40 GENERAL AND PHYSICO-CHEMICAL. 



and which is ordinarily considered as a type of an entire series of syn- 

 theses occurring in the body, where water is eliminated, the number of 

 known syntheses in the animal kingdom has increased considerably. 

 Many of these syntheses have also been artificially produced outside 

 of the organism, and numerous examples of animal syntheses of which 

 the course is absolutely clear will be found in the following pages. Besides 

 these well-studied syntheses, there also occur in the animal body similar 

 processes unquestionably of the greatest importance to animal life, but 

 of which we know nothing with positiveness. We enumerate as examples 

 of this kind of synthesis the re-formation of the red-blood pigment (the 

 haemoglobin), the formation of the different proteins from simpler sub- 

 stances, and the production of fat from carbohydrates. This last- 

 mentioned process, the formation of fat from carbohydrates, is also an 

 example of reduction processes which occur to a considerable extent in 

 the animal body. 



Certain reactions, which are either not reproduceable with dead ma- 

 terial or are only possible under conditions which destroy the cells, belong 

 to the chemical decompositions going on within the living organism. 

 Thus the synthesis of glycogen or of protein has not been accomplished 

 outside of the organism or without the aid of agents prepared by the 

 cells. On the other hand proteins and starches can be split into simpler 

 products without these agents, but for this purpose the action of acids or 

 alkalies of a concentration which would kill the cells is necessary. In 

 certain cases it is possible to bring about such reactions outside of the 

 organism without any injurious effect upon the cells. This is accomplished 

 by the aid of substances which are formed within the cells but have the 

 power of being active after they have left the cells. These substances 

 have been called enzymes or ferments. 



Enzymotic Processes. We must now mention a group of reactions 

 which are more or less related to enzyme action. 



In the first place the so-called hydrolytic cleavage processes in which 

 complex substances are divided into simpler substances with the simul- 

 taneous decomposition of water and the taking up of its constituents. 

 These processes are of the greatest importance in the digestion of the 

 food-stuffs and for making them of value but they are also important for 

 the metabolic processes in general. As examples of such cleavages 

 we will mention the division of proteins into simpler products, the trans- 

 formation of starch into sugar and the cleavage of neutral fats into the 

 corresponding fatty acid and glycerin: 



Tristearin Glycerin Stearic Acid. 



The importance of the hydrolytic cleavage processes for digestion 

 will be discussed in detail in Chapter VIII. 



