AEROBIC RESPIRATION 



is so feeble as to be hardly distinguishable from normal metabolism, 

 and anaerobic organisms need not be necessarily capable of exciting any 

 recognizable fermentation whatever. 



o 



According to their specific nature, obligate aerobes are only capable 

 of existing in the presence of oxygen, obligate anaerobes in its absence, 

 while facultative anaerobes can exist both in the presence and absence of 

 oxygen. No sharp line of demarcation exists, however, and a facultative 

 anaerobe which when fed in a certain manner is only capable of aerobic 

 existence, may under other circumstances grow anacrobically. Indeed 

 a few bacteria hitherto regarded as ob!igate anaerobes have been recently 

 successfully cultivated as aerobes under special conditions. Certain bacteria 

 and yeasts are temporary anaerobes, for after growing in the presence of 

 oxygen they may increase and multiply for a time anaerobically. Even 

 in the absence of oxygen aerobic plants continue to respire for a short 

 time, and this intramolecular respiration is accompanied in Phanerogams by 

 a production of carbon dioxide, alcohol, and other substances. Intramolecular 

 respiration does not suffice in an aerobic plant to maintain life, but may 

 enable it to survive the temporary absence of oxygen. An anaerobe exists 

 by means of an enlarged kind of intramolecular respiration, which may or 

 may not acquire the special features characteristic of fermentation. As 

 will be shown later, a close genetic relationship exists between aerobic 

 respiration, intramolecular respiration, and fermentation, while free oxygen 

 may when present enter for a time into the metabolism of obligate anaerobes, 

 until the disturbance of the normal vital activity induced by its presence 

 becomes so pronounced as to cause death. Even aerobic organisms die 

 in a similar manner when the partial pressure of the surrounding oxygen 

 is raised to a sufficient extent. 



The actual course of respiration within the protoplast is quite obscure, 

 and the visible end-products afford an index only to the activity of the 

 process and to any changes in its character, without giving any indication 

 as to the way in which these are produced. Respiration is of importance 

 only as a factor in metabolism as a whole, and it is not always possible to 

 distinguish clearly between the products of respiration and those which are 

 destined for constructive purposes, although usually the two are clearly 

 defined from one another. 



SECTION 95. Aerobic respiration. 



Except in the case of anaerobic plants aerobic respiration is as 

 necessary for plants as for animals, oxygen being absorbed and carbon 

 dioxide exhaled by all aerobic plants in darkness. Even in resting organs 

 such as bulbs, tubers, &c., respiration still continues, and it is absent from dry 

 but living mosses, lichens, or seeds only so long as they remain in a desiccated 



