CHAPTER 10 

 RESPIRATION AND RELEASE OF ENERGY 



The total requirements of energy and the general source of it in the 

 food have already been discussed in connection with nutrition. How 

 energy is released from food is a separate problem. 



Derivation of Energy. — Ultimately most energy comes from sun- 

 light. Many plants and a few of the simplest animals have chlorophyll, 

 which utilizes solar energy to make sugars. In these sugars, energy is 

 bound up in chemical structure. As sugars are converted into starches, 

 or fats, or proteins, by combining them with other substances, still further 

 energy is stored in these higher products. When plants are devoured by 

 animals, the latter take possession of this potential or stored energy. 

 So it is that all energy of life is traceable to sunlight. Indeed, most other 

 energy in the world comes from the same source. Coal and oils used for 

 fuel got their energy from ancient sunlight. Even the energy of water- 

 falls came from the same source, for it was the energy of the sun which 

 lifted the water to its higher level. About the only energy expended on 

 the earth which is not traceable to sunlight is that of the tides. 



Animals derive some of their energy directly from the sun, for sunlight 

 is one of the most potent of health-giving agencies. In the main, how- 

 ever, they obtain it from food, and for this they are directly or indirectly 

 dependent on plants. To get energy from foods, it is necessary that the 

 latter be chemically decomposed. The foods must be changed into 

 simpler substances whose content of potential energy is smaller. In 

 general, complex substances with large molecules have more energy 

 bound up in their constitution than do simple substances with small 

 molecules. Nearly all chemical reactions which split up molecules into 

 smaller and simpler ones may therefore be depended on to release a 

 certain amount of energy. Proteins, carbohydrates, and fats, on being 

 decomposed, even in the process of digestion, liberate energy. 



There is, however, one type of energy-yielding chemical reaction which 

 is so much more abundant than any other that it is common practice to 

 speak of energy as coming from that source. That type of reaction is 

 oxidation (page 37), the union of oxygen with other elements. The 

 commonest of these unions is that of oxygen with carbon, because carbon 

 is abundant in all the classes of organic compounds — in proteins, but 

 especially in carbohydrates and fats. Carbon dioxide, a very stable 



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