THE CELL 41 



be converted into kinetic energy by the metabolic processes. The 

 container of potential energy for protoplasm is commonly called a 

 FOOD. Foods also have a second value, namely, the providing of ma- 

 terials used in building protoplasm. 



Photosynthesis. The chief difference between green plants 

 and animals is in their methods of obtaining foods, both as a supply 

 of potential energy and as a source of ingredients necessary for the 

 maintenance of the cell. In the green plant cell the green substance, 

 CHLOROPHYLL, is located in plastids known as chloroplastids (Fig. 

 8). It is an activator which causes a reaction to take place between 

 two common and otherwise undistinguished substances, water and 

 carbon dioxide. This reaction can be effected by chlorophyll only in 

 the presence of sunlight, the energy necessary for the combination 

 of the substances being derived from solar energy. The result of 

 the reaction is a carbohydrate; the molecule contains potential 

 energy derived from the sun. The steps in the reaction are not yet 

 thoroughly understood but may be summarized briefly as follows: 



CO2 + 3 H2O (In presence of sunlight) = CHOH + 2. H2O2 

 6 CHOH = CeHisOe (a sugar) 

 2. H2O2 = 1 H2O + O2 



The end results of the reaction, which in reality is a series of reac- 

 tions, are by-products, water and oxygen, and a carbohydrate con- 

 taining potential energy. 



Carbohydrates are formed only in this way, by the trapping of the 

 energy of the sun in green plant cells. The entire supply of carbo- 

 hydrate for all animals and all plants therefore depends on the 

 action of chlorophyll, and the ultimate source of energy for all liv- 

 ing objects is the sun. This process of the synthesis of carbohydrate 

 by chlorophyll and solar energy is called photosynthesis. 



The proteins, which form a most important group of protoplas- 

 mic constituents, are nitrogen-containing compounds. Although the 

 air is about 79 per cent nitrogen, neither plants nor animals are 

 able to utilize the atmospheric nitrogen in building proteins. 



