- Explain how the structure of each lipid impacts its function as it relates to health and disease.
- Compare and contrast the structure of the following types of fats: triglycerides (monounsaturated, polyunsaturated, omega-3, and omega-6), phospholipids, and sterols.
- Explain how the fatty acids you consume relate to the fatty-acid composition of your body fat.
Lipids are unique organic compounds, each serving key roles and performing specific functions within the body. As we discuss the various types of lipids (triglycerides, phospholipids, and sterols) in further detail, we will compare their structures and functions and examine their impact on human health.
Triglyceride Structure and Functions
Triglycerides are the main form of lipid found in the body and in the diet. Fatty acids and glycerol are the building blocks of triglycerides. Glycerol is a thick, smooth, syrupy compound that is often used in the food industry. To form a triglyceride, a glycerol molecule is joined by three fatty acid chains. Triglycerides contain varying mixtures of fatty acids.
Fatty acids determine if the compound is solid or liquid at room temperature. Fatty acids consist of a carboxylic acid (−OOH) group on one end of a carbon chain and a methyl group (−CH3) on the other end. Fatty acids can differ from one another in two important ways—carbon chain length and degree of saturation.
It’s All in the Chain
Fatty acids have different chain lengths and different compositions. Foods have fatty acids with chain lengths between four and twenty-four carbons and most of them contain an even number of carbon atoms. When the carbon chain length is shorter, the melting point of the fatty acid becomes lower—and the fatty acid becomes more liquid.
A methyl group is on the left. A carboxyl group is on the right. The carbon atoms of those are connected to 2 carbon atoms between the methyl and carboxyl groups, so there is a chain of carbon atoms from left to right.
Degrees of Saturation
Fatty acid chains are held together by carbon atoms that attach to each other and to hydrogen atoms. The term saturation refers to whether or not a fatty acid chain is filled (or “saturated”) to capacity with hydrogen atoms. If each available carbon bond holds a hydrogen atom we call this a saturated fatty acid chain. All carbon atoms in such a fatty acid chain are bonded with single bonds. Sometimes the chain has a place where hydrogen atoms are missing. This is referred to as the point of unsaturation.
When one or more bonds between carbon atoms are a double bond (C=C), that fatty acid is called an unsaturated fatty acid, as it has one or more points of unsaturation. Any fatty acid that has only one double bond is a monounsaturated fatty acid, an example of which is olive oil (75 percent of its fat is monounsaturated). Monounsaturated fats help regulate blood cholesterol levels, thereby reducing the risk of heart disease and stroke. A polyunsaturated fatty acid is a fatty acid with two or more double bonds or two or more points of unsaturation. Soybean oil contains high amounts of polyunsaturated fatty acids. Both monounsaturated fats and polyunsaturated fats provide nutrition that is essential for normal cell development and healthy skin.
3 chemical structures are shown. 1. Stearic acid or C subscript 18 H subscript 36 O subscript 2: a saturated fat. 2. Oleic acid or C subscript 18 H subscript 34 O subscript 2: a monounsaturated fat with a double bond. 3. Linoleic acid or C subscript 18 H subscript 32 O subscript 2: a polyunsaturated fat with 2 double bonds. Foods that have a high percentage of saturated fatty acids tend to be solid at room temperature. Examples of these are the fats found in chocolate (stearic acid, an eighteen-carbon saturated fatty acid is a primary component) and meat. Foods rich in unsaturated fatty acids, such as olive oil (oleic acid, an eighteen-carbon unsaturated fatty acid, is a major component) tend to be liquid at room temperature. Flaxseed oil is rich in alpha-linolenic acid, which is an unsaturated fatty acid and becomes a thin liquid at ro