Structure of Fats
Home Up Structure of Fats Variations in Fats and Oils Functions of Triglycerides

 

Structure of Fats

Let's start by reviewing the structure of fats from an earlier lesson. Fats are triesters of glycerol and long-chained carboxylic acids.

Triglycerides

They are formed (as shown below and in Example 5 in your workbook) by dehydration reactions in which the three OH groups of a glycerol molecule react with the carboxylic acid groups of three different fatty acids to form three ester bonds and also three molecules of water. The resulting molecule is called a triglyceride because of the three ester groups that are formed with glycerin. (Note: The bonds coming down from the carbon atoms on the right sight of this diagam are supposed to line up with the single-bonded oxygen atoms below them.)

   H              H              H
                             
   HC   C  C H
                             
     OH           OH            OH
3 H2O +

       H                      H                       H
                                                   
      H C  C  C H
                                                     
  O    OH
\\  /
C

H C H

H C H

H C H

H C H

H C H

H C H

H C H

        C H

        C H

H C H

H C H

H C H

H C H

H C H

H C H

H C H

H C H

H
  O    OH
\\  /
C

H C H

H C H

H C H

H C H

H C H

H C H

H C H

        C H

        C H

H C H

H C H

H C H

H C H

H C H

H C H

H C H

H C H

H
  O    OH
\\  /
C

H C H

H C H

H C H

H C H

H C H

H C H

H C H

        C H

        C H

H C H

H C H

H C H

H C H

H C H

H C H

H C H

H C H

H
O    O
\\  /
C

H C H

H C H

H C H

H C H

H C H

H C H

H C H

        C H

        C H

H C H

H C H

H C H

H C H

H C H

H C H

H C H

H C H

H
O    O
\\  /
C

H C H

H C H

H C H

H C H

H C H

H C H

H C H

        C H

        C H

H C H

H C H

H C H

H C H

H C H

H C H

H C H

H C H

H
O    O
\\  /
C

H C H

H C H

H C H

H C H

H C H

H C H

H C H

        C H

        C H

H C H

H C H

H C H

H C H

H C H

H C H

H C H

H C H

H

 

Shorthand Notation

At times it becomes tedious to draw all of these carbon atoms or write them out along with all the hydrogen atoms that are found and so sometimes you will see structural formulas like this where the triester portion is shown but then the carbon chain is represented by a zigzag line, each corner of which represents a carbon atom with two hydrogen atoms. There maysometimes be a double bond in which case the carbon atom would only have one hydrogen atom. The carbon atoms at the end of the chain have three hydrogen atoms, of course.

Shorthand structure of a fat. [struct1.jpg]

 

Hydrolysis Reaction

Just as the fats can be assembled (or synthesized) by means of a dehydration reaction, they can also be dissembled by hydrolysis reactions to reform the glycerol and fatty acids which were present when it was put together. In our bodies, this reaction is generally carried out in the presence of enzymes that catalyze the hydrolysis reaction.

struct1.jpg (9491 bytes) + 3 H20 Structure of glycerol. [struct4.jpg]
Shorthand structures of three fatty acids. [struct2.jpg]

 

Saponification Reaction

This reaction can take place outside our bodies in the presence of a strong base such as sodium hydroxide. Under those conditions, the reaction is called a saponification reaction and the products are glycerol and also salts of long-chained fatty acids which are also known as soap. (A similar diagram is shown in Example 6 in your workbook.)

struct1.jpg (9431 bytes) + 3 NaOH struct4.jpg (3151 bytes)
Shorthand structure of three fatty acid salts. [struct3.jpg]

 

Monoglycerides and Diglycerides

If only one long-chained carboxylic acid is bonded, it is called a monoglyceride. If only two are bonded, it is called a diglyceride. Only with three is it called a triglyceride.

Shorthand structure of a monoglyceride. [struct5.jpg] Shorthand structure of a diglyceride. [struct6.jpg] Shorthand structure of a triglyceride. [struct7.jpg]
monoglyceride diglyceride triglyceride

 

Phospholipids

It's also possible for a diglyceride to be formed and then instead of the third carboxylic acid being added, phosphate group being added to form a related type of compound called a phospholipid. Phospholipids can be considered to be simply a variation of the fats, but they are really important enough to be considered as a separate category of lipids.

Shorthand structure of a phospholipid. [struct8.jpg]

 

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