PHYSIOLOGICAL CHEMISTRY 297 



CHAPTER IV. 

 THE PROTEINS. 



THIS group of bodies, as their name signifies (protos = first) are bodies, 

 of great importance. They occur in all animal and vegetable cells; 

 indeed they are intimately connected with the life of the cell. Without 

 them as food-stuffs animal cells cannot live. At present, top, they are 

 bodies of purely biological origin, no effort to make them in the 

 laboratory having as yet been successful. Most of the members of the 

 group are amorphous bodies of high molecular weight. The molecule 

 is made up of the elements carbon, hydrogen, nitrogen, oxygen, and 

 sulphur. The amounts of the elements vary considerably in different 

 proteins, as can be seen from the following table : 



Protein. C. H. N. O. S. 



Fibrinogen, - - 52 -93 6 '90 16 '66 22-26 1'25 



Serum albumin, - 52*08 7'10 15*93 21'96 1'90 



Serum globulin, - 52'7l 7'01 15-85 23 -32 I'll 



Keratin, - - - 50'65 6'36 17'14 20'85 5'00 



Elastin, - - - 54-32 6 "99 1675 21-94 



Gelatin, - - - 49'83 6'80 17'97 25*13 07 



The nitrogen and the sulphur are usually contained in two forms, 

 loosely combined and firmly combined. The loosely combined portions 

 can be split off from the molecule by boiling with a caustic alkali 

 (caustic soda, caustic potash, or soda lime). 



All proteins when heated alone give a smell of burnt feathers, due to 

 the evolution of ammonia, pyridine, etc. 



EXPERIMENT I. Evaporate to dryness some of the protein solution 

 provided. Heat strongly. Notice the charring and smell of burnt 

 feathers. 



EXPERIMENT II. The loosely combined Nitrogen. To about five 

 c.c. of diluted egg-white add a few drops of 20 % caustic soda ; warm 

 slowly, and hold a piece of moistened red litmus paper over the mouth 

 of the test tube. The litmus turns blue, showing that ammonia gas is 

 being evolved. The ammonia may also be detected by its smell, or by 

 holding the stopper of the concentrated hydrochloric acid bottle over 

 the mouth of the test tube when fumes of ammonium chloride are 

 formed. 



EXPERIMENT III. The loosely combined Sulphur (lead sulphide 

 test). To about five c.c. of 20 % caustic soda add two drops of lead 

 acetate solution and some solution of egg-white. Boil. A brown to- 

 black colour is developed, due to the lead sulphide which is formed. 



