The Basis of Life 31 



atom has the unique abihty of being able to unite with other carbon 

 atoms to form chains or rings. Possibly this ability is due to the fact 

 that the carbon atom has exactly half the number of electrons (4) in 

 the outer orbit that are needed for saturation; thus it tends to share 

 electrons. Often other elements or radicals are attached to these rings 

 or chains and complex molecules result. 



There are several ways in which the organic compounds differ 

 markedly from the inorganic ones. On the whole they are less stable 

 when heated, do not react readily, tend to be less soluble in water, 

 burn easily to produce carbon dioxide and water, and, in general, are 

 much more complex. 



In biological materials, there are three main classes of organic 

 materials which are of importance. These are the carbohydrates, 

 lipids, and proteins. 



The Carbohydrates. — The chief source of energy for animals is 

 furnished by the carbohydrates. These are the sugars and starches. 

 In all carbohydrates, there are but three elements present : carbon, 

 hydrogen, and oxygen. The latter two are always present in the same 

 ratio as in water, that is, two parts of hydrogen to one of oxygen. 

 Sugars vary greatly in their complexity. The simple ones, known as 

 the monosaccharides, consist of but a single unit. An example of 

 this type is glucose, which has the formula CeHisOe. 



If two monosaccharide units unite, a single disaccharide molecule 

 is formed. A typical disaccharide is sucrose, the chief sugar found in 

 honey, milk, fruits, and in sugar cane. 



The starches are the polysaccharides, that is, many monosaccharide 

 units are hooked together. These molecules are very large and quite 

 insoluble. During digestion these are changed into the simpler, soluble 

 monosaccharides and disaccharides. Starches are an important source 

 of food for both human beings and animals. Potatoes, rice, corn, and 

 many other foods are good sources of starches. Glycogen, a special 

 polysaccharide, is found in the liver of many animals. 



Lipids.— Lipids constitute an important group of organic materials 

 familiar as the fats and fatty acids. In the animal body they play an 

 important role in the storage of energy ; in addition, they appear to be 

 of great structural importance in the living body. 



The lipids have the same three elements as the carbohydrates: 

 carbon, hydrogen, and oxygen. The chief difference between the fats 

 and carbohvdrates is the fact that in fats there is less oxygen in pro- 



