[Chap. XVII RESPIRATION 155 



A third limitation to the method of detecting the rate of respiration 

 by measuring the amount of carbon dioxide released is rehited to the 

 fact that sometimes the food is not completely oxidized to CO2 and 

 H2O. It may be only partially oxidized, with the result that such sub- 

 stances as alcohol and various organic acids are formed. The alcohol and 

 organic acids may accumulate in the plants. Respiration by incomplete 

 oxidation of food occurs to some extent in all plants. It is most easily 

 detected when the supply of oxygen is low. Certain types of plants and 

 plant organs, such as cacti and the large fruits of tomatoes, for instance, 

 usually contain a small percentage of alcohol. 



This sort of respiration is frequently referred to as fermentation. It is 

 particularly conspicuous in many of the non-green plants such as yeast 

 and acetic acid bacteria, in which rather high percentages of the incom- 

 pletely oxidized products (alcohol and acetic acid) accumulate and 

 escape into the surrounding medium. In all such cases of respiration the 

 volume of carbon dioxide released is not likely to be the same as the 

 volume of oxygen consumed. It may be many times greater or many 

 times less. These facts may be illustrated briefly by equations: 



Sugar — -^ Ethyl alcohol + Carbon dioxide + Free heat energy 



674 Calories of 

 bound energy 



enzyme of 



180 gm. CeHizOe > 92 gm. C2H5OH + 88 gm. CO2 + 28 Calories 



yeast 



The facts indicated by the above equation may be demonstrated if a 

 mass of living yeast plants is placed in a small thermos bottle filled with 

 water containing some sugar in solution, and a tightly fitting stopper is 

 inserted. Within a short time, the stopper will be forcibly ejected by the 

 pressure of gas that develops within the bottle, the temperature of the 

 water will have increased a little, and the odor of alcohol may be de- 

 tected. If the gas that is formed is tested by the usual limewater test, it 

 will be found to be carbon dioxide. 



If one end of a tube is put through the stopper of this bottle and the 

 other end beneath the surface of water in another vessel, the carbon 

 dioxide formed can escape, but at the same time air is excluded from the 

 yeast culture in the bottle. Under these conditions, if sufiicient sugar is 

 present, the yeast plants will live and produce alcohol until they are 

 checked by the toxic effect of the alcohol. Varieties of yeast that continue 

 to remain active when the amount of alcohol formed in the vat is equiva- 



