DESTRUCTIVE METABOLISM 405 



when O 2 is present in sufficient quantities, and among the end products 

 two, CO 2 and H 2 O, are characteristic, though formed in very variable 

 quantities in proportion to the O 2 taken up. Anaerobic may replace 

 aerobic respiration in any organism when O 2 is cut off, and may proceed 

 for a long time; but the end products are various and quite different 

 from those of aerobic respiration. Among them are commonly ethyl 

 alcohol and hydrogen, and less CO 2 . Certain minute organisms may 

 pass their whole existence without oxygen, which indeed hinders or alto- 

 gether stops their development, and they are thus restricted to anaerobic 

 respiration. In most organisms, however, anaerobic respiration can 

 be considered only as a makeshift. 



Nature. What then is the fundamental feature of a process that 

 goes on under such different conditions and results in such diverse prod- 

 ucts? So far as now appears, respiration consists in the decomposition 

 of the protoplasm or some of its constituent proteins, either directly, 

 or as a result of the action of an enzyme or of some internal force (stim- 

 ulus) upon it. Inasmuch as the inciting cause is rarely apparent, spon- 

 taneous or self-decomposition is often spoken of, but this merely means 

 that the reason is unknown. 



The view here presented is not the one most generally held at present, but appeals 

 to the author as most consistent with the known facts. Many physiologists consider 

 respiration to consist primarily in the decomposition of foods by the protoplasm 

 or by enzymes, without their assimilation into the living substance. In this case 

 foods are a kind of fuel for the body (see p. 406). It is not denied that some de- 

 composition of protoplasm occurs, but this is slight; as it were, a sort of natural 

 wear and tear in consequence of work. 



Advantage. The advantage of respiration is not certainly known, 

 but as the plant in order to do work must expend energy, the inference 

 is that respiration sets free energy by which that work is performed. 

 Now complex and unstable compounds contain much available potential 

 energy, the store of which is diminished when they decompose, arid the 

 essence of nutritive processes is the building up of those compounds 

 which disappear in respiration. Furthermore, heat, one easily observed 

 form of energy, is generated by respiration, though it is not known that 

 this is of any service to the plant. But the most definite reason for con- 

 necting the release of energy with respiration is that those tissues in 

 which growth or other work is proceeding rapidly are also characterized 



longer tenable, the words aerobic and anaerobic (aer, air; bios, life; an, not), applied 

 first to organisms that live in air or flourish only when it is excluded, are preferable. 



