GENERAL PRINCIPLES OF BIOCHEMICAL ENERGETICS 133 



as the result of passing from one state to another with an increase in 

 disorder, or, if one prefers, a decrease of the ordered state. 



To illustrate more clearly the difference between — AH and — AF, that 

 is, between the heat liberated by a reaction and the maximum possible work 

 which can be done by that reaction, we may consider molecules as con- 

 taining two different types of energy. One is an ordered energy : it unites 

 the atoms to each other by primary or secondary valency bonds. It is this 

 energy which can do work. The other is of a disordered nature (vibra- 

 tional, rotational and translational). What primarily interests the bio- 

 chemist is work, chemical or otherwise, obtainable from a reaction. If the 

 reaction takes place with liberation of a great deal of useful energy, then 

 work may be done. 



It can be seen that it is AF which we wish to know. But it should be 

 noted that AF does not depend solely on the nature of the chemical re- 

 action. It also depends on the concentrations of the reactants, and the 

 direction of the reaction will also depend on these concentrations. It would 

 not be practicable to compile tables of AF for all concentrations, so its 

 value is determined under defined conditions : liquids or solids in the pure 

 state, gas at a pressure of 1 atmosphere, solutions at 0-lM, temperature 

 25°. Concentrations thus defined are assigned a value of unity and, from 

 the AF° value thus defined, it is possible to calculate values of AF for other 

 conditions. 



The use of AF values is limited, as far as the biochemist is concerned. 

 Pardee compares the information they give with that given to a car-driver 

 by a map showing contour lines but no roads. The driver, knowing the 

 power of his engine, can deduce whether he can, or cannot, climb the slope 

 between two points. No information is given him as to the road to take. 



B. Free Energy and the Equilibrium Constant 



Under given conditions of temperature, pH, etc., each of the chemical 

 reactions occurring in the organism is in a stationary state of dynamic 

 equilibrium (see later). An organism of a given species, given age, and 

 under specified conditions, has a definite composition. Undoubtedly, in a 

 species, the compositions of two individuals show differences, but these 

 differences are much smaller between individuals of one species than be- 

 tween individuals belonging to different genera, and less still, when 

 different families, classes or orders, are considered. 



A reversible reaction is usually written thus : 



A-\- B ^C + D 



According to the law of mass action, the velocity of the reaction between 

 A and B is proportional to the product of the active masses of these two 

 compounds, and the reaction velocity from left to right is written : 



