ENZYMS 67 



and other conditions are favorable and time be given 

 for the operation. The statement may safely be made 

 that we do not now know of any chemical change taking 

 place, in plant cells at least, in which enzyms are not 

 involved. The one exception to this general rule is an 

 exception due to our own ignorance rather than to any 

 knowledge to the contrary, namely, the series of chemical 

 reactions taking place in photosynthesis. 



Enzyms. — Generally speaking, these substances occur 

 in such small quantities that they escape chemical analy- 

 sis. It is believed, however, that they are colloidal and 

 possibly proteins. They are soluble, and hence they are 

 dissolved in the cells, presumably in colloidal solution. 

 They act at ordinary temperatures, those at which cell 

 activities are possible. Most of them are decomposable 

 by heat. 



Classification of Enzyms. — The enzyms are so 

 numerous that classification is necessary. They are di- 

 vided according to their physiological or chemical ac- 

 tivities, thus: 



I. The hydrolyzing enzyms (Hydrolases) : those which 

 accomplish the addition of water with a subsequent 

 splitting of the molecules acted upon. Of these we may 

 mention (a) the lipases, enzyms which act upon the fats 

 and oils stored in castor beans, Brazil nuts, corn, hemp, 

 etc., and split them into glycerine and fatty acids, as the 

 seeds sprout; (b) the amylases, enzyms which convert 

 starch into sugar; (c) other carbohydrases, such as ceU 

 lulase, which digests the cellulose parts of cell walls; 

 pectinase, which digests the cementing material in or 

 between the cell walls, particularly of fruits, which forms 

 the jelly made from fruits; invertase, which forms fer- 

 mentable sugars, e.g. glucose, from cane sugar; {d) 

 emulsin and the other enzyms which digest the glucosides 

 found in almonds, willow bark, etc., to form glucose and 

 various other products; (e) the proteases, which act upon 

 protein and nucleo-proteins, converting them into amino- 



