174 A CONCEPTUAL INTRODUCTION TO BIOENERGETICS 



Although the principles are straightforward enough, measurement of the 

 quantities in these expressions is difficult. Let us make some guesses for 

 illustrative purposes. For a normal man in North America the food intake, 

 AJC, is about 3000 Cal/day, and the basal heat loss, q' bm , about 1400 

 Cal/day. These are measured values. Since the Second Law says: 



A JC= A£F + TA§> 



= A5' + q' hm + q' a + 7-AS 

 then 



-3000 = AS' - 1400 + q\. x + 7~AS 



If the food taken in and burned was glucose, for example, XAS can be 

 evaluated as follows. A A JC of -3000 Cal arises from 4.5 moles of glucose 

 (Table 7-3), and therefore 



TA$ = 310degK x 4.5 moles x 514 cal/deg mole = 700 Cal 



Our problem then reduces to q ' cx -f AS' = -2300 Cal. 



The value of total rate of heat loss has been measured for man in many 

 aspects (look ahead to Table 8-1 1), and in an average day q' ex can be at least 

 as large as the basal metabolic heat loss, and usually runs in excess of 2000 

 Cal. Therefore -AS' will be less than 300 Cal. The external work AS' 

 can be roughly estimated, especially for an unskilled laborer. Suppose he is 

 required to dig a hole 8 ft square and 4 ft deep; the work of lifting alone is 

 about 30 Cal, and this represents at most a third of the total work expended 

 in loosening, picking, and lifting operations associated with the job. Loco- 

 motion, eating, and the other daily external expenditures probably account 

 for the rest of the 300 Cal of external work. 



An estimate of the internal work done per day can also be obtained. In 

 our example above, the total free energy available was 3700 Cal (3000 + 

 700). If the efficiency, S , was 37 per cent, then 



AS' + AS' int = 1370 



Of this, about 300 Cal was external work, A^', as we saw above; and there- 

 fore the internal work, A L J' int , which kept the metabolic process running, 

 was about 1170 Cal, 34 per cent of the metabolic heat loss, q' . 



The reader is invited to consider other aspects of man's life and work from 

 this point of view: to put other estimated values into the Second Law and 

 juggle them about, hence to become familiar with both the clarity of concept 

 and the difficulty of successful detailed application at the present state of 

 knowledge. 



