Lecture IV. 33 



and the energy thus rendered available becomes manifested in the 

 various phenomena which we call life. It is only in connection 

 with the substance protoplasm that these phenomena of energy 

 are exhibited. Thus it has been said, " Protoplasm is the physical 

 basis of life". Life appears to be the complex energy transfor- 

 mations and material changes associated with protoplasm. 



The process of anaerobic respiration in yeast is then identical 

 with the process of fermentation or the conversion of sugar into 

 alcohol. The commercial value of yeast centres, of course, on 

 this process. For a long time it was held that fermentation was 

 a property of the protoplasm itself, and that the process could 

 not be carried on apart from living protoplasm, and in fact it was 

 one of the " vital " attributes of the cell. Consequently great 

 interest was aroused when it was discovered that a substance 

 could be extracted from yeast' which, apart from all living proto- 

 plasm, could bring about, or accelerate, the change of sugar into 

 alcohol and carbon dioxide. The substance is zymase, and it 

 is found to act in every way like other enzymes. Thus we come 

 to regard respiration as a process dependent on the still more 

 general characteristic of protoplasm, that of producing organic 

 catalysts the enzymes by means of which the protoplasm can 

 effect changes which apart from these bodies can only be brought 

 about by the application of powerful reagents or of high tempera- 

 tures. 



The number of enzymes extracted from yeast cells, is already 

 considerable and it is likely that these cells possess many more 

 not yet recognised : indeed many, if not most, of the chemical 

 reactions of cells seem accelerated and controlled by these agents 

 which often appear to be formed only -when their need arises. 



PRACTICAL WORK. 



Dissolve a little cane sugar in about 20 c.c. of hot water, add 2 gm. of gela- 

 tine previously swelled in cold water. When the gelatine has completely 

 gone into " solution " and the mixture has cooled to about 30 add a drop of 

 a yeast suspension. Distribute the yeast thoroughly by shaking. While still 

 liquid put drops of this mixture on slides and cover. If the preparation is 

 successful you will see two or three isolated yeast cells in each field of the 

 high power. Subsequent observations will show the successive stages of 

 growth and budding. As the gelatine sets to a gel there is no danger of 

 the successive buds being dislodged from the positions in which they origin- 

 ate, and one can see the successive generations formed during the time of 

 observation. The time occupied from inception to maturity may be noted, 

 by keeping one cell under periodic observation. 



Before leaving the yeast cell it will be well to utilise it to calibrate our 



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