ENERGY CHANGES IN ORGANISMS 455 



10. Vitamins are essential elements in food, serving in some way to 

 make possible the incorporation of foods into the living organization. 

 In the absence of vitamins perfectly appropriate food may be brought to 

 cells which need it, but these cells are unable to make use of it. As a 

 result the body may be starved though it may contain an abundance of 

 food. 



It remains to be noted that the specific character of many substances 

 affects their availability as food; for instance, vegetable fats are less 

 readily assimilated and less serviceable in the animal organism than are 

 animal fats. 



501. Planes of Metabolism. — All animals do not live on the same 

 metabolic plane, some animals demanding a high protein content in their 

 food, others a high carbohydrate content. Not only is this true of differ- 

 ent types, but the individual organism may at any time adjust itself to 

 a metabolic plane different from that upon which it has previously lived 

 and, it may be, different from that of other animals of its kind. This, 

 of course, is true within certain limits, which, however, seem rather wide. 

 Different types of animals are capable of adjusting themselves to great 

 differences in the amount of water they require. An aquatic organism 

 may demand constant immersion; on the other hand, a desert type 

 may find all its needs satisfied by the dew which falls during the night or 

 the little moisture which comes in the form of rain. The clothes moth 

 can go through generations in a dry closet without apparently needing 

 any moisture from without. It is believed that any animal can manu- 

 facture a certain amount of water in its body as a result of oxidation 

 processes. In the case of the clothes moth this metabolic water is appar- 

 ently sufficient in amount to meet the needs of the organism. 



502. Body Heat. — Since all active organisms are constantly carrying 

 on chemical changes which mainly involve oxidation, all produce heat, 

 though the amount of heat produced varies greatly with the size and 

 character of the organism. The most minute organisms produce an 

 amount of heat so small as to be beyond the registering power of the 

 most delicate instrument; larger organisms, however, produce a con- 

 siderably greater amount. Many animals which singly produce an 

 amount that is imperceptible to us may in large groups yield enough to 

 produce a very distinct sensation. That is true, for example, of ants in 

 an ant hill or bees in a hive. The amount of heat produced is determined 

 by the plane of metabolism and the character of the food, as well as the 

 degree of activity of the organism. The higher animals with their 

 more efficient respiratory organs are capable of producing a relatively 

 larger amount of heat than are the lower forms with their less efficient 

 systems, 



503. Heat Regulation, — Since the lower animals have no means of 

 regulating the heat of their bodies, it is radiated as fast as it is produced. 



