BIOLOGICAL ENERGETICS 



41: 



by the equilibrium co7istant, K. For a reaction of the type A + B^ 

 C + D, A' equals [C] X [D] divided by |AJ X IB], where the brackets 

 indicate molar concentrations at equilibrium (review the law of mass 

 action in a textbook of general chemistry) . Thus if K is one, the equi- 

 librium point is reached when half of the starting materials have been 

 converted into products, whereas if K is ten, the reaction is about 90 

 per cent complete. 



The relation between AF and K is given by the equation, 



-^F = RT\nK 



where R is the gas constant (1.987 cal. per degree per mole), T the abso- 

 lute temperature, and In K the natural logarithm of K. Substituting 

 numerical values and converting to ordinary logarithms, this equation 

 becomes at body temperature (37°C) : 



- A/'' =1419 log X 



This equation enables one to calculate the equilibrium point of any reac- 

 tion for which the free energy change is known, and vice versa. For 

 example, if ^F is zero, K is one, but if aF equals —10,000, K is 1.22 X 10'. 

 The latter value indicates an equilibrium point very far to the right. 



Muscle contraction 



The free energy available from oxidation of foodstuffs is not usable 

 by animals and human beings in the form of heat. In other words, the 

 body is not simply a heat engine,^ although it is an engine or a machine 

 in the sense that it converts energy from one form into others. Some 

 heat of course is needed for warmth, but except under the most severe 

 conditions of cold and exposure, more than enough heat for this purpose 

 is always available. Heat production is primarily a consequence of the 

 fact that the body, like other machines, operates at much less than 100 

 per cent efficiency. In fact, excess heat is an important waste product 

 which must be ehminated to maintain health and vigor. 



The question therefore arises as to just how the body does convert 

 the chemical energy of foodstuffs into muscular work and other useful 

 processes. A partial answer to this question has been obtained from 

 studying the chemistry of muscle contraction. It was discovered around 

 1930 that muscle contains two substances which are readily hydrolyzed 

 with the liberation of large amounts of energy. These compounds are 

 adenosine triphosphate (ATP, p. 158) and creatine phosphate (CrP, 

 p. 349). The latter was observed to decrease in amount as contraction 

 occurred and to be completely used up if the muscle was stimulated to 



^ A heat engine is a macliine wliicli operates by virtue of temperature and pressure 

 changes. 



