156 TEXTBOOK OF BOTANY 



lent to 12 per cent or more are the most desirable for the production of 

 alcohol in commercial quantities. 



If the air is not excluded, however, the alcohol will be oxidized rapidly 

 by acetic acid bacteria which were placed in the bottle with the mass of 

 yeast. Instead of having a solution of alcohol we will have a solution of 

 acetic acid (vinegar). The formation of acid cider and the souring of 

 wines are the result of respiration in acetic acid bacteria. These bacteria 

 are also used for the production of commercial vinegar. We may repre- 

 sent the process briefly as follows: 



Alcohol + Oxygen > Acetic acid + Water + Free heat energy 



Enzymes of acetic 



C2H5OH + O2 — '- > CH3COOH + H2O + Free heat energy 



acid bacteria 



Lactic acid bacteria exemplify an interesting type of respiration in 

 which no free oxygen is used, no carbon dioxide escapes, and there is 

 no loss in dry weight. 



Sugar > Lactic acid + Free heat energy 



Enzymes of lactic 



CgHiaOe — > 2 CH3 • CHOH • COOH + Free heat energy 



acid bacteria 



In many organisms respiration results in the formation of an incom- 

 pletely oxidized food, and is usually referred to as fermentation. The 

 different types of fermentation are recognized and named on the basis 

 of the incompletely oxidized product formed. The three examples cited 

 above are called, in order, alcoholic, acetic acid, and lactic acid fermen- 

 tation. The fundamental fact, however, is that this is the way respiration 

 occurs in these particular organisms. In each one food is oxidized in 

 living cells and some of the chemically bound energy in it is liberated. 

 In two of the examples no free oxygen enters into the process, and in 

 two of them no carbon dioxide is liberated. 



Enzyme 



Sugar > 2 Molecules of alcohol + 2CO2 



HHHOHH HH HH 00 



I I I I Enzyme | II II II 



H— C— C— C— C— C— C = O > H— C— C— OH + H-C-C— OH + C + C 





 H H H 



It may be noted that the oxidation-reduction process in alcohoHc and 



HO HH HH 00 

 H H H H 



Carbon reduced Carbon oxidized 



more than in more than in 



sugar sugar 



