Figure 5. Saturated fatty acids have hydrocarbon chains connected by single bonds only. Unsaturated fatty acids have one or more double bonds. Each double bond may be in a cis or trans configuration. In the cis configuration, both hydrogens are on the same side of the hydrocarbon chain.
In the trans configuration, the hydrogens are on opposite sides. A cis double bond causes a kink in the chain. In the food industry, oils are artificially hydrogenated to make them semi-solid and of a consistency desirable for many processed food products.
Simply speaking, hydrogen gas is bubbled through oils to solidify them. During this hydrogenation process, double bonds of the cis — conformation in the hydrocarbon chain may be converted to double bonds in the trans — conformation. Margarine, some types of peanut butter, and shortening are examples of artificially hydrogenated trans fats. Many fast food restaurants have recently banned the use of trans fats, and food labels are required to display the trans fat content. Figure 6.
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. For clarity, the carbons are not shown. Each singly bonded carbon has two hydrogens associated with it, also not shown. Essential fatty acids are fatty acids required but not synthesized by the human body. Consequently, they have to be supplemented through ingestion via the diet.
Omega-3 fatty acids like that shown in Figure 6 fall into this category and are one of only two known for humans the other being omega-6 fatty acid. These are polyunsaturated fatty acids and are called omega-3 because the third carbon from the end of the hydrocarbon chain is connected to its neighboring carbon by a double bond.
Nutritionally important because the body does not make them, omega-3 fatty acids include alpha-linoleic acid ALA , eicosapentaenoic acid EPA , and docosahexaenoic acid DHA , all of which are polyunsaturated. 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, reduce triglycerides in the blood, lower blood pressure, and prevent thrombosis by inhibiting blood clotting. They also reduce inflammation, and may help reduce the risk of some cancers in animals. Like carbohydrates, fats have received a lot of bad publicity.
However, fats do have important functions. Many vitamins are fat soluble, and fats serve as a long-term storage form of fatty acids: a source of energy. They also provide insulation for the body.
Figure 7. Waxy coverings on some leaves are made of lipids. Wax covers the feathers of some aquatic birds and the leaf surfaces of some plants. Because of the hydrophobic nature of waxes, they prevent water from sticking on the surface Figure 7. Waxes are made up of long fatty acid chains esterified to long-chain alcohols. Phospholipids are major constituents of the plasma membrane, the outermost layer of animal cells. Like fats, they are composed of fatty acid chains attached to a glycerol or sphingosine backbone.
Instead of three fatty acids attached as in triglycerides, however, there are two fatty acids forming diacylglycerol, and the third carbon of the glycerol backbone is occupied by a modified phosphate group Figure 8.
Figure 8. A phospholipid is a molecule with two fatty acids and a modified phosphate group attached to a glycerol backbone. Adding a charged or polar chemical group may modify the phosphate. A phosphate group alone attached to a diaglycerol does not qualify as a phospholipid; it is phosphatidate diacylglycerol 3-phosphate , the precursor of phospholipids.
The phosphate group is modified by an alcohol. Phosphatidylcholine and phosphatidylserine are two important phospholipids that are found in plasma membranes. A phospholipid is an amphipathic molecule, meaning it has a hydrophobic and a hydrophilic part. How are fats absorbed and transported by the body? What are some examples of fatty acids?
What are some examples of lipids? What is the role of glycolipids in cells? How does the structure of lipids relate to its function? What are the functions of lipids? See all questions in Lipids. For example, lipid molecules such as steroids do not have a reactive carbonyl or carboxyl group, and thus are not readily hydrolyzed compared to a triglyceride, for example.
However, steroid molecules are fairly intricate, consisting of a tetracyclic ring system with a rigid geometry. In addition to steroids, the other major classes of simple lipids are prostaglandins and terpenes. All three of these simple lipid classes play critical roles in the human body. To connect these molecules to foods students are familiar with, the major lipid component in several fatty foods comes in the form of triglycerides — a complex lipid where 3 fatty acid molecules carboxylic acid are esterified to a molecule of glycerol alcohol.
While the glycerol portion of the triglyceride remains constant, the fatty acid portion of the triglyceride can vary widely. As mentioned above, fatty acids are carboxylic acids that contain long, unbranched hydrocarbon chains.
These chains can vary in length, typically containing between 12 and 20 carbon atoms. Furthermore, fatty acids can be saturated ie no carbon-carbon bonds or unsaturated contains at least 1 carbon-carbon bond. Examples of a saturated and unsaturated fatty acid are shown below.
As you can see from the structural schematics above, the presence of a double bond in the palmitoleic acid dramatically alters the structure of the fatty acid, which, in turn, dramatically alters its melting point. Because of the linear nature of the saturated fatty acid, these molecules can pack more tightly in the solid phase, leading to an increased number of dispersion interactions and hence the requirement of more energy to break them.
Alternatively, the kink resulting from the double bond in unsaturated fatty acids prevents efficient packing, decreasing the overall strength of the dispersion forces holding the fat molecules together. The general rule of thumb is that saturated fatty acids have a higher melting point than unsaturated fatty acids. Triglycerides containing unsaturated fatty acids are liquid at room temperature whereas triglycerides containing saturated fatty acids are solid at room temperature.
This is the difference between an oil and a fat. Also affecting the melting point is the length of the hydrocarbon chain — longer chains have a higher melting point than shorter chains. This is again related to the strength of the dispersion forces that longer chains afford. Naturally occurring oils and fats are usually made up of a mixture of triglycerides, meaning that each glycerol backbone can contain up to 3 different fatty acid molecules. However, there are trends.
For instance, triglycerides from animal fats typically have a higher percentage of saturated fatty acids compared to triglycerides extracted from plants oils.
As such, they also have different impacts on human health. When speaking of the double bonds in naturally occurring unsaturated fatty acids, we are usually referring to a cis double bond formation. Cis bonds are formed in nature as opposed to trans bonds because the enzymes responsible for desaturation reactions — reactions that transform a saturated carbon-carbon bond into an unsaturated carbon-carbon bond — operate in a way that only results in cis bond formation.
If enough autooxidation occurs, the oil will go rancid — this is why food oils tend to have a much shorter shelf life than fats saturated fatty acids are more stable. To address this issue, and prevent food waste, food scientists implemented a method to partially hydrogenate unsaturated oil mixtures until a desired texture was obtained. By reducing the double bonds in an unsaturated fatty acid mixture, the melting point can also be altered.
This change in the fatty acid physical property favors some cooking applications. For instance, Crisco, which was widely used for baking, is actually made up of partially-hydrogenated cottonseed oil.
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