720 METABOLISM 



by the relatively small amounts of lipase present in the stomach, whereas neutral fats 

 themselves are not so. 



Fatty acids also exist in nature in combination not only with the triatomic alcohol, 

 glycerol, but also with monatomic alcohols such as cholesterol. These cholesterol fats 

 differ from the glycerol fats in being very resistant towards enzymes and microorgan- 

 isms. They are therefore used for protective purposes in the animal economy; for 

 example, they occur in the sebum, the secretion of the sebaceous glands, where they 

 serve to moisten the hairs and skin. They are also present in cells, in which it is prob- 

 able they take an important part in forming the skeleton of the cell. Cholesterol is 

 absorbed from the intestine ; it is always present in the blood both in plasma and in 

 corpuscles; and it is an important constituent of bile, from which it may separate out 

 in the bile passages and form calculi (gallstones). 



In the cells themselves the lipoids are represented mainly by compounds of a some- 

 what more complex structure namely, the phospliolipins. As their name indicates, 

 these consist chemically of phosphoric acid combined with neutral fat and with a nitro- 

 genous base, cholin. The best known of the phospholipins is lecithin, which is widely 

 distributed in the animal body (present in blood and bile as well as in all cells). 

 Other phospholipins present in nervous tissue are cephalin, cuorin and sphyngomyelin. 

 There are various lecithins distinguished from one another by the fatty-acid radicles 

 which they contain. Distearyl-lecithin, for example, has the formula: 



CH 2 -0-OC(CH 2 ) 14 -CH 3 



CH -0-OC(CH 2 ) 14 -CH 3 



(stearic acid) 

 CH 2 -0 O 



(glycerol) P 



OH OCH 2 - CH 2 - N(CH 3 ) 2 



(phosphoric j 



acid) OH 



(choline) 



This complex molecule can readily be split up by hydrolysis (warming with baryta 

 water) into: 



glycero-phosphoric acid, CH 2 - OH 

 CH -OH 



CH 2 -0 



O 



P ; choline, N 



OH 



C 2 H 4 OH 



(CII.,) 3 (oxy-ethyl-ammonium 



OH 



hydroxide) ; and fatty acids. 



With hydrochloric acid, choline forms a salt which readily forms a double salt 

 with plantinic chloride. Since this double salt forms characteristic crystals, it is 

 used to identify and separate lecithins. For quantitative purposes, however, it il 

 more suitable to determine lecithin indirectly by the amount of phosphoric acid 

 present in an ethereal extract of the organ or tissue. 



Evidence is constantly accumulating to show that lecithin is an extremely impor- 

 tant constituent of cells; indeed, it seems to be the intermediate stage in the utiliza- 

 tion of neutral fats by protoplasm. Its phosphorus also probably serves as the source 

 of this element for the construction of nucleie acid (see page 669). In nervous tissues 



