92 Introductory Biology 



enzymes are soluble in water and can be evaporated to a dry state and 

 retain their catalytic abilities for a long time. Most enzymes possess an 

 optimum temperature at which they function best, and most are de- 

 stroyed by boiling. They are also influenced by acids, alkalis, and 

 pressure. 



As stated previously, some of the vitamins are components of enzymes. 

 Experimental evidence indicates that the complex chemical reactions 

 associated with respiration are controlled, at least in part by special 

 oxidizing enzymes called respiratory enzymes. It is suggested that the 

 protein genes (hereditary determiners) must have some close relation to 

 enzymes. Probably specific genes (discussed in a later chapter) deter- 

 mine the presence of specific enzymes which are essential for the numer- 

 ous chemical processes in metabolism. 



Enzymes are usually classed after the type of chemical action produced 

 in a particular substance. For example, hydrolytic enzymes (Gr. hydor, 

 water; lysis, loosing) break down substances by causing them to combine 

 with water or build up substances by removing a molecule of water from 

 two simpler molecules of a substance when they combine. 



2 molecules of glucose sugar (CeHi^Oe) + hydrolytic enzyme -^ 

 1 molecule of sucrose sugar (G12H22O11) + H2O 



Proteolytic enzymes (Gr. protos, first; lysis, loosing) break down, or 

 possibly build up, proteins. Enzymes are frequently named by adding 

 -ase to the substance acted on: lipases (Gr, lipos, fat) change fats to 

 glycerine and fatty acids; amylase (diastase) (L. amylum, starch) 

 changes starch and dextrins to maltose (sugar) ; maltase (A.S. mealt, 

 malt) changes maltose to glucose (sugar) ; proteinases convert various 

 types of proteins into amino acids or intermediate products. 



Enzymes are of great economic importance in industries as a few 

 examples will illustrate : the making of breads, cheeses, syrups, glycerine, 

 alcoholic beverages, soy sauce, etc.; the retting (removal) of fibers from 

 the stems of hemp, flax, and other plants; the ripening of tobacco; the 

 preparation of sizing for paper and textiles; the preparation of skins for 

 tanning; the preparation of certain medicinal products, etc. 



III. METABOLISM, AUTOSYNTHESIS, AUTOCATALYSIS 



Metabolism (me -tab' o lizm) (Gr. m^^a^o/^^ to change) includes those 

 chemical activities of living protoplasm which are associated with growth, 

 maintenance, repair, and the constant building of new, living, proto- 



