CELL METABOLISM 75 



equally unchanging. In the absence of any evidence to the contrary, it 

 was generally held, until about twenty years ago, that the constituent 

 molecules of animal and plant cells were relatively static and that, once 

 formed, they remained intact for a long period of time. A corollary of 

 this concept is that the molecules of food which are not used to in- 

 crease the mass of protoplasm are rapidly metabolized to provide a 

 source of energy. It followed from this that one could distinguish two 

 kinds of molecules: relatively static ones that made up the cellular 

 "machinery," and ones that were rapidly metabolized and thus cor- 

 respond to cellular "fuel." 



However, in 1938 Rudolf Schoenheimer and his colleagues at 

 Columbia University began a series of experiments in which amino 

 acids, fats, carbohydrates and water, each suitably labeled with some 

 "heavy" or radioactive isotope, were fed to rats. Schoenheimer's experi- 

 ments, which have been confirmed many times since, showed that the 

 labeled amino acids fed to the rats were rapidly incorporated into body 

 proteins. Similarly, labeled fatty acids were rapidly incorporated into 

 the fat deposits of the body, even though in each case there was no in- 

 crease in the total amount of protein or fat. Such experiments have 

 demonstrated that the fats and proteins of the body cells— and even the 

 substance of the bones— are constantly and rapidly being synthesized 

 and broken down. In the adult the rates of synthesis and degradation 

 are essentially equal so that there is little or no change in the total mass 

 of the animal body. The distinction between "machinery" molecules 

 and "fuel" molecules becomes much less sharp, for some of the ma- 

 chinery molecides are constantly being broken down and used as fuel. 

 From the rate at which the labeled atoms are incorporated it has been 

 calculated that one half of all the tissue proteins of the human body 

 are broken down and rebuilt every eighty days. The proteins of the liver 

 and blood serum are replaced very rapidly, one half of them being 

 synthesized every ten days. The muscle proteins, in contrast, are re- 

 placed much more slowly, one half of the total number of molecules 

 being replaced every 180 days. The celebrated aphorism of Sir Fred- 

 erick Gowland Hopkins, the late English biochemist, sums up this con- 

 cept very succinctly: "Life is a dynamic equilibrium in a polyphasic 

 system." 



24. Special Types of Metabolism 



The metabolic paths just described, by which carbohydrates, fats and 

 proteins are metabolized to carbon dioxide and water, with the con- 

 comitant release of biologically available energy, are common to almost 

 all cells. Certain cells have in addition one or more unique metabolic 

 abilities such as the enzymatic shortening of certain kinds of protein 

 molecules (i.e., muscle contraction), the enzymatic synthesis of sub- 

 stances with specific biologic activities such as hormones, the produc- 

 tion of electricity by specialized organs such as that of the electric eel, 

 or the enzymatic production of light by a variety of fish, insects, molds 

 and bacteria. 



B/o/um/nescence. A number of animals, and some molds and bac- 



