[Chap. XVI 



SYNTHESIS OF FATS AND PROTEINS 



139 



sugar — or its derivatives — must be oxidized during the formation of 

 fatty acids. Sugar, therefore, is the primary source of both the material 

 and the energy in fat synthesis. 



The final step in the process, the condensation of glycerin and fatty 

 acid to fats or oils, may be briefly represented: 



Glycerin 



+ Palmitic acid 



(a fatty acid) 



Tri-palmitin 



(a tat) 



+ Water 



C3H5(OH)3 + 3 C15H31COOH > C.3H5(Ci,H3lC02)3 + 3 H2O 



Certain important facts are much more evident when this process is 

 represented in a little more detail: 



H O HO 



HC^OjH + HOj— CH3,Ci.5 

 O 



HC— OiH + HOl— CH31C15 



O 

 HC— OjH + HOJ— CH31C15 

 H 



HC— O— CH31C15 



O 



I! 



HC— O— CH31C15 + 3H2O 



O 

 HC— O— CH31C15 



H 



Glycerin + 3 Palmitic acid > Tri-palmitin + Water 



The place of union of the 3 molecules of fatty acid with 1 molecule 

 of glycerin by loss of 3 molecules of water is indicated in the above 

 equation. During the digestion of fats just the converse occurs: the 

 3 molecules of water are added and the glycerin and fatt)' acid are 

 separated. When the digestion of fats occurs in the presence of mineral 

 salts, some of the free ions of calcium, potassium, sodium, etc., unite with 

 the fatty acids and form soaps. 



Both the condensation and the digestion represented abo\'e are 

 catalyzed by an enzyme known as lipase. Here again is a reversible 

 reaction catalyzed by an enzyme (glycerin + fatty acids ^ fat or 

 oil + water). Whether condensation exceeds digestion or vice versa 

 depends upon several conditions in the cell. One of these conditions is 

 the water content of the cell. This fact may be demonstrated by placing 

 all these substances in test tubes in which the water content is varied. 

 If the amount of water in the test tube is increased, digestion is promi- 



