Name two structural characteristics that triglycerides and phospholipids have in common

Learning Outcomes

• Identify the basic structural features of fatty acids.
• Distinguish between saturated and unsaturated fatty acids.
• Define essential fatty acids.
• Identify the structural features of a triglyceride.
• Label the type of bond formed between fatty acids and glycerol in triglyceride.

There is a lot of interest these days on healthy diets as well as concerns about heart problems. There is also a strong market for the sales of omega-3 fatty acids, which are said to help lower fat levels in blood. But too many people rely on the supplements to help their hearts and don't understand the chemistry behind it all. Yes, taking omega-3 fatty acids will give you some of the fatty acids your body requires. No, this is not a substitute for eating a healthy diet and exercising. You can't sit in front of the TV set, eating your large pizza, and expect these pills to keep you health. You've got to do things the hard way - eat your vegetables and get some exercise.

A lipid is an organic compound such as fat or oil. Organisms use lipids to store energy, but lipids have other important roles as well. Lipids consist of repeating units called fatty acids. Fatty acids are organic compounds that have the general formula \(\ce{CH_3(CH_2)_{n}COOH}\), where \(n\) usually ranges from 2 to 28 and is always an even number. There are two types of fatty acids: saturated fatty acids and unsaturated fatty acids.

In saturated fatty acids, carbon atoms are bonded to as many hydrogen atoms as possible. This causes the molecules to form straight chains, as shown in the figure below. The straight chains can be packed together very tightly, allowing them to store energy in a compact form. This explains why saturated fatty acids are solids at room temperature. Animals use saturated fatty acids to store energy.

In unsaturated fatty acids, some carbon atoms are not bonded to as many hydrogen atoms as possible due to the presence of one or more double bonds in the carbon chain. Instead, they are bonded to other groups of atoms. Wherever carbon binds with these other groups of atoms, it causes chains to bend (see figure above). The bent chains cannot be packed together very tightly, so unsaturated fatty acids are liquids at room temperature. Plants use unsaturated fatty acids to store energy.

Unsaturated fat is generally considered to be healthier because it contains fewer calories than an equivalent amount of saturated fat. Additionally, high consumption of saturated fats is linked to an increased risk of cardiovascular disease. Some examples of foods with high concentrations of saturated fats include butter, cheese, lard, and some fatty meats. Foods with higher concentrations of unsaturated fats include nuts, avocado, and vegetable oils such as canola oil and olive oil.

Humans need lipids for many vital functions, such as storing energy and forming cell membranes. Lipids can also supply cells with energy. In fact, a gram of lipids supplies more than twice as much energy as a gram of carbohydrates or proteins. Lipids are necessary in the diet for most of these functions. Although the human body can manufacture most of the lipids it needs, there are others, called essential fatty acids, that must be consumed in food. Essential fatty acids include omega-3 and omega-6 fatty acids. Both of these fatty acids are needed for important biological processes, not just for energy.

Although some lipids in the diet are essential, excess dietary lipids can be harmful. Because lipids are very high in energy, eating too many may lead to unhealthy weight gain. A high-fat diet may also increase lipid levels in the blood. This, in turn, can increase the risk for health problems such as cardiovascular disease. The dietary lipids of most concern are saturated fatty acids, trans fats, and cholesterol. For example, cholesterol is the lipid mainly responsible for narrowing arteries and causing the disease atherosclerosis.

Lipids may consist of fatty acids alone, or they may contain other molecules as well. For example, some lipids contain alcohol or phosphate groups. They include

1. triglycerides: the main form of stored energy in animals.
2. phospholipids: the major components of cell membranes.
3. steroids: serve as chemical messengers and have other roles.

One type of lipid is called a triglyceride, an ester derived from glycerol combined with three fatty acid molecules.

Glycerol is a triol, an alcohol which contains three hydroxyl functional groups. A fatty acid is a long carbon chain, generally from 12 to 24 carbons in length, with an attached carboxyl group. Each of the three fatty acid molecules undergoes an esterification with one of the hydroxyl groups of the glycerol molecule. The result is a large triester molecule referred to as a triglyceride.

Triglycerides function as a long-term storage form of energy in the human bods. Because of the long carbon chains, triglycerides are nearly nonpolar molecules and thus do not dissolve readily in polar solvents such as water. Instead, oils and fats are soluble in nonpolar organic solvents such as hexane and ethers.

Contributors and Attributions

• Allison Soult, Ph.D. (Department of Chemistry, University of Kentucky)

A note from Dr. Haas: Lipids are molecules that are mostly nonpolar, but have some polar character. These molecules serve important biological functions, such as providing the principle component of membranes (phospholipid bilayers) and serving as energy storage (fat). The structures of a triglyceride and a phospholipid are shown above. Triglycerides are the things we commonly refer to as “fats” and “oils”. Phospholipids are similar to triglycerides with one important difference.

A skeletal structure of a phospholipid and a triglyceride are shown above. Notice the similarities and differences between the two structures. The phospholipid is similar to the triglyceride in that it contains fatty acid tails attached to a glycerol backbone. However, the phospholipid contains a organic phosphate zwiterion instead of a third fatty acid tail.

Triglycerides are completely insoluble in water. However, due to the ionic organic phosphate group, phospholipids demonstrate properties because the ionic group is attracted to water. Phospholipids have both a polar, hydrophilic end, and a nonpolar, hydrophobic end. Phospholipids are partially soluble in water, meaning that part of the molecule is attracted to water, and part of it is not. Phospholipids form important structures in water when the polar end faces water and the nonpolar end faces away from water. Below is a cartoon version of the phospholipid bilayer in 2D.

Phospholipid bilayers separate aqueous compartments in cells. They act as semipermeable membranes that allow only very small or nonpolar molecules through. Membranes also contain small molecules (ex cholesterol, coenzymeQ) and proteins, which can be embedded in the bilayer or can span from one side of the membrane to the other.Lipid

Every cell is enclosed by a membrane which gives structure to the cell and allows for the passage of nutrients and wastes into and out of the cell. The purpose of the bilayer membrane is to separate the cell contents from the outside environment. The outside of the cell is mostly water and the inside of the cell is mostly water. The cell membrane may be coated with other molecules such as carbohydrates and proteins, which serve as receptor sites for other messenger molecules. Interaction with the cell membrane allows for molecular communication signals to pass from outside to inside of the cell.

Cell membranes are composed of two classes of molecules: lipids and proteins. The proteins serve as enzymes, carry molecules, and provide the membrane with distinctive functional properties. Details of proteins and enzyme structures are given elsewhere. The lipids provide the structural integrity for the cell. The lipids found in the membrane consist of two parts: hydrophilic (water soluble) and hydrophobic (water insoluble). The hydrophobic portion of the lipids is the non-polar long hydrocarbon chains of two fatty acids. The fatty acids are present as esters bonded to glycerol. The third-OH group on glycerol is ester bonded to phosphate hence the term phospholipid. The phosphate ester portion of the molecule is polar or even ionic and hence is water soluble. A simple interaction of several phospholipids is shown in the graphic on the left.

There are two common phospholipids found in the bilayer:

1. Lecithin contains the amino alcohol, choline.
2. Cephalins contain the amino alcohols serine or ethanolamine.

The arrangement of phospholipids in cell membranes has been deduced by X-Ray diffraction data. The phospholipids are arranged as a bilayer (two molecules thick). The phospholipids are stacked with the non-polar hydrocarbon chains pointed inward while the polar ends act as the external surface as shown in graphic on the left. The structure of the bilayer is another application of the solubility principle of "likes dissolve likes".

Most of the fatty acids in the membrane are unsaturated because this allows the membrane to be more flexible (cis bonds are bent) to allow certain molecules through the membrane. However, the interaction of the hydrophobic inside of the layer acts as a barrier for ionic and polar molecules from entering the inside of the cell. In animal cells cholesterol is inserted between the non-polar chains, and makes up about 20% of the molecules of the membrane. This helps to make the membrane more rigid and adds strength.

Lipid Bilayer Graphic: Red/white spheres represent water molecules on the outside surfaces of the bilayer which are hydrophilic (water loving). The gray spheres represent the non-polar hydrocarbon chains, which are hydrophobic or water hating. The purple spheres represent individual phospholid molecules.

References

• Chime from: Dr. William McClure, Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213
• The coordinates are from the work of H. Heller, M. Schaefer, and K. Schulten (1993) "Molecular dynamics simulation of a bilayer of 200 lipids in the gel and in the liquid-crystal phases", J. Phys. Chem. 97, 8343-60.

• Charles Ophardt, Professor Emeritus, Elmhurst College; Virtual Chembook