236 PHYSIOLOGY OF NUTRITION 



cially carbon and hydrogen) as well as oxygen. The oxygen of one part of a complex 

 molecule may oxidize even another part of the same molecule. These primary steps 

 of respiration are called intramolecular respiration, anaerobic -respiration, or fermenta- 

 tion. They proceed under the influence of fermentation enzymes and they result in 

 incomplete oxidation, some or all of the new substances produced being only partially 

 oxidized; but considerable amounts of energy are set free by these fermentation 

 processes. The partially oxidized substances thus formed may accumulate in, and 

 escape from, the organism without further alteration, or their oxidation may be 

 completed by the action of oxidizing enzymes in the presence of an adequate supply of 

 free oxygen, the ultimate products being then all completely oxidized. In the pres- 

 ence of free oxygen, the products of the primary fermentation processes may be 

 different, or complete oxidation may occur before they are formed. The final steps of 

 the respiration process are called normal respiration or aerobic respiration. In aerobic 

 respiration, free oxygen is absorbed, and completely oxidized substances are produced, 

 with the setting free of much more energy than was previously made available by the 

 anaerobic processes. Intramolecular or anaerobic respiration, or fermentation, occurs 

 in every living cell; but there are many kinds of cells in which aerobic respiration does 

 not occur, either because the cells lack the necessary oxidizing enzymes or else because 

 free oxygen is not supplied. Some microorganisms are unable to live at all in the 

 presence of free oxygen (obligate anaerobes), and their respiration is consequently 

 limited to the primary fermentation processes; these forms give off products, some or 

 all of which are incompletely oxidized. Organisms that thrive in the presence of free 

 oxygen may give off some incompletely oxidized substances even with a good supply of 

 oxygen — as if these substances diffused out of the cells before the completion of 

 oxidation. With an inadequate supply of oxygen, fermentation products generally 

 become conspicuous, as in the roots of ordinary plants in poorly aerated soil (see 

 Chapter V, Section 5). 



The chemistry of respiration may be pictured superficially by means of the follow- 

 ing equations, which are to be considered simply as illustrations of the kinds of proc- 

 esses that are apparently involved. 



I. Fermentation or anaerobic processes: — 



1. Oxygen from the same molecule. 



Glucose Sugar Ethyl Alcohol Carbon Dioxide 



C 6 Hi 2 6 = 2 C0H5OH + 2 C0 2 



(oxidation (oxidation 



incomplete) complete) 



2. Oxygen from another compound. 



Glucose Sugar Water Hydrogen Carbon Dioxide 



C 6 H 12 6 + 6 H 2 = 12H2 + 6 C0 2 



(oxidation (oxidation 



incomplete) complete) 



II. Aerobic processes (with free oxygen): — 



Glucose Sugar Oxygen Water Carbon Dioxide 



i. C 6 Hi 2 6 + 6O2 = 6 HoO + 6 C0 2 



Ethvl Alcohol Oxvgen Water Carbon Dioxide 



2. C 2 H 5 OH + 3 2 = 3 H 2 + 2 C0 2 



Hydrogen Oxygen Water 



3. 2 H 2 + 2 =2 H 2 



When a certain amount of any substance is transformed by fermentation, the 

 quantity of energy set free is much less than when aerobic respiration occurs and 

 oxidation is complete. The fermentation of a gram-molecule of glucose (forming 2 



