PHENOMENA OF LIFE 13 



pendent existence. When the essential parts of a living animal 

 are separated from one another, they soon lose their identity 

 and ultimately disintegrate into simple inorganic compounds. 

 Non-living bodies have no common unit of structure which is 

 comparable to the cell, and their parts show no interdependence. 



(5) Growth in living organisms is usually by intussusception, 

 i.e. by the addition of new particles among those already present; 

 this results in a swelling of every part, and does not necessitate 

 a change in form. Non-living bodies usually grow by accretion, 

 i.e. the addition of successive layers on the outside. 



Growth in any living thing involves a complex series of changes. 

 The chemical compounds which make up the bodies of animals 

 are extremely labile; they are constantly breaking down into 

 simpler substances or becoming more complex by the addition 

 of new materials. There is no time during the life of any indi- 

 vidual when elaborate chemical reactions are not taking place. 

 Metabolism is the term used to include this great complex of 

 incessant changes. Those processes which use energy to build 

 up compounds are said to be anabolic; those which destroy sub- 

 stance to produce energy are termed katabolic. 



Animals are primarily katabolic organisms. They cannot 

 make organic compounds from simple inorganic substances; in 

 this respect they differ from plants which manufacture starch 

 from carbon dioxide and water. Since animals must have organic 

 food, it follows that plant products are necessary. Before animal 

 growth is possible, food must be converted into living substance. 

 By the process of digestion food is prepared for absorption. Some 

 substances that cannot be digested are passed out of the body as 

 faces. After absorption, food is carried to some part of the body 

 where it is needed; here it is transformed into living substance 

 by the process of assimilation. 



In order that metabolic activity may go on without ceasing, a 

 constant supply of energy is necessary. This energy is in part 

 furnished by oxidation, i.e. the chemical union of oxygen with the 

 living substance in a manner which may be compared to the 



