7. Respiration 



R 



.espiration in the most general sense includes all 

 cellular oxidations which yield energy, or could yield energy, to the 

 cell. In this chapter, after touching on a few general problems, we 

 shall consider primarily those reactions which are or may be part of 

 organized respiratory systems. Many individual reactions described 

 in the preceding chapter undoubtedly yield energy, but their relation 

 to the major pathways of respiration is not yet known. 



Respiration may be aerobic or anaerobic. Aerobic respiration is 

 defined by the fact that the ultimate hydrogen acceptor is molecular 

 oxygen; in anaerobic respiration, or fermentation, hydrogen from 

 substrate is transferred to compounds other than oxygen (107). Use 

 of the term "fermentation" to describe an aerobic process is mislead- 

 ing. 



The maximum energy available from a reaction or sequence of 

 reactions is, of course, the free energy change; thus, complete aerobic 

 oxidation of glucose to carbon dioxide and water involves a change 

 in free energy of approximately —700,000 calories per mole under 

 standard conditions. Some fraction of this energy is trapped by the 

 cell and used for energy-requiring reactions. So far as we now know, 

 the size of the fraction so trapped depends on the ability of the or- 

 ganism to couple the oxidation to phosphorylation of adenosine mono- 

 phosphate or adenosine diphosphate to yield adenosinetriphosphate 

 (ATP). In turn, the hydrolysis of ATP is coupled to energy-requiring 

 reactions. The common practice of describing ATP and similar com- 



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