Inside the Cell 



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We can now turn our attention from the reproductive apparatus of 

 the nucleus to the other parts of the cell, known as the cytoplasm, 

 in which its ordinary everyday business of living takes place. As we 

 have seen, the cell is a chemical factory which takes in food materials 

 and uses them for its own purposes. One of its needs is energy, which 

 it gets by burning sugars. This requires a whole series of enzymes 

 by which controlled oxidations are performed. As I have already 

 said, these oxidative processes are coupled with other processes 

 which give rise to 'high energy' phosphate compounds. The most 

 important of these is adenosine triphosphate (atp), which is the main 

 carrier of chemical energy within the organism^ It not only provides 

 the energy required for the functioning of muscles, but it also, as we 

 shall see, takes a direct part in the synthetic activities of the cells, 

 including those which give rise to the proteins and nucleic acids. 



A typical sugar such as glucose contains six carbon atoms. The 

 first stage is its degradation to two molecules of lactic acid, a 'three- 

 carbon' compound which is oxidized to pyruvic acid. Pyruvic acid 

 is one of the main sources of energy in the cell. The way in which it 

 is used was worked out by Krebs, who showed that it is the fuel of 

 a kind of chemical engine, which operates through a cyclic series of 

 operations known as the Krebs or 'citric acid' cycle. The result of 

 this is that the pyruvic acid is oxidized to carbon dioxide and water; 

 while the energy obtained in this process is stored in adenosine 

 triphosphate (atp), which goes into the pool available for all syn- 

 thetic processes and for the performance of muscular work. 



A rather general picture, due to H. Lettre, showing how this fuel 

 station within the cell operates, is given in Fig. 11. atp is obtained not 

 only from the oxidation of pyruvic acid, but also, to a lesser extent, 

 from the initial conversion of the sugar to pyruvic acid. No less than 



^ ATP consists of adenine and d-ribose, combined with three molecules of phos- 

 phoric acid. See right-hand side of Appendix 1, Fig. 2. 



