232 PHYSIOLOGY OF NUTRITION 



§13. Circulation of Energy in Nature. — The circulation of energy is quite 

 different from that of matter. The available supply of energy upon the earth 

 is inadequate for a long continuation of plant and animal life, which would soon 

 cease were it not for the continuous influx of energy from the sun. From the law 

 of the conservation of energy it is clear that the solar energy stored up in poten- 

 tial form by the photosynthetic process in green plants must be completely 

 liberated by the reverse process (the formation of carbon dioxide and water), 

 as this occurs in combustion or in plant and animal respiration. The carbon 

 dioxide thus produced can, of course, enter again into organic compounds, 

 but that portion of the energy liberated by respiration and fermentation that 

 takes the form of heat is almost entirely lost from the organism and does not 

 again become available for organic synthesis; it becomes dissipated into space 

 and is gone forever from the earth. Thus vital activity upon our planet is 

 directly dependent upon the sun, from which new supplies of energy must 

 continually come if life is to be long continued. 



This process of energy dissipation may be illustrated somewhat as follows. 

 If a small beaker of hot water is poured into a large tank of cold water, the cold 

 water is warmed but little; supposing the original temperatures to be 95 and 5 , 

 respectively, the temperature of the tank may perhaps rise to 6° when the hot 

 water is added. At first the heat energy is concentrated, or intensive, in the 

 beaker; later it is dissipated, or extensive, in the tank. The coefficient of 

 energy dissipation, that fraction of the original energy that can no more be con- 

 verted into mechanical work, is termed entropy, and entropy always tends 

 toward a maximum. In this process of the dissipation of the intensive energy 

 of our solar system, plants play a direct role. 



Summary 



1. General Discussion of Fermentation and Respiration. — All changes and move- 

 ments of materials involve changes with respect to energy, and the material trans- 

 formations and movements that occur within the living plant are not exceptions to this 

 principle. Some of the processes of living tissues result in the setting free of energy, 

 while others of these processes result in energy fixation or accumulation. The 

 photosynthesis of carbohydrates from carbon dioxide and water, in chlorophyll-bearing 

 tissue, results in the transformation of radiant energy (sunlight) into the stored potential 

 energy of the combustible compounds that are formed; this is therefore an energy- 

 fixation process, and the fixed energy remains in the plant. Transpiration is a 

 process by which kinetic energy (light, heat) is transformed into the potential form, as 

 the latent heat of water vapor; it is an energy-fixation process, but the fixed energy 

 leaves the plant. Whenever a carbohydrate is oxidized in the living tissue, energy is 

 set free, just as when wood is burned. Some of the energy resulting from this oxida- 

 tion escapes from the plant, but much of it is consumed in other chemical processes 

 and is retained for a longer or shorter period. Most of the chemical transformations 

 that occur in the living protoplasm take place because of, and at the expense of, energy 

 that is set free by oxidations. The processes that set energy free in this way, in the 

 living organism, are grouped together as the process of respiration. 



It appears that the oxygen consumed in the primary steps of respiration is generally 

 not free oxygen (0 2 ) ; it is derived from compounds that contain other elements (especi- 



