160 GENERAL BIOCHEMISTRY 



A + B -> Y + Z + A/^' 



For convenience the last sign is omitted and known values of AF may 

 be inserted, 



A + B-^Y + Z AF= 3,000 cal. 



with chemical energy expressed in calories like heat. 



When the total free energies of the reactants exceed those of the 

 products, the reaction is said to be spontaneous and will occur without 

 an external supply of energy. This situation is arbitrarily represented 

 with a negative sign in front of the value of AF. On the other hand, 

 if the free energy of the products exceeds that of the reactants, energy 

 must be supplied to keep the temperature up and the reaction going. 

 In such cases as the hypothetical reaction above, the change in free 

 energy is assigned a plus symbol or the sign is omitted. Practically 

 all reactions fall into one group or the other with few possessing 

 AF = and then only over a limited range of temperature. The 

 presence of enzymes or other catalysts changes the rate of reaction but 

 apparently does not change the value of AF. 



A mathematical expression relates changes in free energy with heats 

 of reaction. In practice the extra free energy required to make a 

 reaction take place may be supplied as heat at the expense of either 

 the system or the environment. In the former case, when energy is 

 drawn from the reaction system without replacement from the environ- 

 ment, the temperature falls until the reaction slows down and finally 

 ceases. In the latter case reaction proceeds until the reactants are 

 consumed or equilibrium is reached. 



Spontaneous reactions yield the excess free energy for direct utiliza- 

 tion in subsequent reactions or for conversion to an equivalent amount 

 of heat. This heat may produce a general increase in temperature, 

 diffuse to the sites of reactions consuming free energy, or both. 



Cells are distinctly limited in the temperatures they can withstand. 

 Thus, if the free-energy change of a reaction were very large and were 

 all transformed to heat, a fatal rise in temperature might occur. 

 Likewise, the substitution of heat for a high free-energy requirement 

 could lead to an excessive temperature. Therefore, reactions involving 

 interconversions of heat and chemical energy can safely occur in a cell 

 only if the free-energy changes are comparatively small. Many reac- 

 tions in biological materials fit this condition. 



Other processes produce relatively enormous changes in free energy 

 as illustrated by the complete oxidation of glucose. 



CeHisOe + 6O2 -^ 6CO2 + 6H2O AF = -688,000 cal. 



