UPON ENZYMES AND LIVING CELLS 205 



it is found that at a certain minimal temperature the activity 

 just begins to be perceptible, as the temperature rises the activity 

 increases with it until an optimum point is reached, lying usually, 

 as in the case of the enzyme, a few degrees above the usual tempera- 

 ture of the action of the organism, and beyond the optimum 

 temperature the life of the organism becomes again more sluggish, 

 its activities lessened, and if the high temperature is maintained it 

 dies. The optimum point of activity for the living cell probably 

 arises in the same manner as in the case of some enzymes, by the 

 simultaneous action of two opposing causes viz., (1) the hastening 

 of all chemical reactions by rise in the temperature, (2) a similar 

 hastening from the same cause of the by-reactions which lead to 

 a using up and diminishing of the cell substances which act as 

 catalysts to these reactions. As a result of this it follows that in 

 the earlier stages of the range of temperature, as the temperature 

 rises, the chemical reactions in the cell will increase in velocity, 

 while as yet there will be no appreciable destruction of the more 

 stable cell substance, or of the catalysts. But in the later stages, 

 at the optimum point and beyond it, destruction of catalysts, 

 cellular enzymes, and cell substance will also proceed at an ever- 

 increasing rate, and although the velocity of reaction of the cata- 

 lysed reactions is still increasing, the diminution in concentration of 

 catalyst more than outbalances this, and the cell activities are 

 lessened. 



The same differences are to be noted at the two extreme ends 

 of the range in the case of enzymes and living cells ; as the tempera- 

 ture falls, the enzyme and living cell merely become dormant and 

 temporarily pass out of activity, but neither is killed unless the 

 fall in temperature is enormous compared to the rise in tempera- 

 ture which would cause total permanent loss of activity or death 

 upon the other side of the active range. Nor is the reason far to 

 seek; the lower limit is reached by gradual fall in activity until 

 the zero point is reached, while the upper limit is reached by gradual 

 increase in activity, accompanied by gradual destruction finally 

 surpassing increase in activity, until the cell stops from destruction 

 in hyper- activity. 



The level at which living cells are rapidly destroyed by increased 

 temperature closely resembles that at which enzymes are similarly 

 destroyed, and lies around the coagulation temperature of proteins. 

 As in the case of enzymes, the point varies with the nature of the 



