166 GENERAL BIOCHEMISTRY 



ever, respiration requires a sujDply of oxygen, and not all environ- 

 ments contain this gas. In sewage, swamps, certain soil zones, heaps 

 of decaying biological matter, intestinal tracts of animals, or any 

 place having limited contact with air but densely populated with 

 metabolizing cells, there is little or no oxygen. Hence respiration 

 cannot serve to supply energy, and only the mechanisms of fermenta- 

 tion operate. Thus fermentation permits life in otherwise unfavorable 

 situations. The metabolism of the higher plants and animals is of the 

 respiratory type, and these species depend upon oxygen. Certain 

 microorganisms called anaerobes ferment their food materials and re- 

 quire environments very low in oxygen. Others can operate either way, 

 respiring when oxygen is present and fermenting when oxygen is ab- 

 sent. Organisms of this last type are termed facultative on the basis of 

 their diverse capabilities. 



METABOLIC OXIDATIONS 



The free energy of foods becomes available to a cell as a result 

 of a series of oxidation-reduction reactions. In respiration the cell 

 catalyzes the stepwise oxidation of glucose to carbon dioxide and water 

 in the presence of oxygen, which is reduced during the same course 

 of events. Thus oxidation and reduction are paired, one reactant 

 undergoing one of these changes and another reactant the other. 



Consideration of the overall reaction of a typical fermentation is a 

 little more complex on the surface. The fermentation of glucose 

 actually oxidizes part of the molecule to carbon dioxide and reduces 

 the rest to ethanol, for example. Thus the two changes are again 

 paired. If fermentation is indicated by the individual reactions, there 

 are always at least two reactants, one being oxidized and one reduced. 

 Hence the commonly used term, metabolic oxidations, must always 

 imply the simultaneous occurrence of reductions. 



Oxidation and Reduction 



These reactions may occur as the result of one of several different 

 changes in the arrangement of the valence electrons of the substances 

 affected. For the present purpose, oxidations may be most simply con- 

 sidered as involving one of the following changes: 



1. Loss of electrons by transfer from one molectde or ion to an- 

 other with the formation of a new electrovalent bond as in 



Na + CI -^ Na+ + CI" 



