CHAPTER IX 



RESPIRATION AND FERMENTATION 



SECTION 94. Introductory. 



METABOLISM involves processes both of construction (anabolism) and 

 destruction (katabolism), the latter yielding the continual supply of energy 

 necessary for vital activity. Kinetic energy may be liberated not merely 

 by physiological combustion involving a consumption of free oxygen, but 

 also by chemical decompositions taking place without the aid of the latter l . 

 Energy is liberated entirely in the latter manner in certain bacteria and 

 fungi, which are thus able to exist anaerobically. The existence of such 

 organisms is hardly in agreement with the old dogma ' no life without 

 respiration' unless we assume, as is here done, that respiration includes 

 all metabolic processes which involve a liberation of energy. According 

 to the manner in which the latter occurs, it is possible to distinguish 

 between aerobic or oxygen-respiration and chemical, anaerobic, or intra- 

 molecular respiration. 



With the exception of water, carbon dioxide is the sole continually 

 excreted end-product of aerobic respiration in the higher plants. This 

 limitation is purely an adaptive modification, for many aerobic fungi and 

 bacteria may excrete such products as acetic, oxalic, or citric acid, 

 ammonia, &c. The higher plants have a very marked power of recon- 

 structing proteids, and hence their metabolism is unaccompanied by any 

 such excretion of urea as occurs in the higher animals. During anaerobic 

 respiration, however, a variety of end-products may be formed, and the 

 removal of these is essential for continued vital activity (Sects. 77, 92). 

 Many aerobic and anaerobic plants are able to obtain a supply of energy by 

 decomposing the greater part of their food into such products as alcohol, 

 butyric or acetic acid, &c. This process, known as fermentation, is simply 

 a form of metabolism characterized by the special nature and disproportionate 

 amount of the products, and it exists mainly for the purpose of obtaining 

 a supply of energy. The fermentative activity of many aerobic organisms 



1 Cf. Pfeffer, Studien z. Energetik, 1892. 



