424 



BIOLOGICAL ENERGETICS 



When allowance is made for these facts, the average physiological fuel 

 value of carbohydrates, proteins, and fats becomes, in round numbers, 

 4, 4, and 9 Cal. per gram, respectively. Note that these values are ex- 

 pressed in kilocalories and that they refer to the total energy obtained 

 by the organism, both in the form of heat and in the form of ATP or 

 other ~P compounds. However, no allowance for entropy changes is 

 included, and as explained previously these changes are likely to pro- 

 vide additional energy. The entropy changes involved in the metabolism 

 of most foodstuffs are not known. 



The physiological fuel values of a list of common foods are given in 

 the Appendix (Table A-1, p. 434). A study of this list emphasizes the 

 tremendous effect of two components, namely fat and moisture, on the 

 calorific value. Watery foods like fresh fruits and vegetables contribute 

 very few calories, whereas concentrated, dried foods, especially those high 

 in fat such as nuts, chocolate, vegetable oils, etc., have very high energy 

 value. Sherman has pointed out that an ounce of olive oil is equal in 

 fuel value to over three pounds of lettuce! 



Energy requirements 



Basal Metabolism. The most conspicuous use for food energy is in 

 the performance of muscular work. However, even at rest, energy is 

 required by a living animal or human being to keep various vital func- 

 tions in operation. These include, not only such obvious processes as 

 breathing, heart action, and blood circulation, but also the maintenance 

 of a certain minimum muscle tension or tonus (even when lying down 

 and completely "relaxed") and tlie normal operation of the organs of 

 digestion, secretion, and excretion. The kidneys, for example, use energy 

 to excrete waste products, and energy is needed to cause digested foods 

 to pass through the intestinal wall. Even the synthesis and maintenance 

 of the protein molecules which make up body tissues require energy, 

 since the equilibrium point of the reaction 



protein + water ^ amino acids 



lies far to the right, and energy must be supplied to shift it to the left. 



The minimum rate of energy metabolism to provide for such functions 

 is called the basal metabolism, and the energy thereby consumed is con- 

 sidered to be the minimum energy requirement. The basal metabolism 

 is measured about 12 hours after eating so that no digestion of food is 

 taking place, and with the subject lying down in a room of comfortable 

 temperature. Under these conditions all the energy being used by the 

 body appears directly in the form of heat since no external work of any 

 sort is being done. 



The basal metabolism can be measured by direct calorimetry, that is, 



