30 Practical Plant Biology. 



substrates and the products become concentrated on this surface. 

 This surface concentration or adsorption seems essential in 

 enzyme action. As colloids their actions are readily localised 

 in the cell since they will not tend to become distributed 

 throughout the substance of the colloidal protoplasm. It is 

 also known that the action of some enzymes is greatly enhanced 

 by the presence of certain dissolved substances while the ex- 

 istence of inhibiting substances is also indicated. These charac- 

 teristics render enzymes extraordinarily suited to the delicate 

 and localised actions needed in the cell. We may easily imagine, 

 granted agents with these characteristics, how very varied re- 

 actions may take place in close proximity to one another but 

 so completely isolated from one another in the colloidal proto- 

 plasm as not to interfere in the slightest. The enzyme by means 

 of which yeast accelerates the hydrolysis of proteins has been 

 called endotryptase. Possibly this substance is a mixture of 

 several enzymes as its action extends to several different proteins 

 disintegrating them into amino-acids, and thus rendering them 

 accessible to, and capable of assimilation by, the yeast cell. 



Invertase is another enzyme produced by yeast. It is produced 

 very widely throughout the vegetable kingdom and being easily 

 extracted lends itself to experiment apart from the cell. Invertase 

 is responsible for the reversible change of cane sugar into dextrose 

 (or glucose) and levulose, which may be indicated as follows : 



C 12 H 22 O n + HoO ^ C,H 12 6 + C 6 H 12 6 

 Cane sugar. Dextrose. Levulose. 



Under normal conditions the change is complete or almost so. 

 The final product, being nearly entirely a mixture of the two 

 hexoses (dextrose and levulose), is called invert sugar. The 

 change itself is called inversion. It is easily studied owing to 

 the fact that cane sugar does not react with the blue alkaline 

 copper tartrate solution (Fehling's solution) to produce the red 

 copper oxide, while invert sugar does this when heated with 

 that reagent. Thus a solution of cane sugar, which may be 

 tested with the Fehling's solution beforehand, will after the 

 action of invertase give this reaction. 



The invertase may easily be obtained by adding a few cubic 

 centimetres of toluene to 100 gm. of pressed yeast and keep- 

 ing the mixture in a loosely stoppered bottle for three weeks 

 at a temperature of 30 C. The toluene kills the yeast without 

 injuring many of the enzymes which it contains. The proteo- 

 lytic enzymes destroy the cells and the greater number of other 



