CHAPTER LXV 



FUNCTIONS OF ANIMAL ORGANISMS 

 GENERAL PHYSIOLOGY 



The energy changes discussed in the preceding chapter have been 

 seen to be intimately associated with the metabohc activities of the 

 organism. These in turn cause or accompany the exercise of the various 

 functions. It is logical, then, to consider next certain facts in regard to 

 metabolism and to review the general physiology of animals. 



506. Chemical Cycles. — In the carrying on of metabolism an animal 

 takes certain foods into its body, breaks them down in dissimilation 

 into simpler waste substances, and then eliminates these wastes, adding 

 them to the air, the water, or the soil. These wastes, together with the 

 bodies of dead organisms, are reduced to very simple substances by 

 decomposition due to the action of bacteria. Plants take these sub- 

 stances, or in some cases the animal wastes, from the air, water, and soil 

 and build up again from them the foods which the animal needs. Thus 

 the elements which these various substances contain may be followed 

 through cycles, partly related to the inorganic environment, partly to the 

 life of plants, and partly to the life of animals. In that portion of 

 the cycle occurring in plants and animals the element is involved in the 

 chemical changes included in metabolism. Of the elements found in 

 animal organisms certain ones which are of peculiar interest or importance 

 may be cited as examples. 



The carbon cycle (Fig. 297) may be begun with carbon dioxide which 

 the plant takes from the air or water and by photosynthetic processes 

 builds into proteins, fats, and carbohydrates. The animal uses these as 

 foods in building up protoplasm and as sources of heat and muscular 

 energy. Carbon dioxide is expired into the air. The carbon compounds 

 in other waste products and in the bodies of dead animals are decomposed, 

 yielding carbon dioxide, which is added to the air, to be used once more 

 by plants. Of all chemical elements carbon is the one which enters into 

 the greatest number and variety of combinations, such combinations 

 being particularly characteristic of living things. Organic chemistry, 

 and particularly that portion of it known as biochemistry, is the chemis- 

 try of carbon compounds. 



Atmospheric nitrogen can be utilized only by nitrogen-fixing bacteria 

 (Fig. 298). When thus fixed, however, the nitrogen is in the form of 

 compounds with other elements which may be used by any plant and be 



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