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Triglycerides

Triglycerides are the most widespread of the lipid molecules. They are generally what people think of when they think about fats.

Triglycerides have two components. They always contain a glycerine molecule (Figure 1). Glycerine is a molecule with three carbon atoms and three alcohol groups, which are -OH groups, the same as those found in ethanol, the type of alcohol that is found in alcoholic beverages and carbohydrates. One -OH group in attached to each carbon atom and all triglycerides contain this same glycerine molecule, hence the name having the suffix -glyceride.

Picture of a molecular model of glycine.
Figure 1: Glycerine, a three carbon triol (compound with three alcohol groups) is the key component of triglycerides, as it is this consistent molecule which all triglycerides contain and to which the rest of the triglyceride is built around, as the other components of the triglyceride are bound to this molecule.

The other component of triglycerides are fatty acid molecules (Figure 2). Unlike glycerine which remains consistent, fatty acids can vary, which gives triglycerides a flexible composition, as the fatty acids which are incorporated can be varied to affect the composition of the fatty acids incorporated into the triglyceride. Fatty acids though have one vital and consistent feature, in that they always contain a carboxylic (-COOH) group and always have a long carbon chain.

Picture of a molecular model of butyric acid.
Figure 2: Butyric acid is an example of a fatty acid that is found in butter, hence the similar sounding name. As a fatty acid, it is relatively short, containing only four carbon atoms, which is only two carbon atoms more than ethanoic acid (commonly known as vinegar). Other fatty acids are considerably longer, and may have features such as one or more double bonds, rather than just single bonds.

The alcohol and carboxylic acid groups of the glycerine and fatty acids respectively are able to form ester bonds (Figure 3). These ester bonds are condensation reactions that produce a water molecule, formed by the loss of the two hydrogen atoms and one oxygen atom when the bond between the molecules is formed.

Once the glycerine forms ester bonds with three fatty acids, the triglyceride molecule is complete. The large carbon chains contained in the triglyceride make the molecule hydrophobic, which means they are sequestered away from water, in order to minimize the area of contact between the triglyceride and water molecules.

Picture of a molecular model of a triglyceride.
Figure 3: A triglyceride composed of a glycerine molecule and the fatty acids (from top to bottom): oleic acid (a monounsaturated fatty acid), stearic acid (a saturated fatty acid) and linoleic acid (a polyunsaturated fatty acid). The fatty acids are joined to the single glycine molecule, and form a large, hydrophobic molecule.

Triglycerides are important molecules in organisms. Like polysaccharides, they are long term energy storage, and can be stored in fat cells. They are released into the bloodstream and can be metabolized slowly to produce energy. They also have insulating properties, helping animals retain heat. They are also hydrophobic molecules, allowing compounds that are not soluble in water to be transported in them. Overall, they are an important molecule for many biological processes.