178 A CONCEPTUAL INTRODUCTION TO BIOENERGETICS 



and it is reversible. Left to right, it couples in wherever free energy is needed 

 throughout the living system. Right to left it becomes "charged back up," 

 ready to supply energy at another site. 



Now the living system is not wasteful of free energy without a good pur- 

 pose, such as to keep the system warm in a cold environment. Thus most 

 endergonic processes occur in steps of about 8 kcal/mole, or slightly less, 

 making full use of the free energy of the hydrolysis reaction. Likewise the 

 oxidation of foods also goes in steps of slightly more than 8 kcal/mole each, 

 so that the charging reaction is also not wasteful. Indeed, the very complex 

 sets of steps in the oxidation of carbohydrates, fats, and proteins seem de- 

 signed so that at several stages of each the ADP + P can couple in and be 

 condensed into ATP. This is the principle of the Krebs (citric acid) cycle, 

 for instance, in which it is estimated that 38 ATPs are reformed per mole- 

 cule of glucose oxidized to C0 2 and H 2 0. This number permits an estimate 

 of the efficiency of the recharge process to be made: 



8 kcal/mole of ATP x 38 ATP's inn „ 



'- x 100 = 37 per cent 



824 kcal/mole of glucose 



This efficiency is very respectable, especially since the reactions are going 

 very fast. By contrast, a steam or diesel engine could probably do 20 to 

 30 per cent on glucose (for a short while!), and up to about 35 per cent on 

 gasoline or oil; solar batteries can convert only about 10 per cent; and 

 thermoelectric converters about 5 per cent from the fuel (including nuclear, 

 or radioactive fuels). Other (like ATP/ADP) electrochemical devices — eg. 

 batteries and fuel cells — are able to give very high efficiences (>80 per 

 cent) if operated slowly, much less if required to operate very fast. 



A simple calculation (note the approximations) will emphasize the im- 

 portant point of how efficient the human machine really is. Man's basal 

 metabolic rate is about 70 kcal/hr. If this is all expended through ATP, the 

 turnover (charge-recharge) rate is 70/8 ~ 9 moles ATP/hr. If we assume 

 that a 150-lb man of density about 1 g/cc contains on the average 10~ 4 moles 

 ATP per liter, the turnover time for ATP is: 



150 1b x454g/lb x 1 1/1000 g x 10~ 4 moles/1 „- 



— 2 — x 3600 sec/hr ~ 30 sec 



9 moles/hr 



That is, each ATP molecule in the body is hydrolyzed and reformed about 

 once every 30 sec! At this speed of discharge and charge, a man-made bat- 

 tery would have an efficiency well below 1 per cent. Indeed, it would burn 

 up in the attempt! Hence 37 per cent in the living system is truly remark- 



^ , , , , • , / c 70 kcal/hr 



able. To supply the basal energy, it burns the equivalent ot ~ 1 / g 



~4 kcal/g 



glucose each hr, 24 hr a day. 



