FATS AND PHOSPHATIDES 267 



Fats give the test because of the glycerine which they contain. 

 Fatty acids and pure soaps do not give the test, a result which 

 might be expected from the fact that they contain no glycerine. 



II. Phosphatides 



The phosphatides contain glycerine, phosphoric acid, choline 

 and fatty acid radicles. Chemically they are closely allied to the 

 fats (see lecture notes) . A lecithin will be studied as an example 

 of this group. 



Lecithins occur probably in all animal cells and are especially 

 abundant in the brain, which contains also other members of the 

 group of phosphatids and from which these substances may be 

 prepared. For laboratory purposes, lecithin may be obtained 

 from egg yolk. 



Lecithin. 



10. Preparation. The yolk of one egg is allowed to stand with 

 30 c.c. ether over night. To 30 c.c. of ether extract of egg yolk add 

 50 c.c. of alcohol. If a precipitate forms (another extractive sub- 

 stance if present in large amounts may be thrown down by alco- 

 hol) filter. Evaporate on the water bath, dissolve the residue in 

 15 c.c. ether and filter. Add 40 c.c. acetone to the filtrate. The 

 lecithin is precipitated. Filter it off. The filtrate contains chol- 

 esterol. Use the lecithin for the following tests : 



11. Add a small amount to 1 c.c. of water. Note opalescence. 

 The mixture may be filtered unchanged, although the lecithin 

 does not form a true solution. 



12. On a small portion make the acrolein test. 



13. Fuse about % of the lecithin with fusion mixture, dissolve 

 in dilute nitric acid and test for phosphate with ammonium 

 molybdate. 



14. Saponify l / 3 of the lecithin by heating in a flask on the 

 water bath with 20 c.c. of alcoholic NaOH for about 15 minutes, 

 replacing the alcohol if necessary. Evaporate to dryness and dis- 

 solve the soap in water. Boil and observe the soap bubbles. 



