396 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



LOCALISATION OF THE ENZYMES AND THE 

 CHEMICAL CHANGES IN THE ORGANISM. 



In the unicellular plants and animals the various chemical changes 

 in the organic compounds must occur in the single cell. The colloidal 

 substances are ingested and broken down by enzymes into crystalline 

 and diffusible substances. These diffuse into the cell and are built up 

 into colloids, which again are broken down and converted into other 

 crystalline substances capable of diffusing out of the cell. In the 

 process of evolution of the higher plants and animals a differentiation 

 of groups of cells into special tissues or organs has occurred. Like 

 the single cell, the cells of each organ must originally have been able 

 to effect all the chemical changes, but during evolution the power 

 of effecting some of them has become lost, whilst the power of effecting 

 others has become increased. Individual organs can effect most 

 changes, but not all ; some are effected by only one organ. Thus, 

 for example, in animals, the organs of the alimentary canal are chiefly 

 concerned in the hydrolysis of the proteins, carbohydrates and fats ; 

 the liver is concerned mainly in the metabolism of fats and carbo- 

 hydrates, the conversion of ammonia into urea and the destruction and 

 removal of blood pigment; the kidney is a filter which removes 

 'vaste products; the ductless glands produce special substances to 

 maintain the general equilibrium of the organism. The sequence of 

 chemical processes taking place in the alimentary canal of animals is 

 termed digestion. 



In plants, the differentiation is much less than in animals : the 

 growing points and the cambium layer in the stems are the active 

 tissues, also the cells in the flower, or tuber, which produce the seed and 

 embryo. Specialised cells are found in the insectivorous plants in Ne- 

 penthes in the lining membrane of the pitchers, in Drosera in special 

 tentacles. Certain cells in the surface of the scutellum of Zea Mais 

 have a glandular appearance, but these glands have no lumen. 



Very frequently in plants the enzyme may be present in one cell 

 and its substrate in an adjoining cell. They come into contact when 

 the cell walls are broken, artificially by crushing the tissue or by the 

 action of anaesthetics. 



Sucroclastic enzymes are present in greater variety and more 

 abundantly in plants ; proteoclastic enzymes in animals. This accords 

 with the general composition of the organic matter of plants and 

 animals. 



