7.2: Lipids

Identifying Lipids & Types of Lipids

Unlike other classes of biomolecules, lipids have a wide variety of structures. The thing that they have in common is a property rather than a structure: lipids are soluble in nonpolar solvents. Therefore, you can generally recognize lipids by looking for a large nonpolar region, i.e. lots of C-C and C-H bonds. 

Lipids can generally be sorted into two categories: those that are made from fatty acids and those that are not. In this course we will examine triacylglycerols and phospholipids (which are made from fatty acids) and steroids (which are not). 

Fatty Acids and Triacylglycerols

Fatty acids are long molecules composed of a carbon chain (with an appropriate number of hydrogen atoms) and a carboxylic acid group on one end. The long chain, consisting of 12 - 20 carbon atoms, gives the fatty acid its ability to dissolve in nonpolar solvents. The carboxylic acid end allows the fatty acid to react with glycerol to form triacylglycerides. 

Relative melting points for fatty acids can be estimated based on the length of their carbon chains and the number of cis double bonds. As you saw with hydrocarbons, longer carbon chains can experience stronger (London) dispersion forces and therefore require more heat to melt and therefore have higher melting points. Saturated fatty acids have higher melting points than unsaturated fatty acids because the molecules can align better to form stronger dispersion forces. Double bonds, especially cis- double bonds, cause the chains to bend. Compare the structures of arachidic acid on the left and the two oleic acid structures on the right. 

        

Arachidic acid is a saturated fatty acid where all the C-C bonds are single bonds. The oleic acid fatty acids are unsaturated due to their double bonds. Of these molecules the cis-oleic acid has the lowest melting point (13.4 ℃) because the molecules cannot line up well. The dispersion forces are therefore weaker in cis-oleic acid compared to trans-oleic acid (melting point = 43 ℃) and arachidic acid (melting point = 75.5 ℃). 

The triacylglycerols formed when fatty acids react with glycerol are also called triglycerides and are frequently measured in blood tests to determine heart disease risk. The chemical equation below shows how glycerol (a molecule with three carbons and an alcohol functional group on each carbon) reacts with three fatty acids to form a triacylglycerol (or triglyceride) and three water molecules. 

Source: Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013.

The structure below shows a triacylglycerol. The three carbons on the left came from the glycerol. Each one reacted with a fatty acid, which is where the “tri” comes from in triglyceride. The remnants of the fatty acids’ carboxylic acid groups can be seen connected to the three carbon atoms. Towards the right are three long nonpolar carbon chains. Remember that these line structures have a carbon atom at each corner and at the end of the line. The triacylglycerol has three ester functional groups which formed when the alcohol groups of glycerol reacted with the carboxylic acid groups of the fatty acids (the extra oxygen and hydrogen atoms formed water as shown in the chemical equation above). 

If the carbon chains contain one or more double bonds, such as the second and third chains in the figure above, then they are unsaturated fatty acids. If all the bonds are single bonds, as in the first carbon chain, then it is a saturated fatty acid. That is, it is saturated with hydrogen atoms. 

 

Phospholipids

Phospholipids have a structure similar to triacylglycerides. Both are formed from a glycerol molecule and fatty acids, but whereas the triacylglycerols contain three fatty acid chains, the phospholipids contain two fatty acids and a phosphate/alcohol group. 

In the structure shown below, the two fatty acid chains are shown at the bottom of the picture (black and white, hydrophobic tails). The phosphate group is shown near the top in yellow (phosphorus) and red (oxygen) along with a polar head group (hydrophilic head). 

 

Credit: Mariana Ruiz Villarreal (LadyofHats) for the CK-12 Foundation

Also shown in the figure above is how phospholipids constitute a majority of the molecules in the membranes of cells. Two layers of phospholipids align with the nonpolar tails pointing towards one another. The polar heads of the molecules face the extracellular fluid outside the cell and the cytosol inside the cell, both of which are aqueous and therefore polar.  

Steroids

Though they are lipids, the structure of steroids is quite different than triacylglycerols or phospholipids. The basic structure, shown below, is a series of fused rings: three six-membered rings and a 5-membered ring.

 

A variety of different functional groups can be attached to this backbone to form different steroids. The three diagrams below show three common steroids: cholesterol (top), estrogen (bottom left), and testosterone (bottom right). 

Notice that all three have the four fused rings characteristic of steroids, however, each has a different combination of functional groups. 

 

Other Lipids

There are several other classes of lipids. These will be covered in CHEM 30B.