Fatty acid
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Fatty acids are a class of compounds containing a long hydrocarbon chain and a terminal carboxylate group (-COOH). They have the general structure CH3(CH2)nCOOH. Fatty acids belong to a diverse class of biological molecules called lipids, which are generally water-insoluble but highly soluble in organic solvents.
Fatty acids function as fuel molecules and serve as components of many other classes of lipids, such as phospholipids and glycolipids, important building blocks of biological membranes. Fatty acid derivatives also serve as hormones and intracellular messengers.
Fatty acids can be either saturated or unsaturated:
- Saturated fatty acids have no double bonds between the carbon atoms of the fatty acid chain (hence, they are fully saturated with hydrogen atoms).
- Unsaturated fatty acids have one or more double bonds. The presence of double bonds generally reduces the melting point of fatty acids, enhancing the fluidity of unsaturated fatty acids and their derivatives.
Make some kind of diet/bio-function point about degree of unsaturation and fatty acids generally
Chemical structure of fatty acids
Fatty acids are a group of carboxylic acids (organic acids with a terminal carboxyl group). Fatty acids are distinguished by two important characteristics: (1) the length of their hydrocarbon chain and (2) degree of unsaturation. Variations in chain length and degree of unsaturation affect the chemical properties of fatty acids as well as the other types of lipids they comprise (yuck!!!).
Chain length
Fatty acid chains in naturally occurring triglycerides are typically unbranched and range from 14 to 24 carbon atoms, with 16- and 18-carbon lengths being the most common. Fatty acids found in plants and animals are usually composed of an even number of carbon atoms, because their biosynthesis in these organisms involves acetyl-CoA, a coenzyme carrying a two-carbon-atom group. Bacteria, however, possess the ability to synthesize odd- and branched-chain fatty acids. Consequently, ruminant animal fat, such as in cattle, contains significant proportions of branched-chain fatty acids, due to the action of bacteria in the rumen.
Fatty acids with long chains are more susceptible to intermolecular forces of attraction (in this case, van der Waals forces), raising their melting point. Long chains also yield more energy per molecule when metabolized.
Degree of unsaturation
Fatty acids may also differ in the number of hydrogen atoms that branch off of the chain of carbon atoms.
Saturated fatty acids
When each carbon atom in the chain is bonded to two hydrogen atoms, the fatty acid is said to be saturated. Saturated fatty acids do not contain any double bonds between carbon atoms, because the carbon molecules are "saturated” with hydrogen; that is, they are bonded to the maximum number of hydrogen atoms. Saturated fatty acids form straight chains and, as a result, can be packed together very tightly, allowing living organisms to store chemical energy very densely.
Unsaturated fatty acids
Monounsaturated fatty acids contain one double bond near the middle of the chain, creating a "kink" in the chain. One of the carbon atoms, bonded to only one hydrogen atom, forms a double bond with a neighboring carbon atom.
Polyunsaturated fatty acids may contain between two and six double bonds, resulting in multiple "kinks." As the degree of unsaturation increases, the melting points of polyunsaturated fatty acids become lower.
The double bonds in unsaturated fatty acids may occur either in a cis or trans isomer, depending on the geometry of the double bond. In the cis conformation, the hydrogens are on the same side of the double bond, whereas in the trans conformation, they are on the opposite side. A trans double bond between carbon atoms makes the molecule less 'kinked' in comparison to fatty acids with cis double bonds. Trans fatty acids are characteristically produced during industrial hydrogenation of plant oils. Research suggests that amounts of trans fats correlate with circulatory diseases such as atherosclerosis and coronary heart disease more than the same amount of non-trans fats, for reasons that are not well understood.
Fatty acids are important components of membrane lipids
Fatty acid chains are important components of phospholipids and glycolipids, which are constituents of biological membranes. Biological membranes differentiate the cell from its environment, or define compartments within the cell (called organelles).
The differences in geometry between the various types of unsaturated fatty acids, as well as between saturated and unsaturated fatty acids, play an important role in the construction of biological structures, such as the cell's plasma membrane and the intracellular membranes of organelles. For example, the presence of short chains and/or unsaturated chains generally enhances the fluidity of biological membranes. At reduced temperatures, some organisms may vary the type and relative amounts of fatty acids to maintain the flexibility of their membranes. These changes in membrane lipid components contribute to the survival of plants, bacteria, and hibernating animals during winter.
Triglyceride is the storage form of fatty acids
Fatty acids are also important components of another type of lipid, triglyceride (commonly known as fat), which consist of three fatty acid chains bonded to a glycerol backbone. A triglyceride is an ester of glycerol; i.e., a molecule formed from a condensation (water-releasing) reaction between the three hydroxyl (-OH) groups of glycerol and the carboxyl groups of the three fatty acid molecules.
Triglycerides play an important role in metabolism as highly concentrated energy stores; when metabolized, they yield more than twice as much energy as carbohydrates and proteins (approximately 9 kcal/g versus 4 kcal/g). Triglycerides make such efficient energy stores because they are (1) highly reduced and (2) nearly anhydrous (i.e., because they are relatively nonpolar, they do not need to be stored in hydrated form).
Fatty acids function as a fuel source
When they are not attached to other molecules, fatty acids are known as free fatty acids. Free fatty acids are an important source of fuel for many tissues since they can yield relatively large quantities of ATP, the chemical energy currency of the cell. Although many cell types can use either glucose or fatty acids for fuel, heart and skeletal muscle prefer fatty acids. On the other hand, the brain cannot use fatty acids as a source of fuel. During starvation or periods of low carbohydrate intake, the brain relies instead on glucose or on ketone bodies produced by the liver from fatty acid metabolism.
Fatty acid derivatives serve as hormones and intracellular messengers
The human body can produce all but two of the fatty acids it needs. Since these two fatty acids, the polyunsaturated fatty acids linoleic acid and alpha-linolenic acid, cannot be synthesized and must be supplied in the diet, they are called essential fatty acids. They are widely distributed in plant and fish oils. In the human body, essential fatty acids are primarily used to produce hormone-like substances that regulate a wide range of functions, including blood pressure, blood clotting, blood lipid levels, the immune response, and the inflammation response to injury infection.
Related topics
Fatty acids in the diet
Naturally occurring fats contain varying proportions of saturated and unsaturated fatty acids, which in turn determine their relative energy content and melting point. The following table gives the fatty acid and cholesterol composition of some common dietary fats.[1] [2]
| Saturated | Monounsaturated | Polyunsaturated | Cholesterol | Vitamin E | |
|---|---|---|---|---|---|
| g/100g | g/100g | g/100g | mg/100g | mg/100g | |
| Animal fats | |||||
| Lard | 40.8 | 43.8 | 9.6 | 93 | 0.00 |
| Butter | 54.0 | 19.8 | 2.6 | 230 | 2.00 |
| Vegetable fats | |||||
| Coconut oil | 85.2 | 6.6 | 1.7 | 0 | .66 |
| Palm oil | 45.3 | 41.6 | 8.3 | 0 | 33.12 |
| Cottonseed oil | 25.5 | 21.3 | 48.1 | 0 | 42.77 |
| Wheat germ oil | 18.8 | 15.9 | 60.7 | 0 | 136.65 |
| Soya oil | 14.5 | 23.2 | 56.5 | 0 | 16.29 |
| Olive oil | 14.0 | 69.7 | 11.2 | 0 | 5.10 |
| Corn oil | 12.7 | 24.7 | 57.8 | 0 | 17.24 |
| Sunflower oil | 11.9 | 20.2 | 63.0 | 0 | 49.0 |
| Safflower oil | 10.2 | 12.6 | 72.1 | 0 | 40.68 |
| Rapeseed oil | 5.3 | 64.3 | 24.8 | 0 | 22.21 |
Nomenclature
In IUPAC nomenclature, the name of a fatty acid is derived from its parent hydrocarbon by substituting the suffix -oic for the final -e. (In common nomenclature, the suffix is usually -ic.) For example, —
The notation C18:0 means that the carbon chain of the fatty acid consists of 18 carbon atoms and does not contain any double bonds, whereas the notation C18:1 describes an 18-carbon chain with one double bond.
There are two methods for describing the position of a double bond in the hydrocarbon chain:
- cis/trans-Delta-x or cis/trans-Δx: The double bond is located on the xth carbon-carbon bond, counting from the carboxyl end. The cis or trans notation indicates whether the molecule is arranged in a cis or trans conformation. In the case of a molecule having more than one double bond, the notation is, for example, cis,cis-Δ9,Δ12.
- Omega-x or ω-x : Alternatively, the position of a double bond can be counted by starting from the distal end, with the ω carbon (methyl carbon) as position one. Sometimes, the symbol ω is substituted with a lowercase letter n, making the notation n-6 or n-3.
ReferencesISBN links support NWE through referral fees
- Krogh, D. 2005. Biology: A Guide to the Natural World, 3rd edition. Upper Saddle River, NJ: Pearson.
- Purves, W., D. Sadava, G. Orians, & H. C. Heller. 2004. Life: The Science of Biology, 7th edition. Sunderland, MA: Sinauer.
- Stryer, L. 1995. Biochemistry, 4th edition. New York, NY: W.H. Freeman.
External links
- Chemical Structure of Fats and Fatty Acids
- Plant Oils and Fats, from the Cyberlipid Center Web site
- Fat content and fatty acid composition of seed oils. Retrieved 2006-10-07. From Udo Erasmus' book, Fats that Heal Fats that Kill
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- ↑ Food Standards Agency (1991). "Fats and Oils", McCance & Widdowson's The Composition of Foods. Royal Society of Chemistry.
- ↑ Ted Altar. More Than You Wanted To Know About Fats/Oils. Sundance Natural Foods Online. Retrieved 2006-08-31.
