92 PLANT PHYSIOLOGY 



is required from outside the cell. By the aid of certain 

 enzymes produced by the cell the carbohydrates or other 

 substances used in respiration are started in their disin- 

 tegration and proceed in it until simpler compounds and 

 some carbon dioxide are produced. Thus glucose is usually 

 decomposed into alcohol and carbon dioxide, the end 

 results being in accordance with the following formula: 



= 2C 2 H 5 OH+2C0 2 . 



It is probable that the reaction is not as simple as this, 

 but that there are many steps in the process. This proc- 

 ess sets free a certain amount of energy. In the produc- 

 tion of alcohol and carbon dioxide from sugar by the yeast 

 plant it is this anaerobic stage of respiration that takes 

 place. Corresponding decomposition processes occur in 

 various kinds of bacterial fermentation and decay, the 

 intermediate and end products varying with the com- 

 position of the substance fermented and the kind of 

 organism. 



140. The aerobic stage consists usually of the oxid- 

 ation of the rather complex compounds produced in the 

 anaerobic stage to simpler compounds, this also being 

 accompanied by the liberation of energy in large 

 amounts. This process also is probably carried on by 

 the aid of enzymes and it may be that the use of the 

 oxygen is rather to get rid of harmful products instead 

 of being the agent which sets free the energy. Taking 

 the case illustrated in the preceding paragraph the 

 alcohol is broken down and combined with oxygen to 

 form carbon dioxide and water. The final results, but 

 not the intermediate stages, are shown by the following 

 formula 



C 2 H 5 OH+6O = 2CO 2 +3H 2 O. 

 Alcohol -f- oxygen = carbon dioxide + water, 



