8o ELEMENTS OF BIOLOGY 



and paramoecium, or in the alimentary system of Man. To obtain 

 a fair concept of how protoplasm works without incorporating an 

 understanding of the nature of enzymes as catalysts is impossible. 



Some important characteristics of enzymes now concern us. In 

 the first place, the rate of a chemical reaction that is controlled by 

 enzymes depends on the concentration and effectiveness of the 

 enzyme, rather than the amount of the substances which are being 

 chemically changed. Thus the rate of oxidations in the cells of the 

 human body is not appreciably afTected by the presence of excess 

 food and excess oxygen, the ability of the oxidative enzymes to 

 transfer oxygen being the determining factor. Secondly, an enzyme- 

 controlled reaction may be slowed down or completely stopped by 

 some agent or condition which adversely affects the enzyme. For 

 example, respiration in the cell may be almost completely arrested 

 by prussic acid even in the presence of oxygen, the acid almost com- 

 pletely destroying the ability of the enzymes to transfer oxygen. 



Third, each enzyme has its own characteristics as regards the 

 optimum acidity, temperature, and so on, for its action. For exam- 

 ple, the enzyme in the saliva of Man operates best when the medium 

 is slightly alkaline; the enzyme in the digestive fluid in the stomach 

 operates best when acid is present in dilute solution. 



Fourth, the products of an enzyme reaction may affect the activ- 

 ity of the enzyme. A good example of this is found in the metab- 

 olism of the yeast plant. If a sugar solution is planted with yeast 

 and a small amount of alcohol is added, the sugar is converted to 

 alcohol and carbon dioxide much more rapidly than in the absence 

 of alcohol. So the product of the reaction accelerates the reaction. 

 This type of enzyme action is known as autocatalysis, and many 

 biological reactions are thought to be of this nature. On the other 

 hand, when the alcohol concentration becomes too high, the activity 

 of the enzyme is lowered thereby, and the reaction automatically 

 stops itself by means of its own products. 



