''Common Pathivays of Cellular 

 Metabolism 



SINCE World War II, when radioactive car- 

 bon (C 14 ), radioactive phosphorus (P 32 ), heavy 

 oxygen (O 18 ), and other distinctively labeled 

 atoms began to be available for "tagging" 

 organic molecules, biochemists have suc- 

 ceeded more and more in unraveling the 

 complexities of intermediary metabolism in 

 many kinds of cells. Much was known pre- 

 viously about the initial and the final prod- 

 ucts of metabolism; but now many of the 

 intermediary products are coming into focus. 

 Intermediary metabolism, to be sure, is a 

 difficult field, which extends beyond the 

 scope of general biology. However, some 

 general aspects of the field are very important 

 and these must be considered. 



One broad conclusion seems inescapable. 

 Cells generally, regardless of type, display a 

 number of basic patterns of metabolism that 

 are common to them all. Bacteria, yeasts, and 

 molds, and the cells of animals and plants 

 — both simple and complex — all possess cer- 

 tain sets of enzymes, which determine the 



basic pathways of their metabolism. There 

 are, of course, a number of metabolic dif- 

 ferences, which distinctively characterize the 

 cells of typical animals, plants, and other or- 

 ganisms, but these differences are superim- 

 posed upon a basic pattern that seems to be 

 common -to all protoplasm, -Perhaps- -this 

 indicates that organisms generally have had 

 a common evolutionary origin. At least it 

 seems unlikely that the same systems could 

 have arisen independently in so many differ- 

 ent types of living things. 



ISOTOPE METHODS 



Each of the isotopes of a particular atom 

 possesses an identical system of planetary 

 electrons but a different atomic center. Usu- 

 ally the isotopic center is heavier, owing to 

 the presence of one or more supernumerary 

 neutrons (Fig. 8-1). In the case of hydrogen, 

 for example, the dominant form in nature, 

 which may be specified as H 1 , has a single 



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