212 TEXTBOOK OF PLANT PHYSIOLOGY 



the optimum point at which it proceeds at its highest rate and, 

 finally, the maximum point at which the reaction ceases. (For a 

 further discussion of cardinal points see Art. 89.) An essential 

 distinction of the enzymes from the living protoplasm is the fact 

 that their optimum points lie considerably higher, i.e., at 45 to 

 50° C. and in some enzymes even at 60° C. The optimum for 

 living cells usually does not exceed 25 to 35° C, while a tempera- 

 ture of 45 to 50° C. is lethal. The complete destruction of the 

 enzymes is attained at a still higher temperature. The usual 

 method of checking the enzymatic action is by boiling. 



The actual acidity, pH, of the medium exerts a great influence on 

 the work of the enzymes. Each enzyme has an optimum concen- 

 tration of hydrogen ions, as well as an upper and a lower limit, 

 exceeding which, the enzyme is either wholly destroyed or rendered 

 inactive. This fact was noticed long ago in animal physiology. 

 The proteases of the digestive canal have been divided in two 

 sharply differing groups: the pepsins working in an acid, and the 

 trypsins working in an alkaline medium. In availing oneself of 

 the modern methods of determining the concentration of hydrogen 

 ions it has been possible to establish the optimum pH for all of 

 the more important enzymes of the animal and plant. It has been 

 found that the optimum pH for pepsin of the gastric juice is 1.2 to 

 1.6, for the invertase of yeast 4.2 to 4.6, for the diastase of malt 

 4.3 to 4.5, for emulsin 4.4, for the protease of yeast 6.7 to 8.5, for 

 the trypsin of the pancreas 7.5 to 9.5. The first four enzymes pre- 

 fer an acid reaction, the last two, an alkaline medium. These 

 values are not absolutely constant. They depend on the tempera- 

 ture, the substance on which the enzyme acts, the presence of cer- 

 tain chemical substances, etc. The position of the optimum may 

 be slightly changed. Still the influence of pH on the activity of 

 the enzymes is so great that it is quite impossible to study the 

 action without precisely determining the acidity of the medium. 

 There is no doubt that the acidity is of equally great importance 

 for the enzymes acting in the living cell, and that changes in acidity 

 may induce a sharp acceleration or a retardation in a series of 

 enzymatic processes. It must not be forgotten that the different 

 parts of the same cell may show different acidities. The proto- 

 plasm is always somewhat alkaline, while the cell sap is generally 

 acid. 



A very characteristic peculiarity of enzymes is the specificity of 



