138 UNITY AND DIVERSITY IN BIOCHEMISTRY 



molecule having an energy-rich bond, ATP, which acts by phosphorylating 

 one of the reactants : — 



glucose + (glycogen)^ -^ (glycogen)n+i + HgO AF° = +5,000 cal 



The reaction can be split up into the following stages : 



glucose + ATP -^ G-6-P + ADP + H+ AF° = -8,000 cal 



G-6-P ^ G-l-P AF° = +1,800 cal 



G-l-P + (glycogen)^ -> (glycogen)„+i + H3PO4 AF° = + 200 cal 



The sum of these reactions leads to the following : 

 glucose + (glycogen)„ + ATP -> (glycogen)„+i 



+ ADP + H3PO4 + H+ AF° = -6,000 cal 



IV. THE PHOSPHATE CYCLE 



Research carried out during recent years has revealed that the energy 

 stored in food which is broken down during cellular metabolism is gradu- 

 ally liberated in "packets" of energy which can be stored and utilized at a 

 later date. The direct oxidation of glucose by oxygen : 



6O2 + CeHiaOe -> 6CO2 + 6H2O AF° = -686,000 cal 



will release a considerable amount of energy in the form of heat, that is to 

 say, in a non-utilizable form. But, as Pardee has remarked, one does not 

 start a car by putting a match to the petrol tank. 



The energy economy of living organisms rests on the fact that anaerobic 

 and aerobic metabolism can give rise to energy-rich bonds, which can be 

 stored as energy-rich bonds of ATP and this energy can be used by the cell 

 to do various forms of work including the chemical work of biosynthesis. 



V. BIOLOGICAL OXIDO-REDUCTIONS AND THE GENERATION 



OF ENERGY-RICH BONDS 



Energy-rich bonds are generated by the conduction of certain definite 

 electrons along definite channels. During this passage of electrons their 

 energy level is lowered and energy-rich bonds are formed at their expense. 

 So an organism is an electro-chemical machine whose structure is that of 

 a special type of electronic conductor. 



A. Oxido-Reduction Potentials 



Consider a solution of ferric chloride. In this solution there are Fe+'*"+ 

 ions, CI- ions and water molecules. The solution is stable, however it is 

 capable of accepting electrons with the conversion of Fe+++ to Fe++ : we 

 can say that it has a certain (negative) electron pressure. We can measure 

 this pressure, provided that we have a reference system which we can place 

 together with the solution in an electric circuit. Let us place in the ferric 



