GENERAL PRINCIPLES OF BIOCHEMICAL ENERGETICS 



147 



liberating inorganic phosphate ; the concentration of glucose on this side of 

 the membrane is above that existing on the right of the membrane ; so the 

 glucose diffuses to the right of the membrane. The membrane is made up 

 of cells and its thickness is the same as these cells. Thus, the free energy of 

 a pyrophosphate bond is used for each molecule of glucose transported, 

 and it is converted into work done against the concentration gradient. 



Low 



concentration 



ATP ADP 



Hexokinase 



G- 



Membrane 



; 



Phosphatase 



- G 



High 

 concentration 



Fig. 29 (Cantarow and Schepartz)— Transfer of a glucose molecule against the 

 concentration gradient at the price of an ATP pyrophosphate bond. 



VIII. CHEMICAL EQUILIBRIUM AND THE STATIONARY STATE 



Let us recall the definition of a reversible reaction : it is a reaction in 

 which the reacting substances are not completely used up in forming the 

 products of the reaction. If, for example, we mix equimolecular amounts 

 of ethyl alcohol and acetic acid, water and an ester will be formed according 

 to the equation 



C0H5OH + CH3COOH = CH3COOC2H5 + H2O 



Whatever the duration of the reaction, at equilibrium a third of the ethyl 

 alcohol and a third of the acid will remain unchanged, in the presence of 

 the reaction products from the other two-thirds. 



If, on the other hand, we mix equimolecular amounts of the ester and 

 water, ethyl alcohol and acetic acid will be formed 



CHgCOOCoHg + H2O = C2H5OH + CH3COOH 



But two-thirds of the reactants will remain unchanged in the presence 

 of the products from the remaining third. 

 So, one can write 



C2H5OH + CH3COOH ^ CH3COOC2H5 + H2O 



The reaction is reversible. Whether the reaction begins from the left or 

 the right it will reach equilibrium when there are twice as many ester 

 molecules as there are alcohol molecules. 



