ii 4 BIOLOGICAL CHEMISTRY 



A resting seed shows no enzymic activity, but when it is 

 exposed to proper conditions of temperature and moisture 

 growth occurs with the utilisation of the food stores. During 

 the process of growth more enzyme can be extracted than 

 from the resting seed. On this fact depends the process of 

 making malt. Barley seeds are allowed to germinate and the 

 growth stopped after several days. The extract of these 

 germinated seeds forms the well-known malt extract which 

 contains a large amount of enzyme together with the products 

 of its activity. 



In some seeds the enzymes are formed by epithelial cells 

 forming what is termed the scutellum, a layer of tissue lying 

 between the embryo and the stored food (endosperm). The 

 manner in which the enzyme is set free from its precursor 

 (zymogen) is not known, but it is of interest that Reychler* 

 has found that dilute acid liberates the active enzyme. 



The processes of food transference are more complicated in 

 animals and the details are better worked out than in plants. 

 We shall therefore devote our attention to the processes of 

 digestion and absorption in mammals and refer to special 

 process in plants and other animals afterwards. 



Digestion in animals exhibits one marked difference from 

 food transference in plants. Animals use mechanical processes 

 to reduce the coarse particles of food to a state of fine sub- 

 division. This is a great advantage as the rate of solution of 

 a solid is dependent on its surface and when a solid is sub- 

 divided its surface is increased, hence digestion is accelerated 

 by the subdivision of the food. In addition, stirring and other 

 movements occur which facilitate digestion and absorption 

 by removing the concentrated solution from the surface of the 

 solid particles, thus allowing more of the substance to dissolve 

 and by promoting the contact of fresh portions of the food 

 with the enzymes and with the wall of the alimentary canal. 

 These movements cannot be dealt with from the chemical 

 side, they must be studied with the allied subject of animal 

 physiology. 



The epithelial cells which produce the enzymes are collected 

 into groups called glands. The active substances can be 

 obtained from these glands by extraction, with glycerine, salt 

 solution or water, and the enzymes can be purified by precipi- 

 tation with alcohol or by carrying them down in absorption 

 with certain insoluble materials. The extracts or the purified 

 enzymes obtained from them can be studied in their relation 



* A. Reychler, Ber,, 1889, vol. 22, p. 414. 



