SOME OF THE LIFE PROCESSES 123 



released upon the oxidation of the carbon in the coal as 

 having been stored there by the sun millions of years ago, 

 when the coal was the growing tissue of a tree. We may 

 borrow from physics two other terms which will help us in 

 getting the notion of the states in which energy may exist. 

 Energy at rest — as, for example, chemical affinity (that 

 is, the readiness of the carbon to combine with oxygen) — 

 is called potential energy ; energy in action, as heat, light, 

 electricity, and motion, is called kinetic energy. As we have 

 already seen, potential energy may become kinetic energy, 

 and kinetic energy may become potential energy. 



Carbohydrates, and especially fats, are capable of com- 

 bining with a relatively large amount of oxygen because of 

 the small proportion of that element in those compounds 

 and the large proportion of carbon. Carbohydrates, chiefly 

 glycogen, one of the many kinds of sugar in nature, are fre- 

 quently made from proteins by the protoplasm in the cells 

 and are stored in the liver cells of the higher animals, to be 

 later transported to their muscle cells and stored in them 

 until needed. In animals that have no liver, as an earth- 

 worm, the glycogen is stored in the cells of the lining of the 

 body cavity and in the muscles. Fats, made either from 

 carbohydrates or from protein food by the protoplasm, or 

 stored directly in cells from the fatty acids absorbed through 

 the intestine, are reserve material and are capable of sup- 

 plying energy when there is need of it. 



When carbohydrates and fats are oxidized the resulting 

 compounds are carbon dioxide and water, as these foods 

 contain only carbon, hydrogen, and oxygen. Proteins are 

 far more complex. All proteins contain at least carbon, 

 hydrogen, nitrogen, oxygen, sulphur, and usually, in addi- 

 tion, phosphorus. When they are oxidized many different 

 compounds result. The best known of these are uric acid, 

 carbon dioxide, and water. All these compounds are wastes. 



