ions of sodium, calcium, 

 and potassium are all three present in the proper proportions, it will 

 beat for a considerable time. Slightly less than 1 per cent of pro- 

 toplasm by volume consists of inorganic salts. The inorganic minerals 

 also include certain dissolved gases, mainly oxygen and carbon dioxide. 



Organic Compounds. Within the cell there are a number of dif- 

 ferent kinds of organic compounds. Three great classes of these or- 

 ganic compounds can be broken down with a release of energy. They 

 are: 



Carbohydrates. These are composed of carbon, hydrogen, and oxy- 

 gen, with the hydrogen and oxygen present in the ratio of 2:1, which 

 is the same as in water. The sugars, starches, and celluloses come under 

 this heading. Simple sugar (CgH^Oe) is the product of food manu- 

 facture in a green plant, but usually these molecules are combined to 

 form more complex sugars or starch for storage. Simple sugar is also 

 found in animal cells, but may be converted into glycogen (animal 

 starch) for storage. Cellulose is a very complex carbohydrate which 

 forms the cell wall of many plants. It is not found within the proto- 

 plasm itself. 



Fatty compounds. These are also composed of carbon, hydrogen, 

 and oxygen, but the oxygen is present in much smaller quantity in pro- 

 portion to the carbon and hydrogen. A typical animal fat from beef 

 tallow (tristearin) has the formula: C57H 110 C>6. The true fats (lipins) 

 can be broken down into two kinds of smaller molecules known as 

 glycerol and fatty acids. There are other fatty compounds (lipoids) 

 which contain these same two molecules, but contain other molecules 

 as well. The outer plasma membrane of cells contains lipoids as well 

 as the other cell membranes, such as the nuclear membrane. 



