CHEMICAL CHANGES IN PLANTS AND ANIMALS. 639 



usually obtained from the fresh pancreas of the hog. It consists 

 principally of amylopsin, myopsin, trypsin, and steapsin. It is a 

 yellowish or grayish, almost odorless powder, soluble in water to the 

 extent of 90 per cent., insoluble in alcohol. It has the power to 

 digest proteins, and should convert not less than twenty-five times its 

 own weight of starch into sugar. 



Experiment 73. Introduce 7.5 grammes of starch into a flask, add 120 c.c. 

 of distilled water, and boil until a translucent mixture results. Cool the 

 resulting paste to 40.5 C. (105 F.), and add to it 0.3 gramme of pancreatin, 

 previously dissolved in about 10 c.c. of distilled water at 40.5 C. (105 F.). 

 Shake the flask well, maintaining the temperature of the mixture at 40.5 C. 

 (105 F.) during five minutes ; at the end of this time all of the starch should 

 be converted into substances soluble in water, and a thin liquid be produced. 

 Mix 2 drops of tenth-normal iodine V. S. with 60 c.c. of distilled water, and 

 add to it 2 drops of the warm converted starch solution ; no color should result, 

 or, at most, a wine-red color, showing the presence of dextrin and maltose. 

 The appearance of a blue or purple color indicates the presence of unconverted 

 starch and that the pancreatin is below the standard i. e., that of converting 

 not less than 25 times its own weight of starch into substances soluble in water. 



54. CHEMICAL CHANGES IN PLANTS AND ANIMALS. 



Difference between vegetable and animal life. As a general 

 rule, it may be stated that the chemical changes in a plant are pro- 

 gressive or constructive, in an animal regressive or destructive. That is 

 to say, plants take up as food a small number of inorganic substances 

 of a comparatively simple composition, convert them into organic 

 substances of a more and more complicated composition with the 

 simultaneous liberation of oxygen, while animals take up as food 

 these organic vegetable substances of a complex composition, assim- 

 ilating them in their system, where they are gradually used (burned 

 up) and finally discharged as waste products, which are identical (or 

 nearly so) with those substances serving as plant food. 



QUESTIONS. To which class of substances is the term protein applied, and 

 which elements enter into their composition? How are proteins classified, and 

 how do these groups differ from each other? Describe the five color-reactions 

 of proteins. Mention the conditions necessary for the coagulation of a protein 

 solution by heat ; and state how coagulation differs from precipitation. De- 

 scribe the products of proteolysis. Describe the nucleoproteins. Give a full 

 description of the nature and action of enzymes. State the composition and 

 properties of tyrosine and leucine. Define hydrolytic cleavage. Mention two 

 enzymes that are official; state their sources and their function in the process 

 of digestion. 



