66 PLANT PHYSIOLOGY 



16. Influence of the Condition of the Medium upon the 

 Activity of Enzymes. Specificity of Function of Enzymes and 

 Their Classification. — The condition of the medium greatly 

 affects the activity of enzymes. The same enzyme may exhibit 

 a high degree of activity under certain conditions, while under 

 others it may be entirely inactive. 



Of the external conditions that exert a great influence on the 

 rate of enzymatic reactions, temperature must be considered 

 of primary importance. The effect of temperature on the 

 activity of an enzyme has much in common with its influence 

 on vital processes. With an increase of temperature, the reac- 

 tion first accelerates; then a certain optimum temperature is 

 reached at which the reaction proceeds at its highest rate. With 

 a further increase of temperature, the reaction begins to decrease; 

 and, finally, the destruction of the enzyme occurs. In short, on 

 the temperature curve of enzymatic action there are three cardi- 

 nal points characteristic of vital processes: the minimal point, 

 at which the reaction only begins to show a perceptible activity; 

 the optimal point, at which it proceeds at its highest rate; and, 

 finally, the maximal point, at which the reaction ceases. (For 

 a more detailed discussion of cardinal points, see Art. 30.) In 

 distinguishing enzymes from the living protoplasm, the fact 

 should be noted that their optimal points lie considerably higher, 

 i.e., at 45 to 50°C. and in some enzymes even at 60°C. The 

 optimal for living cells usually does not exceed 25 to 35°C., 

 while a temperature of 45 to 50°C. is usually lethal. The com- 

 plete destruction of the enzymes is attained at a still higher 

 temperature. The usual method of stopping enzymatic action 

 is by boiling. 



The activity of enzymes is likewise determined to a high 

 degree by physicochemical factors of the medium, especially by 

 the hydrogen-ion concentration. In examining the proteolytic 

 enzymes, the fact has already been emphasized that each group 

 works only at definite pH values and outside these limits becomes 

 inactive. For other enzymes likewise, definite optimal pH 

 values have been determined at which they display their greatest 

 acti\'ity. For amylase (diastase) of malt, this pH will be about 

 4.6 to 5.2; for maltase, 6.1 to 6.8; for invertase (sucrase), 4.2: 

 etc. However, it should be observed that these magnitudes of 

 r-H are not absolute. They may change considerably under 



