21.4:
Structure of Lipids
Lipids are a group of hydrophobic molecules that include triglycerides, which are used to store energy, and phospholipids, which are the major structural components of cell membranes.
Triglycerides and most phospholipids consist of glycerol and fatty acid chains.
Fatty acids have methyl groups at one end of the chain and carboxyl groups on the opposite end. The carbon connected to the carboxyl group is called the alpha carbon, and the methyl group carbon is known as the omega carbon.
Fatty acids vary in length and the presence of double bonds in the hydrocarbon chain. Those with double bonds are unsaturated fatty acids and those with only single bonds are saturated fatty acids because they are saturated with the largest number of hydrogen atoms possible.
Fatty acids often have common names but can be systematically named according to the number of carbon atoms and the number and position of the double bonds in the carbon chain.
There are several common numbering methods. The carboxyl referencing system counts the position of all the double bonds from the carboxyl carbon, which is numbered as 1. The omega referencing system counts the position of the double bond closest to the omega carbon, with the omega carbon numbered as 1.
For example, linolenic acid, an omega 3-fatty acid, has 18 carbons and double bonds at positions 9,12, and 15, counting from the carboxyl end. This will be called 18:3 Δ9, 12, 15 according to the carboxyl referencing system and 18:3 (ω-3) according to the omega referencing system.
Unsaturated fatty acids can occur in two configurations: cis and trans. In the cis configuration, the hydrogens on the carbons involved in the double bond are on the same side of the bond, whereas in the trans configuration, the hydrogens are located on the opposite sides.
Structurally, cis fatty acids have bent chains, whereas trans fatty acids have straight chains.
Consumption of trans fats has been found to be responsible for various cardiovascular diseases, whereas consumption of cis fatty acids, such as omega-3 and omega-6, are known to be beneficial to health.
Triglycerides, commonly known as fats, are primarily used for the storage of energy. They are composed of three fatty acids linked to glycerol through ester linkages between the hydroxyl ends of glycerol and the carboxyl ends of the fatty acids resulting in the formation of a nonpolar molecule.
If all the three fatty acids are of the same type, they are known as simple triglycerides. If the three fatty acids differ, they are known as mixed triglycerides.
Another type of lipid, phospholipids, is an important structural feature of biological membranes.
They have hydrophilic heads consisting of phosphate groups modified with a hydrophilic alcohol group and hydrophobic fatty acid tails. The composition of these heads and tails can vary, resulting in different types of membrane lipids.
Glycerophospholipids are a common type of phospholipid that consists of two fatty acids and a highly polar group attached to each carbon of glycerol through ester and phosphodiester linkages, respectively.
21.4:
Structure of Lipids
Lipids include a diverse group of compounds that are largely nonpolar in nature. This is because they are hydrocarbons that include mostly nonpolar carbon-carbon or carbon-hydrogen bonds. Non-polar molecules are hydrophobic (“water fearing”), or insoluble in water. Lipids perform many different functions in a cell. Cells store energy for long-term use in the form of fats. Lipids also provide insulation from the environment for plants and animals. For example, they help keep aquatic birds and mammals dry when forming a protective layer over fur or feathers because of their water-repellant hydrophobic nature. Lipids are also the building blocks of many hormones and are an important constituent of all cellular membranes. Lipids include fats, oils, waxes, phospholipids, and steroids.
General Structure of Lipids
A fat molecule consists of two main components—glycerol and fatty acids. Glycerol is an organic compound (alcohol) with three carbons, five hydrogens, and three hydroxyl (OH) groups. Fatty acids have a long chain of hydrocarbons to which a carboxyl group is attached, hence the name “fatty acid.” The number of carbons in the fatty acid may range from 4 to 36. The most common are those containing 12–18 carbons. In a fat molecule, the fatty acids attach to each of the glycerol molecule’s three carbons with an ester bond through an oxygen atom. Joining three fatty acids to a glycerol backbone in a dehydration reaction forms triacylglycerol. The three fatty acids in the triacylglycerol may be similar or dissimilar.
A phospholipid is another common type of lipid. It is an amphipathic molecule, meaning it has a hydrophobic and a hydrophilic part. The fatty acid chains are hydrophobic and cannot interact with water; whereas, the phosphate-containing group is hydrophilic and interacts with water. The hydrophilic head groups of the phospholipids face the aqueous solution. The hydrophobic tails are sequestered in the middle of the bilayer.
Fatty Acids
Fatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between neighboring carbons in the hydrocarbon chain, the fatty acid is saturated. Stearic acid is an example of a saturated fatty acid.
When the hydrocarbon chain contains a double bond, the fatty acid is unsaturated. Oleic acid is an example of an unsaturated fatty acid. Most unsaturated fats are liquid at room temperature and are called oils. If there is one double bond in the molecule, then it is a monounsaturated fat (e.g., olive oil), and if there is more than one double bond, then it is a polyunsaturated fat (e.g., canola oil). Long straight fatty acids with single bonds generally pack tightly and are solid at room temperature. Animal fats with stearic acid and palmitic acid (common in meat) and the fat with butyric acid (common in butter) are examples of saturated fats.
Fatty acids can additionally be classified into Cis and trans. Cis and trans indicate the configuration of the molecule around the double bond. If hydrogens are present in the same plane, it is a cis fat. If the hydrogen atoms are on two different planes, it is a trans fat. The cis double bond causes a bend or a “kink” that prevents the fatty acids from packing tightly, keeping them liquid at room temperature. Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated fats. Unsaturated fats help to lower blood cholesterol levels; whereas, saturated fats contribute to plaque formation in the arteries.
Trans Fats
The food industry artificially hydrogenates oils to make them semi-solid and of a consistency desirable for many processed food products. During this process, double bonds of the cis– conformation in the hydrocarbon chain may convert to double bonds in the trans– conformation.
Margarine, some types of peanut butter, and shortening are examples of artificially hydrogenated trans fats. Recent studies have shown that an increase in trans fats in the human diet may lead to higher levels of low-density lipoproteins (LDL), or “bad” cholesterol, which in turn may lead to plaque deposition in the arteries, resulting in heart disease.
Omega Fatty Acids
Essential fatty acids are those that the human body requires but does not synthesize. Consequently, they have to be supplemented through ingestion via the diet. Omega-3 fatty acids fall into this category and are one of only two known for humans (the other is omega-6 fatty acid). These are polyunsaturated fatty acids and are omega-3 because a double bond connects the third carbon from the hydrocarbon chain’s end to its neighboring carbon.
Alpha-linolenic acid is an example of an omega-3 fatty acid. It has three cis double bonds and, as a result, a curved shape. Salmon, trout, and tuna are good sources of omega-3 fatty acids. Research indicates that omega-3 fatty acids reduce the risk of sudden death from heart attacks, lower triglycerides in the blood, decrease blood pressure, and prevent thrombosis by inhibiting blood clotting. They also reduce inflammation and may help lower the risk of some cancers in animals.
This text is adapted from Openstax, Biology 2e, Chapter 3.3: Lipids.