Exploring the Molecules of Life: Lipids
What is the difference between vegetable oil and lard, animal fat?
Why do you think hydrogenated vegetable oil, like Crisco, is a solid?
Lipids, components of all plants and animals, are a class of compounds that contain fatty acids or their derivatives and are generally soluble in non-polar solvents. The largest group of lipids are the fats and oils. This activity will introduce you to the various types of lipids, their structure and function.
Fatty acids contain the typical carboxylic acid group (-COOH) as shown on acetic acid to the left below. How would you characterize the carbon chain on palmitic acid, a fatty acid, on the right compared to acetic acid (found in vinegar)?
Fatty acids are just long chain carboxylic acids, where the number of carbons typically varies from 12 to 22 as even numbers. Here are the solubilities for a number of fatty acids at 20oC in a polar solvent, such as water, and a non-polar solvent, such as benzene.
Acetic acid (2 carbon chain) and butanoic acid (4 carbon chain) have an infinite solubility in water. What is the trend in water?
How would you characterize the solubility behavior of fatty acids in benzene, a non-polar solvent?
Now look at the structure of oleic acid shown below on the left. What is different compared to palmitic acid? How about linoleic acid on the right? Be sure to rotate both molecules.
Palmitic acid has all carbon-to-carbon single bonds and is referred to as a saturated fatty acid. While oleic and linoleic acids contain carbon-to-carbon double bonds are are referred to a unsaturated fatty acids.
What is the difference between a fat and an oil? Examine the graph of melting points for the saturated and unsaturated groups of fatty acids.
How do they differ? Room temperature or 25oC is shown as a dashed line.
It is a simple matter of state. The fats are solids since at room temperature their fatty acids melting points are greater than 25oC , while the oils, which contain many unsaturated fatty acids are liquids, due to the room temperature being above the unsaturated melting points. How about the trend in the unsaturated fatty acids? Look at the melting points when plotted against the number of double bonds.
The melting points of unsaturated fatty acids are influence by the number of double bonds in the fatty acid. In the solid state, the fatty acid molecules must line up in the crystalline soild. Which type of fatty acid molecule, the saturated or unsaturated, do you think can align itself in the easiest fashion? Look at their structures!
Double bonds present another characteristic that distinguish the saturated and unsaturated fatty acids. How would you characterize the reactivity based on the two reactions below?
H3C-CH3 + H2 no reaction
H2C=CH2 + H2 H3C-CH3 (the hydrogen adds across the double bond)
The double bond in the unsaturated fatty acids is a site of reactivity and, as well as, causes the molecular shape to be twisted and give the lower melting points. The hydrogenation of vegetable oils to reduce the number of double bonds is what converts the oils to solids. Brominated vegetable oil, where bromine is added across the double bonds, is found in Mountain Dew!
A fat or oil is a combination of glycerol (on the left), a trialcohol (3 -OH groups), and 3 fatty acids molecules. The fat or triglyceride is shown on the right.
The reaction of the alcohol and acid forms an ester and water. This process is called esterification. The reverse of this reaction is important in the digestion of fats/oils.
R-OH + R'-COOH R'-COO-R + HOH
Look at the structures of lecithin given below. In general how do these two structures differ from fats? Can you find the glycerol?
Lecithin is a phospholipid, one of the fatty acids in a fat is replaced by a phosphate group (on the left below) with a choline group (on the right below) containing a quaternary ammonium compound. Can you find the phosphorus and nitrogen in the structures?
The left above lecithin is saturated while the right one is unsaturated. What do you notice about the orientation of the phosphate group with a choline group containing a quaternary ammonium compound part of the molecule?
In a typical fat molecule, the non-polar long carbon chains are aligned up next to each other. The polar branch of the phospholipid orients away from the non-polar area, the long carbon chains.
The phosholipids form the lipid bilayers that make up cell membranes. Remember that a cell is in and contains an aqueous (water) environment. Describe the bilayer structure below. If you right click and go to Options and then unselect the "Display Hydrogen Atoms" and unselect "Display Hetero Atoms Group" you will remove the water molecules. Why are the water molecules found here?
Notice the alignment of the phosholipids in the bilayer. The polar ends of the molecules are oriented to the top and bottom, while the non-polar long carbon chains are interior. The polar water molecules are attracted to the polar ends of the phosholipids in the bilayer. The polar ends are oriented to the outside and interior of the cell, where the aqueous environmental occurs.
What is the thickness of the lipid bilayer?
Another type of phospholipid that coats the nerve axons is sphingolipids (on the right below), which are composed of the alcohol sphingosine (on the left below). Describe the sphingolipid.
Steroids are another group of lipids. They have the base structure shown to the left below. The most common steroid is cholesterol which is shown to the right. What property do you think places steroids into the class of lipids?
Here is the fat substitute, olestra. Why is it fat like?
How many fatty acids can you count on the molecule of olestra above?
Can you determine the molecule that the fatty acid chains are attached to?
The fatty acids can vary from 6 to 8 on olestra (eight of them are shown on the molecule above). The fatty acids are esterified to the -OH groups on sucrose, common table sugar. This makes the large olestra molecule indigestible compared to smaller fat/oil molecules.
Most of the structures on this page are from molecules collection available at the NSF-funded C4 Project at Cabrillo Community College.
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