SYNTHESIS, STORAGE AND BREAKDOWN 129 



were proteins, though examination of the relatively pure preparations 

 of enzymes that have been obtained in later years has not in every 

 case confirmed this belief. It is, however, very difficult to prepare 

 enzymes free from contamination with other cell-contents. Enzymes 

 may be extracted from plant tissues with water or other solvents 

 after the cells have been disrupted by pounding, or the cell- 

 membranes rendered more permeable by pre-treatment with alcohol, 

 or by freezing. For example, a crude extract containing the enzyme 

 diastase can be prepared from germinating cereal grains by mashing 

 the grains in water and standing the mixture aside for a time. Experi- 

 ment shows that the extract is able to effect the conversion of starch 

 into sugar, so that evidently an enzyme may retain its activity after separ- 

 ation from the plant. Extracts so prepared contain a mixture of en- 

 zymes in additions to other cell-contents, and special methods have 

 to be adopted in order to isolate a particular enzyme. 



The following is a list of the better known enzymes of the plant, 

 together with the reactions which they characteristically catalyse 

 outside the plant : — 



Invertase catalyses Sucrose -> Glucose + Fructose. 



Diastase ,, Starch -> Maltose. 



Maltase ,, Maltose -> Glucose. 



Lipase ,, Fats -> Fatty acids + Glycerine. 



Proteases ,, Proteins -> Amino-acids. 



Zymase ,, Sugars -> carbon dioxide + Alcohol. 



Every plant cell almost certainly contains all these enzymes and 

 others not mentioned here. 



Many of the reactions proceeding within the plant are reversible, 

 that is they may proceed in either direction. For example during 

 the daytime starch is formed in the chloroplasts of the leaf from 

 sugars, while at night the starch is converted back into sugars. 

 Under the conditions usually obtaining in experiments with enzymes 

 outside the plant, it is the breaking-down actions (presented in the 

 above table) that are most manifest : but in certain cases it has been 

 shown that the opposite, that is a building-up or synthetic property, 

 is also possessed by enzymes. It is assumed that this finding applies 

 to all enzymes concerned with reversible reactions. Circumstances 

 prevailing within the cell will decide the predominating activity of 

 such an enzyme at any moment, the relative concentration of sub- 

 stances concerned in the reaction being especially important. Thus 

 in a reaction A +± B, if substance A is considerably in excess of B, 



B.B. I 



