70 RESPIRATION 



Reductive processes also may result in the liberation of 

 energy fit for use by the plant ; thus the fermentation of sugar 

 by yeast, in which process the carbohydrate is converted into 

 alcohol and carbon dioxide without the agency of atmospheric 

 oxygen,* is a case in point. 



The energy freed in these processes is derived from the 

 molecular energy of the substances disintegrated and is gener- 

 ally expressed in terms of heat units. Thus the complete oxi- 

 dation of i gram molecule of maltose liberates 1351 kg. 

 calories. 



The fermentation of sugar by yeast also is accompanied by 

 the evolution of heat, although to a much lesser extent, a gram 

 molecule of maltose yielding 42 7 kg. calories, taking Brown's f 

 value of the heat of fermentation of i gram of maltose as being 

 125 calories. The alcoholic fermentation of sugar is therefore a 

 wasteful process for obtaining energy when compared with 

 oxidation, since by its means nearly thirty-one times as much 

 sugar must be consumed to obtain as much energy as is yielded 

 by the direct oxidation of sugar. The significance of this 

 figure of the heat of fermentation of maltose may be realized 

 by Brown's observations that between the temperatures of 14 

 and 1 6 C. the time required by a yeast cell to ferment its own 

 weight of sugar varies from eighteen and a half to nineteen 

 and a half hours and that the heat generated during one hour 

 is sufficient to raise the temperature of the cell by 15 or 16 C. 

 From such observations Brown estimates that at 30 C. yeast 

 can ferment its own weight of maltose in 2 '2 hours and the 

 potential rise in temperature in the cell in one hour will be 

 75'5 C. under adiabatic conditions, figures indicative of an in- 

 tense metabolism and, apparently, a great waste of energy. 

 Brown suggests that the explanation for this waste is to be 

 found in the fact that brewers' yeast is a cultivated plant grown 

 under unnatural conditions. The wild yeasts lead a quiet life 

 on the skin of, say, the grape : rupture of the skin of the fruit 

 provides a nutrient medium eminently suitable for growth and 

 reproduction accompanied by a free access of oxygen. A 

 period of intense activity immediately supervenes and budding 

 takes place, under the continued action of the oxygen, with 



* Vol. I., p. 377. f Brown: "Ann. Bot.," 1914, 28, 197. 



