130 PEOVisiON OF energy: fermentation 



whole molecule, in phospho-enol-pyruvic acid the energy is 

 concentrated in the phosphate bond. 



A second energy-rich phosphate bond is built up during the 

 oxidation of glyceraldehyde-phosphate with the uptake of 

 inorganic phosphate to yield l.S.diphosphoglyceric acid. In 

 the presence of ADP, the diphosphoglyceric acid yields 

 S.phosphoglyceric acid and ATP. If the complete cycle is 

 now inspected from the point of view of phosphate bond 

 formation (Fig. 10) it can be seen that the initial phosphoryla- 

 tion of glucose to hexosediphosphate involves the utilisation of 

 two molecules of ATP, and that the hexosediphosphate then 

 splits to yield two molecules of triosephosphate each of which 

 is eventually converted to pyruvic acid, liberating two molecules 

 of ATP in the course of the metabolism. Thus two mole- 

 cules of ATP are required to start a cycle which yields four 

 molecules of ATP, a net gain of two molecules of ATP. The 

 pyrophosphate bond ( — P — — P — ) of ATP is energy-rich and 

 this appears to be the form in which the cell stores its energy 

 until it is required. The synthetic mechanisms of the cell 

 require energy which is obtained by reactions involving phos- 

 phorylated intermediates obtained, in turn, by interaction with 

 the extra ATP formed by the glycolysis cycle. The cycle can 

 thus be looked upon as a machine for taking in glucose and 

 phosphate at a low energy level, winding the energy up into 

 specific bonds, and then transferring that energy, in the form 

 of such bonds, to a suitable store while discarding the waste 

 product as, in this case, fermentation products. 



OH OH OH 



CH,0 — P-O— P — — P-OH 



I II II II 







HO HO 

 Adenosine-tri-phosphate 



Bacteria differ from other tissues mainly in the way in which 

 they dispose of the pyruvic acid thrown out as a waste product 

 from the energy machine and so, in considering the further 



