METABOLISM 119 



materials and that animals are not. Given a simple food like 

 glucose, however, both animals and plants are able to construct 

 therefrom an endless variety of more complex foods and of other 

 organic compounds. There is thus a constant circulation of 

 various materials through air and soil and through the bodies of 

 green plants, of animals and of bacteria, a process by which 

 organic substances are continually being built up and broken 

 down. This Organic Cycle is graphically represented in Fig. 65. 



Foods may be divided into three main classes, which we call 

 carbohydrates, fats, and -proteins. These food types differ from 

 each other in physical structure and chemical composition as well 

 as in the parts which they play in nutrition. 



A. Carbohydrates. — Carbohydrates are substances composed 

 entirely of carbon, hydrogen, and oxygen, in which the hydrogen 

 atoms are about twice as numerous as those of oxygen. Glucose 

 (C6H12O6), the product of photosynthesis, is an example of a very 

 simple carbohydrate. To this group of foods belong the various 

 sugars, starches, and celluloses, which comprise the great bulk of 

 the food of animals and plants. Carbohydrates are the chief 

 source of energy for all organisms and provide most of the build- 

 ing material for the plant body. 



The sugars are soluble carbohydrates. Three are more 

 common in plants than others. These are glucose or grape 

 sugar, C6H12O6, the direct product of photosynthesis; /nodose or 

 fruit sugar, identical in chemical formula with glucose but differ- 

 ing in the arrangement of its atoms and in certain physical charac- 

 teristics; and sucrose (cane sugar or beet sugar), with the formula 

 C12H22O11, and produced from the simpler sugars by the removal 

 of a molecule of water, thus: 



2C6H12O6 — H2O = C12H22O1] 



These three types of sugar are all common in plants, though in 

 any particular species one is usually more abundant than the 

 others. Glucose and fructose form the bulk of the sugar of 

 fruits and of the nectar of flowers, from which honey is derived, 

 and are common elsewhere, glucose probably occurring in every 

 living plant cell. Sucrose is abundant in the sugar-cane and 

 sugar-beet and is therefore the type of sugar with which we are 

 most familiar. Sugars are stored in many plants as reserve 

 foods, but are also widely distributed throughout the plant body, 

 because of the fact that those carbohydrates which are insoluble 



