LIPIDES 



79 



acid derivative might react with a carbohydrate and a nitrogenous 

 base to form a glycoHpide. The most abtmdant of all esters formed in 

 living tissues, however, is the reaction product of these activated 

 fatty acids with the trihydric alcohol, glycerol. The resulting prod- 

 ucts, glycerides, make up the hundreds of different animal and vege- 

 table fats. 



Since glycerol has three h)droxyl groups (see formula below) it is 

 possible to have one, two, or three ester linkages in the same glyceride 

 molecule. In other words, it is possible to have mono-, di-, or triglyc- 

 erides, examples of which are shown below. 



O 



O 



CHoOH CH.— O— C— (CH2)i4CH3 CH.— O— C— (CH2)i4— CH3 

 CHOH CHOH CHOH O 



CHoOH 



glycerol 



CH2OH 



1-moiiopalinitin 

 (a monoglyceride) 



o 



CH2— O— C(CH2)i4— CH3 



1,3-dipalmitiii 

 (a diglyceride) 



CHo— O— C(CH2)i4— CH3 

 O 



CH— O— C(CH2)i4— CH3 



o 



CH2— O— C(CH2)i4— CH3 



1.2,3-tripalinitin 

 (a triglyceride) 



As can be seen from the example, there is a structural isomer of 

 1-monopalmitin, namely, 2-monopalmitin. These isomeric forms have 

 different physical properties such as melting point, boiling point, 

 viscosity, and density. Since it is possible that either or both isomers 

 might be present in a naturally occurring fat, the consistency of the 

 fat can be affected materially by the amount and type of isomer 

 present. In a similar fashion the two isomeric forms of dipalmitin, 

 if present in a fat, will (ontribute to its overall physical form. 



Thus far we have considered the glyceride forms in which one 

 particular fatty acid has combined with one, two, or all three hydroxyl 

 groups of glycerol. Such compounds are called simple glycerides. If 

 two or three different fatty acids are combined with a glycerol mole- 

 cule, the compound is called a mixed glyceride. Examples of mixed 

 glycerides are shown on page 80. 



