CARBON METABOLISM 241 



requirements is said to ]>e dissimilated. Comparatively 

 little is known about the processes involved in assimi- 

 lation, but dissimilation has been more intensively 

 studied. 



Dissimilation, particularly under aerobic conditions, 

 is often a catalytic oxidation process. The oxidation of 

 ethyl alcohol to acetic acid by the various vinegar organ- 

 isms is a case in point ; it is a partial oxidation, for the 

 organism is not able to carry the reaction further to form 

 carbon dioxide and water. Wieland, it will be remembered, 

 proposed that these oxidations were in reality dehydro- 

 genations, since it was possible to replace free oxygen 

 by reducible substances like methylene blue or quinone 

 which can act as hydrogen acceptors . Bacteria and other 

 organisms are able to bring about these dehydrogenations 

 by means of the enzymes, dehydrogenases, dehydrases or 

 hydrogen transport ases, which they produce and which 

 activate the hydrogen atoms of the substrate, ethyl 

 alcohol, acetaldehyde, etc., which is to be oxidised. The 

 vinegar organisms, species of the genus Acetobacter, 

 usually oxidise substances other than alcohols completely 

 to carbon dioxide and water ; but one species at least is 

 loiown, A. suboxydans, which is capable of oxidising 

 partially many other substances, of which the following 

 examples will serve as illustrations : — 



/OH 



1. {a) CHa.C^ H + ^ OHg.cf + H,0 



\. • \H " 



(ethyl alcohol) (acetaldehyde) 



/OH 

 ^O / OH 



(b) CHg.cf + HoO > CHg.Cr — OH + > OHg.c/ + HoO 



^H \\ ^O 



\h 



(acetaldehyde hydrate) (acetic acid) 



