PHOTOSYNTHESIS: GENERAL 



21 



the processes of decay. This continuous transfer of materials 

 from the organic to the inorganic realm and vice versa results in 

 an organic cycle, which is represented diagrammatically in Fig. 2. 

 Foods.— A very large percentage of the organic materials are 

 built up into what are called foods, which have two main purposes: 

 (1) they supply energy when burned and (2) they supply material 

 used in construction. There are three classes of foods which are 



Free (inorganic) T 

 carbon dioxi de 1 



Forces of decay 

 (bacteria) 



Intermediate 

 decomposition products 



Fig. 2. — Illustrating the organic carbon cycle. 



structurally different and which fulfill the different needs of the 

 plant, — the carbohydrates, fats, and proteins. 



The simplest of these are the carbohydrates, of which the 

 starches and sugars are the most important. They are composed 

 of carbon, hydrogen, and oxygen and, when burned in the plant, 

 liberate energy and heat the same as when burned in an animal or 

 in a stove. In the plant, carbohydrates also play an important 

 role in construction. The cellulose cell wall so characteristic of 

 plants is a carbohydrate. Animals, on the contrary, use very 

 little carbohydrate except for supplying energy. 



Fats are used in the plant almost exclusively as sources of energy 

 and very little for construction. Proteins are used largely in the 

 manufacture of protoplasm and little, if at all, as sources of energy. 

 Both the fats and proteins will be discussed more fully in a later 

 chapter. 



Source of All Food.— All of the foods for both plants and 

 animals are built up by green plants. The green plant is the source 

 of all food, and this truth is one of the most outstanding facts in 



