84 



OXIDATION-REDUCTION POTENTIALS 



phate groups ( — ph). On the other hand in adenosine di- or triphosphate energy-rich 

 phosphate bonds (r-^ph) occur with an energy content of some 12,000 calories. In 

 adenosine triphosphate the first phosphate is joined by an energy-poor bond, whilst 

 the second two are joined by energy-rich bonds. 



Adenosine— PO2H.O ^ POgH.O r^ PO2H.OH 



Creatine phosphate and arginine phosphate contain similar r-^ph linkings. 

 Acetyl phosphate and phospho-enol pyruvic acid contain r-^ph groups joined to 

 carboxyl or acidic enol groups. 



The importance of these r^pli groups lies in the fact that cells utilise the free 

 energy of metabolic reactions to build up the linkings, and the free energy can be thus 

 stored to be available when required. The energy required to generate such bonds 

 usually arises from the electron transfers in oxidation-reduction reactions, a change of 

 potential of 0-25 volts giving rise to some 12,000 calories, which is the free energy 

 change involved in the formation or rupture of one r^ph. 



Adenosine triphosphate (ATP) constitutes a principal agent in phosphate 

 transfers ; when one r-^ph is transferred to another compound the diphosphate 

 (ADP) remains and this can be reconverted to ATP by reaction with creatine phos- 

 phate. AJ)P still contains one r-^ph. 



The free energy available in r-'ph groups can be utilised for muscular work and 

 for essential metabolic reactions and these r^ph linkings constitute one of the chief 

 methods of storage of free energy. 



The phosphate energy cycle may be diagrammatically represented as follows : — 



1 1,0 00 r 



ATP^ 





-9 3,000 



A 



^ 



Ester 



I norgon i c 



I 



Phosphate C ycle 



Fig. 16 

 Phosphate energy cycle 



Some of the details of mechanism will be discussed in succeeding pages. 



ANAEROBIC MUSCLE GLYCOLYSIS 



The route by which glycogen is converted to hictic acid in muscle by anaerobic 

 glycolysis is shown in Table 19. The reactions shown are as follows : — 



(1) Glycogen in the presence of inorganic phosphate is converted by the 

 enzyme phosphorylase to the Cori ester, glucose-1-phosphate. 



