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. Thusglucoseis usually 
decomposed into alcohol and carbon dioxide, the end 
results being in accordance with the following formula: 
CeHi20¢ = 2C,.H;OH+ 2COsz. 
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.H;OH+60 =2C0.+3H.,0. 
Alcohol+ oxygen = carbon dioxide+ water, 
a, 
~_ 
Ae 
