Section III 



CHAPTER 2 



Cellular Metabolism 



H. B. STEINBACH AND F. MOOG 



In the life of a cell, morphological arrange- 

 ments and chemical processes bear a quite 

 different relationship to the integrity of the 

 unit. Parts of cells, or structures within cells, 

 may be shifted around, in many cases with- 

 out marked changes in the life of the whole. 

 But slight alterations in any one of the many 

 chemical events that make up the total me- 

 tabolism usually call forth severe, and often 

 fatal, effects. Traces of drugs, vitamins, and 

 poisons may alter the entire course of a 

 cellular unit or, properly applied, may mo- 

 mentarily and reversibly block some special 

 function. 



Not only individual cells, but also organi- 

 zations of cells, are thus dependent on the 

 maintenance of their normal metabolic con- 

 stitution. The development of embryos par- 

 ticularly illustrates this dependency. Experi- 

 mental morphogenesis has amply demon- 

 strated that an embryo may go on developing 

 in spite of radical alterations in the numbers 

 and orientations of its parts. Chemical in- 

 terference with the metabolism of the em- 

 bryo, on the other hand, readily leads to 

 serious abnormalities or death. 



Since the classic studies of Lavoisier, the 

 gross similarity between the chemical ac- 

 tivities of a living substance and of a burning 

 candle has occupied the attention of many 

 workers. Only in recent years has the con- 

 sequent emphasis on the respiratory aspects 

 of cellular metabolism begun to give way to 

 study of those aspects of cell metabolism of 

 most interest to the general biologist, i.e., 

 mechanisms for utilizing the energy of the 

 burning foodstuffs and the linkage of these 

 mechanisms with oxidative processes. Much 

 careful work has shown us a great deal about 

 the nature of the burning process; the out- 

 lines of how energy evolved is used by the 

 cell, however, have by the present time 

 been only vaguely sketched in. 



Our knowledge of special details of metab- 

 olism is voluminous, and it would be futile 



to attempt a comprehensive factual coverage 

 of the subject in one chapter. On the other 

 hand, a few general mechanisms (which may 

 be regarded as models for a variety of met- 

 abolic processes) are emerging from the 

 welter of biochemical investigations. The at- 

 tempt to use such principles as have been 

 established shovdd be made only against a 

 background of special knowledge. While a 

 general treatment may be a useful intellec- 

 tual guide, the planning and execution of 

 specific experiments on cellular metabolism 

 requires critical application of specific knowl- 

 edge, hard work, and considerable familiarity 

 with experimental techniqvies (and their 

 limitations!). 



In general terms, cellular metabolism may be 

 diagrammed as shown in Figure 2. Reduced carbon 

 chains represent the available fuel burned by cells 

 and, in terms of heat content or, more accurately, 

 free energy, are first encountered by animal cells at 

 a relatively high energy level. By a series of steps, 

 electrons* are removed, leaving CO2 (oxidized car- 

 bon) as ultimate residue. The electrons are com- 

 bined finally with electron acceptors, usually oxy- 

 gen molecules, in which case water is formed by 

 union with protons. As electrons progress stepwise 

 towards a terminal state, other cellular processes 

 are proceeding whereby carbon chains are built up 

 and degraded, units are fixed into cellular struc- 

 tures, and energy is made available for movements 

 of various kinds, for production of electricity, for 

 syntheses and for other types of utilization of en- 

 ergy. The oxidative trail releases energy, the syn- 

 thetic trail requires energy. 



Frequently the energy-releasing side of the pic- 

 ture is termed catabolism, the synthetic side anab- 

 olism. This distinction is no longer of use since all 

 cellular processes are known to be closely linked. 

 However, it is still fruitful to consider oxidation 



* Hydrogen may be regarded as composed of a 

 proton and an electron. In normal combination with 

 other elements, the electron serves as a binding 

 agent. With the electron removed, H* results, which 

 is free to react with the buffers always present in 

 protoplasm. Thus all instances of hydrogen transfer 

 can also be regarded as electron transfers. 



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