Chapter 16 



BIOLOGICAL ENERGETICS 



All living organisms must have a continuous supply of energy in a 

 usable form. The study of energy sources, utilization, and quantitative 

 requirements is called biological energetics. The great bulk of all food 

 consumed goes to meet this need. In all cases the energy is derived from 

 chemical reactions carried out by the living cell, whereby the foodstuffs 

 are converted into products of lower energy content. The difference 

 between the energy content of the foods eaten and waste products excreted 

 represents approximately the energy which may be used (with greater 

 or lesser efficiency) by the organism. 



The chemical reactions on which living things depend for their energy 

 supply are many and varied. Lower forms frequently live under anaer- 

 obic conditions, carrying out reactions which do not involve oxygen. 

 Thus for example, glucose is converted into carbon dioxide and alcohol 

 by yeast, or into lactic acid by lactic acid bacteria. Such conversions 

 yield relatively little energy, and the yeast or bacteria accordingly are 

 forced to metabolize large amounts of the foodstuff (here glucose) . The 

 higher animals and man, on the other hand, are aerobic organisms and 

 oxidize their foodstuffs to the stage of carbon dioxide and water. Since 

 this represents complete combustion, much larger amounts of energy are 

 liberated, and less food per unit weight of living tissue is needed. 



The energy used by living things appears partly in the form of heat, 

 partly as muscular work, and partly in many other forms such as elec- 

 trical, chemical, and light energy. It has become customary, however, 

 to express all of these forms in terms of heat units, or calories. A calorie 

 (cal.) is the amount of heat needed to raise the temperature of one gram 

 of water one degree Centigrade, specifically from 14.5 to 15.5°C. The 

 kilocalorie (Cal.) is one thousand times larger.^ The energy difference 

 between foods and waste products may be expressed quantitatively by 

 means of these units. For example, the combustion of one mole (180 g.) 

 of glucose gives 673,000 cal. The heat change accompanying a reaction 

 is represented by the symbol. A//' (A = change; H = heat). It is given 



^ The British Thermal Unit (BTU) is the amount of heat re<iuired to raise the 

 temperature of one pound of water 1°F. One BTU equals 252 cal., or 1 Cal. equals 

 very nearly 4 BTU. Another energy unit is the foot-pound, the amount of work 

 done in lifting one pound through one foot. One calorie equals slightly more than 

 three foot-pounds. 



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