148 UNITY AND DIVERSITY IN BIOCHEMISTRY 



In a reversible reaction it is the difference in the reaction velocities in 

 opposing directions which regulates the position of equilibrium. In effect, 

 all reactions are reversible, but if the velocity from left to right is very great 

 and the velocity from right to left very small, then the reaction may be 

 considered as irreversible. 



However, even though the reaction velocity is appreciable in either 

 direction, one of the reaction products may be constantly removed, and in 

 such a case a reversible reaction can go to completion in one direction. For 

 example, if we react iron and water vapour in a closed vessel we shall have 

 the reversible reaction : 



3Fe + 4H2O ^ FegOi + W^ 



Now, if we pass the water vapour over the iron, the hydrogen will be 

 swept away and the reaction will go to completion from left to right : 



3Fe + 4H2O ^ Fe304 + 4H2 



If, on the other hand, we pass a stream of hydrogen over the heated iron 

 oxide, it will be the reaction from right to left which will go to completion, 

 the current of gas sweeping away the water vapour : 



FegO^ + 4H2 -> 3Fe + 4H2O 



The equilibrium of a reversible reaction is the resultant of velocities in 

 the two directions. Temperature influences these velocities but it does not 

 change them relative to each other, nor consequently the position of 

 equilibrium. The presence of a catalyst will influence the velocities but 

 likewise will not alter the point of equilibrium. It is the concentrations of 

 the substances present which control the direction of a reversible reaction, 

 as stated by the law of mass action. 



In living organisms, it happens frequently that the products of a rever- 

 sible reaction are used in another reaction, or that they are removed from 

 the site of their formation. Inside the cell, a reversible reaction often goes 

 to completion in one direction and becomes, in effect, an irreversible 

 reaction. 



A closed system can only do work at the cost of an increase in entropy. 

 And irreversible reactions can only take place in one direction, that of an 

 increase in entropy. The reactions will continue until equilibrium is 

 established. 



The situation is quite different in an open system or in an assembly of 

 open systems, as is the case for a cell or an organism. In such an open 

 system, the organism takes complex organic molecules from the environ- 

 ment, liberates free energy from these molecules, and rejects the products 

 of the reaction. It can maintain its entropy constant and even decrease it 

 so that its constituents become more ordered. It has been described as 



