166 A CONCEPTUAL INTRODUCTION TO BIOENERGETICS 



Now AH = q may be positive or negative depending upon which is larger, 

 the enthalpy of the final or of the initial state. The former characterizes an 

 endothermic reaction; the later an exothermic reaction. As a general rule 

 anabolic reactions are endothermic; catabolic reactions are exothermic. 

 More specifically, the synthesis of proteins in the metabolism of the living 

 svstem is endothermic; the combustion of glycogen and other food stores is 

 exothermic. 



For chemical reactions the value of q or AH, the "heat of reaction," can be 

 measured calorimetrically. and quite accurate values obtained. For in- 

 stance, for the simplest reaction 



H 2 + 1/2 O, = H 2 



the heat of reaction 



&H = #final - ^initial 



= H( 1 mole FTO) - H( 1 mole H 2 + 1/2 mole 2 ) 



and although the absolute value of the enthalpy (or internal energy) for 

 neither reactants nor product is known (Who knows how to determine the 

 sum of all the potential energies in the nucleus, for example?), the difference, 

 AH, can be obtained with great precision: —57,798 cal/mole at 25°C, the 

 minus sign indicating that the reaction is exothermic. 



An especially useful heat reaction is the heat of formation, AH., the 

 enthalpy change which occurs during the reaction by which the molecule of 

 interest is formed from its elements. Actually the example above was a 

 formation reaction. Another now follows: 



6CW + 6H 2 (£) + 3 2 (£) = C 6 H,,0 6 (glucose) 



AH f = -279,800 cal/mole 



From a table of heats of formation, heats of reaction can be computed as 



AH = (A//,) producls - (AH f ) reactants 



The heat of combustion or burning of glucose could be computed, from 

 heats of formation, from the following reaction: 



C 6 H 12 6 + 6<J 2 (g) = 6H 2 0(1) + 6C0 2 (g) AH = -669.580 cal/mole 



The fuel value of foods is usually expressed in units of thousands of calories: 

 i.e., kilocalories (kcal). kilogram calories (kg cal), or Calories (Cal). Hence 

 the fuel value of glucose is 669.58 kcal/mole. Other examples are given in 

 Table 7-1 (A), from which is readily apparent the origin of the very useful 

 "4-9-4 rule": the fuel values of carbohydrate, fat, and protein, are respec- 

 tively, about 4, 9, and 4 Cal/g. 



