496 ANIMAL BIOCHEMISTRY 



verlcil to heal, niosL ol it is dissipated slowly to the eiuironment. As 

 already noted, many cclluhu reactions require energy, and of these a 

 number require amounts far beyond the energies available at the level 

 of heat within the biological range of temperatures. Thus animals 

 must assimilate materials of high chemical energy and degrade them 

 by processes utilizing this energy both chemically and thermally. 

 Therefore, animals depend directly or indirectly upon the photosyn- 

 thetic plants which convert radiation to chemical energy in forms 

 that animals can use. The meat-eating animals are, of course, depend- 

 ent on the herbivores and through them on the green plants. In this 

 sense all animals arc parasitic and could not survive by themselves 

 on an earth without plants. 



ENERGY CONTENTS OF FOODS 



Although inorganic materials are necessary to them, animals can- 

 not recover chemical energy from these materials. Water, carbon 

 dioxide, and minerals are important in animal metabolism but as 

 solvent, reactants, catalysts, and structural materials. Organic com- 

 pounds then must serve as the energy sources, falling into the three 

 major food classes, carbohydrates, lipides, and proteins with their 

 related compounds. The metabolic reactions undergone by these 

 materials have already been discussed. Some of the energy-yielding 

 steps have been mentioned also, but neither the overall energies of 

 foodstuffs nor the total needs of animals have been considered. 



First there is one point of theoretical importance worthy of men- 

 tion. As noted above, animals must have chemical energy, usually 

 referred to as available free energy. However, free-energy changes are 

 not easily measured when complex systems like animals are involved. 

 Therefore, heat contents are measured instead, usually by calorimetric 

 procedures, although heat content and free energy are not the same 

 thing. 



The thermodynamic ccjualion 



AF = AH - T AS 



compares the change in free energy AF with the change in total heat 

 content AH for any process. T is the absolute temperature and AS 

 the change in entropy, which is a measure of disorder or randomness 

 in the system. From this equation it is clear that free energy and heat 

 changes may differ markedly, and in fact they do in many chemical 

 reactions. Fortunately, in the overall metabolic processes of animals, 



