When we endeavour to apply the results of the investigations of the fermentation of sugar by yeast-juice, zymin, etc., to the process which goes on in the living cell, considerable difficulties present themselves. A scheme of fermentation in the living cell can, however, easily be imagined, which is in harmony with these results. According to the most simple form of this ideal scheme, the sugar which has diffused into the cell unites with the fermenting complex and undergoes the characteristic reaction with phosphate, already present in the cell, yielding carbon dioxide, alcohol, and hexosephosphate. The latter is then decomposed, just as it is in yeast-juice, but more rapidly, and the liberated phosphate again enters into reaction, partly with the sugar formed from the hexosephosphate and partly with fresh sugar supplied from outside the cell. The main difference between fermentation by yeast-juice and by the living cell would then consist in the rate of decomposition of the hexosephosphate, for it has been shown that yeast-juice in presence of sufficient phosphate can ferment sugar at a rate of the same order of magnitude (from 30 to 50 per cent.) as that attained by living yeast.

The difference between the two therefore would appear to lie not so much in their content of fermenting complex as in their very different capacity for liberating phosphate from hexosephosphate and thus supplying the necessary conditions for fermentation.

A simple calculation based on the phosphorus content of living yeast [Buchner and Haehn, [1910, 2]] shows that the whole of this phosphate must pass through the stage of hexosephosphate every five or six minutes in order to maintain the normal rate of fermentation, whereas in an average sample of yeast-juice the cycle, calculated in the same way, would last nearly two hours.

Wherein this difference resides is a difficult question, which cannot at present be answered with certainty.

In the first place it must be remembered that a very great acceleration of the action of the hexosephosphatase is produced by arsenates (p. [79]), and this suggests the possibility that some substance possessing a similar accelerating power is present in the yeast-cell and is lost or destroyed in the various processes involved in rendering the yeast susceptible to phosphate. The great variety of these processes—extraction of yeast-juice by grinding and pressing, drying and macerating, heating, treating with acetone and with toluene—renders this somewhat improbable, and so far no such substance has been detected. [p124]

A comparison of living yeast, zymin, and yeast-juice shows that these are situated on an ascending scale with respect to their response to phosphate. Taking fructose as the substrate in each case, yeast does not respond to phosphate at all (Slator), the rate of fermentation by zymin is approximately doubled (p. [46]), and that by yeast-juice increased ten to forty times, whilst the maximum rates are in each case of the same order of magnitude. Euler and Kullberg, however, have observed an acceleration of about 25 per cent. in the rate of fermentation of yeast in presence of a 2 per cent. solution of monosodium phosphate, NaH2PO4 [[1911, 1], [2]].

The high rate of fermentation by living yeast and its lack of response to phosphate may possibly be explained by supposing that the balance of enzymes in the living cell is such that the supply of phosphate is maintained at the optimum, and the rate of fermentation cannot therefore be increased by a further supply.

A further difference lies in the fact that yeast-juice and zymin respond to phosphate more strongly in presence of fructose than of glucose, whereas yeast ferments both sugars at the same rate (p. [131]), and this property has been shown to be connected with the specific relations of fructose to the fermenting complex. It seems possible that these differences are associated with the gradual passage from the complete living cell of yeast, through the dead and partially disorganised cell of zymin to yeast-juice in which the last trace of cellular organisation has disappeared and the contents of the cell are uniformly diffused throughout the liquid. Living yeast is, moreover, not only unaffected by phosphate but only decomposes hexosephosphate extremely slowly (Iwanoff).

Some light is thrown on these interesting problems by the effect of antiseptics on fermentation by yeast-cells and by yeast-juice. The action of toluene has hitherto been most completely studied, and this substance is an extremely suitable one for the purpose since it has practically no action whatever on fermentation by yeast-juice. The experiments of Buchner have, in fact, shown that the normal rate of fermentation and the total fermentation produced, are almost unaffected by the presence of toluene even in the proportion of 1 c.c. to 20 c.c. of yeast-juice. What then is the effect of toluene on the living yeast-cell? When toluene in large excess is agitated with a fermenting mixture of yeast and sugar, the rate of fermentation falls rapidly at first and then more slowly until a relatively constant rate is attained which gradually decreases in a similar manner to the rate of fermentation by yeast-juice. Thus at air temperature (16°) 10 grams of [p125] yeast suspended in 50 c.c. of 6 per cent. glucose solution gave the following results when agitated with toluene:—

Time after
Addition of
Toluene,
Minutes		C.c. of
CO2 per
Minute.		Time.C.c. per
Minute.
04·661·6
14 81·2
23·3120·85
32·6240·8
42 320·5
51·8 constant