168 A CONCEPTUAL INTRODUCTION TO BIOENERGETICS 



The quantity of heat given off by living animals can be measured either 

 calorimetrically or by the C0 2 produced, (The two measurements agree!), 

 and when measured under conditions of a carefully defined rest, give a value 

 related to the internal work required to keep the living system alive. This 

 basal metabolic rate is about 70 kcal/hr, (about 1400 kcal/day) for a normal 

 man. In other units, the basal metabolic rate amounts to about 0.1 horse- 

 power (hp) continuously. 



It is readily apparent that if an animal is ill, certain processes are running 

 at too high a rate; heat energy accumulates, and the temperature rises. The 

 rate of energy loss is increased. By contrast with the normal animal in which 

 A 11 = 0, and q = w, in the ill animal w is much larger than q, the quantity 

 (q — w) is negative, and A l U is negative. Thus the animal lives at the ex- 

 pense of its internal energy, with resulting loss of weight — about 2 lb/day 

 for a human, assuming complete breakdown of assimilative processes and 

 food stored as glycogen and ignoring water loss. The quantities Tl and JC 

 decrease with time before the "turn" or "crisis, " then increase more or less 

 slowly back to normal because the animal begins to assimilate again during 

 the recovery period. 



The ideas outlined in the preceding paragraphs show the versatility and 

 the usefulness of the First Law, that energy must be conserved, but of course 

 do not illustrate all its facets. Note parts B and C of Table 7-1 for other 

 examples. 



More Detailed Consideration of the Second Law. Free Energy and Entropy 



The Second Law of Thermo, does not violate the first, but rather extends 

 it. It says: Whenever energy is transformed from one kind into another, only 

 a fraction of the internal energy (enthalpy, if pressure is constant) change is 

 available for doing useful work; the rest remains as heat energy of the mole- 

 cules left at the completion of the reaction. Corollaries, although seemingly 

 unrelated, are the following: heat energy always passes from the hot to the 

 cold body; water always runs downhill; if energy available for doing work 

 can decrease during the course of a process, the process will proceed spon- 

 taneously, although not necessarily at a fast rate. (That last phrase is a very 

 important one!) 



In algebraic terms, the Second Law can be expressed as: 



AH = AF + Q 



Here AF is the maximum available work, the "free" energy, which can be 

 extracted from AH, and Q is the unavailable energy. Note that both AF 

 and Q as does AH, have units kcal/mole (i.e., Cal/mole). 



The word "maximum" needs amplification. It is a fact of common ex- 

 perience that any mechanical job can be done in several ways, some ways 



