NUTRITION OF TEAST. 189 



down, with the production of carbon dioxide, water, and nitro- 

 genous waste (katdbolism\ and the concomitant liberation of 

 energy. The work to be done by the yeast-cell is plainly 

 limited. The manufacture of new and of surplus protoplasm 

 and the protrusion of buds require work, partly chemical, 

 partly mechanical; but most of the liberated energy probably 

 appears as heat. In point of fact, great activity of yeast is 

 accompanied by a rise of temperature, as may beproved by 

 placing a thermometer in " rising" dough or fermenting fruit- 

 juice. 



Outgo. Barring the outgo of energy already mentioned, and 

 the probable excretion of carbon dioxide and nitrogenous waste, 

 but little can be said concerning the outgo of a yeast-cell. The 

 ordinary excretions are so masked by the presence of foreign 

 matters in the liquids which yeast inhabits that little is known of 

 the real course of events. To the consideration of conditions 

 which entail these difficulties we may now pass, merely pausing 

 to caution the student against the supposition that the evolution 

 of carbon dioxide in fermentations represents to any great ex- 

 tent the normal respiration of the yeast cells. 



Mineral Nutrients of Yeast. It has been shown (pp. 148, 181) 

 that Pteris and Protococcus, inasmuch as they possess chlorophyll 

 can live upon simple inorganic matters such as CO 2 , H a O, and 

 nitrates, out of which they are able to manufacture for them- 

 selves energized foods such as starch. Yeast is unable to do 

 this, as might be supposed from the fact that it is destitute of 

 chlorophyll. And yet yeast does not require proteid ready- 

 made as all true animals do, for experiments have shown that it 

 can live and grow in a liquid containing only mineral matters 

 plus some such compound of nitrogen as ammonium tartrate 

 (C 4 H 4 (NH 4 ),O.). Upon a much less complex organic compound 

 of nitrogen such as a nitrate it cannot thrive, thus showing its 

 inferiority in constructive power to Protococcus and all green 

 plants, on the one hand, and its superiority to Amoeba and all 

 animals, on the other. 



Pasteur's fluid, composed of water and salts, among which is ammonium 

 tartrate (above), will suffice to support yeast. It will support a much more 

 vigorous growth if sugar be added to it. But if ammonium nitrate is sub- 

 stituted for ammonium tartrate yeast will refuse to grow in the fluid. 



