Difference between revisions of "Fat" - New World Encyclopedia
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| − | + | Chemically speaking, fats are ''triglycerides''—uncharged [[ester]]s of the three-carbon [[alcohol]] [[glycerol]])—that are solid at room temperature (20° C). Triglycerides that are liquid at room temperature are referred to as ''[[oil]]s''. They belong to a larger class of biological molecules called [[lipid]]s, which are generally water-insoluble but highly soluble in organic solvents. | |
| − | + | Triglycerides are known primarily as efficient energy stores in [[animal]]s, to be mobilized to meet the energy needs of the organism. Some [[plant]] [[species]], such as [[avocado]]s, [[olive]]s, and [[nut]]s, have substantial amounts of triglycerides in [[seed]]s or [[fruit]]s that serve as energy reserves for the next generation. | |
| + | |||
| + | They play a variety of roles (insulation, transport, biosynthesis). | ||
| + | |||
| + | Health/controversy? | ||
==The chemical structure of fats== | ==The chemical structure of fats== | ||
| − | [[Image:Trimyristin.png|thumb|right|150px|The chemical structure of [[trimyristin]], a triglyceride.]] | + | [[Image:Trimyristin.png|thumb|right|150px|The chemical structure of [[trimyristin]], a triglyceride.]]Triglycerides consist of three [[fatty acid]] chains bonded to a glycerol backbone. Fatty acids are a class of compounds that consist of a long hydrocarbon chain and a terminal carboxyl group (-COOH). A trigylceride is an [[ester]] of glycerol; that is, it is a molecule formed from a [[condensation]] (water-releasing) reaction between the three hydroxyl (-OH) groups of the alcohol glycerol and the carboxyl groups of the three fatty acid molecules. |
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| − | Fatty acids | + | Fatty acids are distinguished by two important characteristics: (1) chain length and (2) degree of unsaturation. The chemical properties of triglycerides are thus determined by their particular fatty acid components. |
===Chain length=== | ===Chain length=== | ||
| − | Fatty | + | 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 typically composed of an even number of carbon atoms, due to the biosynthetic processes of these organisms. [[Bacteria]], however, possess the ability to synthesize odd- and branched-chain fatty acids. Consequently, [[ruminant]] animal fat 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 | + | 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=== | ===Degree of unsaturation=== | ||
| − | Fatty acids may also differ in the number of hydrogen atoms that branch off of the chain of carbon atoms | + | [[Image:Myristic acid.png|thumb|left|500px|Two-dimensional representation of the saturated fatty acid [[myristic acid]].]] |
| + | [[Image:Myristic-acid-3D-vdW.png|thumb|left|500px|Three-dimensional representation of the saturated fatty acid [[myristic acid]].]] | ||
| + | Fatty acids may also differ in the number of hydrogen atoms that branch off of the chain of carbon atoms: | ||
| + | *When each carbon atom in the chain is bonded to two hydrogen atoms, ithe fatty acid is said to be [[saturation|saturated]]. ''Saturated fatty acids'' do not contain any [[double bond]]s between carbon atoms, because the carbon molecules are "saturated” with hydrogen; that is, they are [[covalent bond|bonded]] to the maximum number of hydrogen atoms. | ||
| + | *''[[Monounsaturated fatty acids]]'' contain one double bond near the middle of the chain. One carbon atom near the middle of the chain is bonded to only one hydrogen atom, so it forms a double bond with a neighboring carbon atom. | ||
| + | *''[[Polyunsaturated fatty acids]]'' may contain between two and six double bonds. As degree of unsaturation increases, the melting points of polysaturated fatty acids become lower. | ||
| + | [[Image:rasyslami.jpg|frame|Several fatty acid molecules]] | ||
| − | + | 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'' [[Conformational isomerism|conformation]], the hydrogens are on the same side of the double bond, whereas in the ''trans'' conformation, they are on the opposite side. | |
| − | Natural sources of fatty acids are rich in the ''cis'' isomer. In contrast, [[trans fat]]s are popular with manufacturers of [[Food preservation|processed foods]] because they are less vulnerable to [[rancidity]] and more solid at room [[temperature]] than ''cis'' fats. However, trans fats reduce the fluidity (and functionality) of [[Cell (biology)|cell]] [[membrane]]s. Trans-fats have been associated with many health problems, but their biochemistry is poorly understood. | + | Natural sources of unsaturated fatty acids are rich in the ''cis'' isomer. In contrast, [[trans fat]]s are popular with manufacturers of [[Food preservation|processed foods]] because they are less vulnerable to [[rancidity]] and more solid at room [[temperature]] than ''cis'' fats. However, trans fats reduce the fluidity (and functionality) of [[Cell (biology)|cell]] [[membrane]]s. Trans-fats have been associated with many health problems, but their biochemistry is poorly understood. |
==Types of fats and their chemical properties== | ==Types of fats and their chemical properties== | ||
| − | + | Naturally occurring fats contain varying proportions of saturated and unsatured fatty acids, proportions that determine their relative energy content and melting point. ''Saturated fats'', such as [[butter]] or [[lard]] contain a high percentage of saturated fatty acids, which fats can stack themselves in a closely packed arrangement. Thus, saturated fats freeze easily and are typically solid at room temperature. In contrast, ''unsaturated fats'', such as [[olive oil]], which contains a high percentage of monounsaturated fatty acids, have lower melting points than saturated fats. Each double-bond in an unsaturated fatty acid introduces a kink into the molecule that prevents unsaturated fatty acids from stacking efficiently. The kinks decreases intermolecular forces between the molecules, making it more difficult for unsaturated fats in the “cis” formation to freeze; they are typically liquid at room temperature. As mentioned above, “trans”-fats have properties resembling those of saturated fats. | |
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| − | + | Since an unsaturated fat contains fewer carbon-hydrogen bonds than a saturated fat with the same number of carbon atoms, unsaturated fats will yield slightly less energy during metabolism than saturated fats with the same number of carbon atoms. | |
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==Fats function as long-term energy stores== | ==Fats function as long-term energy stores== | ||
| − | Triglycerides play an important role in [[metabolism]] as energy stores | + | Triglycerides play an important role in [[metabolism]] as highly concentrated energy stores; when metabolized, they yield more than twice as much energy as [[carbohydrate]]s and [[protein]]s (approximately 9 [[calorie|kcal]]/g v. 4 kcal/g). Triglycerides make such efficient energy stores because they are (1) [[reduced]] and (2) nearly anhydrous (because they are nonpolar they don't need to be stored in hydrated form). |
| − | + | In animals, a type of loose connective tissue called [[Adipose tissue|adipose]] contains [[adipose cells]], specialized cells that form and store droplets of fat. Depending on current [[physiological]] conditions, [[adipocytes]] store fat derived from the diet and liver or degrades stored fat to supply [[fatty acids]] and [[glycerol]] to the [[circulatory system|circulation]]. When energy is needed, stored triglycerides are broken down to release glucose and free fatty acids. The glycerol can be converted to [[glucose]], another energy source, by the liver. The hormone [[glucagon]] signals the breakdown of the triglycerides by hormone-sensitive lipase to release free fatty acids. The latter combine with [[albumin]], a protein in blood plasma, and are carried in the bloodstream to sites of utilization, such as heart and skeletal muscle. | |
| − | + | In the [[intestine]], triglycerides ingested in the diet are split into glycerol and fatty acids (this process is called [[lipolysis]]) (with the help of [[lipase]]s and [[bile]] secretions), which can then move into [[blood vessel]]s. The triglycerides are rebuilt in the blood from their fragments and become constituents of lipoproteins, which deliver the fatty acids to and from adipose cells. | |
| − | ==Other | + | ==Other roles include insulation, transport, and biosynthesis== |
| − | [[ | + | The fat deposits collected in adipose tissue can also serve to cushion [[organ]]s against shock, and layers under the skin (“subcutaneous fats”) can help to maintain body temperature. Subcutaneous fats insulate animals against the cold because of the low rate of heat transfer in fat, a property especially important for animals living in cold waters or climates, such as whales, walruses, and bears. |
| − | + | The class of ''fat-soluble vitamins''—namely, [[Vitamin]]s [[Vitamin A|A]], [[Vitamin D|D]], [[Vitamin E|E]], and [[Vitamin K|K]]—can only be digested, absorbed, and transported in conjunction with fat molecules. [describe function of these vitamins] | |
| − | + | Dietary fats are sources of the [[essential fatty acid]]s [[linoleate]] and [[linolenate]], which cannot be synthesized internally and must be ingested in the diet; they are the starting point for the synthesis of various other unsaturated fatty acids. Polyunsaturated fatty acids are also precursors of the [[eicosanoids]], known as local hormones because they are short-lived, altering the activity of the cell in which they are synthesized and in nearby cells. [function] | |
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| − | Polyunsaturated fatty acids are precursors of the | ||
==Fats in human health and nutrition== | ==Fats in human health and nutrition== | ||
| − | In the human body, high levels of triglycerides in the bloodstream have been linked to [[atherosclerosis]], and, by extension, | + | In the human body, high levels of triglycerides in the bloodstream have been linked to [[atherosclerosis]], and, by extension, the risk of [[Coronary heart disease|heart disease]] and [[stroke]]. However, the negative impact of raised levels of triglycerides is lower than that of LDL:HDL ratios. The risk can be partly accounted for by a strong inverse relationship between triglyceride level and HDL-cholesterol level. |
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==References== | ==References== | ||
* Donatelle, Rebecca J. 2005. ''Health: The Basics,'' 6th edition. San Francisco, CA: Pearson. | * Donatelle, Rebecca J. 2005. ''Health: The Basics,'' 6th edition. San Francisco, CA: Pearson. | ||
| + | *Krogh, David. 2005. ''Biology: A Guide to the Natural World,'' 3rd edition. Upper Saddle River, NJ: Pearson. | ||
| + | *Purves, William, David Sadava, Gordon Orians, & H. Craig Heller. 2004. ''Life: The Science of Biology,'' 7th edition. Sunderland, MA: Sinauer. | ||
* Stryer, Lubert. 1995. ''Biochemistry,'' 4th edition. New York, NY: W.H. Freeman. | * Stryer, Lubert. 1995. ''Biochemistry,'' 4th edition. New York, NY: W.H. Freeman. | ||
Revision as of 02:32, 9 October 2006
| Types of Fats in Food |
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| See Also |
- For other uses, see Fat (disambiguation).
Chemically speaking, fats are triglycerides—uncharged esters of the three-carbon alcohol glycerol)—that are solid at room temperature (20° C). Triglycerides that are liquid at room temperature are referred to as oils. They belong to a larger class of biological molecules called lipids, which are generally water-insoluble but highly soluble in organic solvents.
Triglycerides are known primarily as efficient energy stores in animals, to be mobilized to meet the energy needs of the organism. Some plant species, such as avocados, olives, and nuts, have substantial amounts of triglycerides in seeds or fruits that serve as energy reserves for the next generation.
They play a variety of roles (insulation, transport, biosynthesis).
Health/controversy?
The chemical structure of fats
Triglycerides consist of three fatty acid chains bonded to a glycerol backbone. Fatty acids are a class of compounds that consist of a long hydrocarbon chain and a terminal carboxyl group (-COOH). A trigylceride is an ester of glycerol; that is, it is a molecule formed from a condensation (water-releasing) reaction between the three hydroxyl (-OH) groups of the alcohol glycerol and the carboxyl groups of the three fatty acid molecules.
Fatty acids are distinguished by two important characteristics: (1) chain length and (2) degree of unsaturation. The chemical properties of triglycerides are thus determined by their particular fatty acid components.
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 typically composed of an even number of carbon atoms, due to the biosynthetic processes of these organisms. Bacteria, however, possess the ability to synthesize odd- and branched-chain fatty acids. Consequently, ruminant animal fat 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:
- When each carbon atom in the chain is bonded to two hydrogen atoms, ithe 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.
- Monounsaturated fatty acids contain one double bond near the middle of the chain. One carbon atom near the middle of the chain is bonded to only one hydrogen atom, so it forms a double bond with a neighboring carbon atom.
- Polyunsaturated fatty acids may contain between two and six double bonds. As degree of unsaturation increases, the melting points of polysaturated 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.
Natural sources of unsaturated fatty acids are rich in the cis isomer. In contrast, trans fats are popular with manufacturers of processed foods because they are less vulnerable to rancidity and more solid at room temperature than cis fats. However, trans fats reduce the fluidity (and functionality) of cell membranes. Trans-fats have been associated with many health problems, but their biochemistry is poorly understood.
Types of fats and their chemical properties
Naturally occurring fats contain varying proportions of saturated and unsatured fatty acids, proportions that determine their relative energy content and melting point. Saturated fats, such as butter or lard contain a high percentage of saturated fatty acids, which fats can stack themselves in a closely packed arrangement. Thus, saturated fats freeze easily and are typically solid at room temperature. In contrast, unsaturated fats, such as olive oil, which contains a high percentage of monounsaturated fatty acids, have lower melting points than saturated fats. Each double-bond in an unsaturated fatty acid introduces a kink into the molecule that prevents unsaturated fatty acids from stacking efficiently. The kinks decreases intermolecular forces between the molecules, making it more difficult for unsaturated fats in the “cis” formation to freeze; they are typically liquid at room temperature. As mentioned above, “trans”-fats have properties resembling those of saturated fats.
Since an unsaturated fat contains fewer carbon-hydrogen bonds than a saturated fat with the same number of carbon atoms, unsaturated fats will yield slightly less energy during metabolism than saturated fats with the same number of carbon atoms.
Fats function as long-term energy stores
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 v. 4 kcal/g). Triglycerides make such efficient energy stores because they are (1) reduced and (2) nearly anhydrous (because they are nonpolar they don't need to be stored in hydrated form).
In animals, a type of loose connective tissue called adipose contains adipose cells, specialized cells that form and store droplets of fat. Depending on current physiological conditions, adipocytes store fat derived from the diet and liver or degrades stored fat to supply fatty acids and glycerol to the circulation. When energy is needed, stored triglycerides are broken down to release glucose and free fatty acids. The glycerol can be converted to glucose, another energy source, by the liver. The hormone glucagon signals the breakdown of the triglycerides by hormone-sensitive lipase to release free fatty acids. The latter combine with albumin, a protein in blood plasma, and are carried in the bloodstream to sites of utilization, such as heart and skeletal muscle.
In the intestine, triglycerides ingested in the diet are split into glycerol and fatty acids (this process is called lipolysis) (with the help of lipases and bile secretions), which can then move into blood vessels. The triglycerides are rebuilt in the blood from their fragments and become constituents of lipoproteins, which deliver the fatty acids to and from adipose cells.
Other roles include insulation, transport, and biosynthesis
The fat deposits collected in adipose tissue can also serve to cushion organs against shock, and layers under the skin (“subcutaneous fats”) can help to maintain body temperature. Subcutaneous fats insulate animals against the cold because of the low rate of heat transfer in fat, a property especially important for animals living in cold waters or climates, such as whales, walruses, and bears.
The class of fat-soluble vitamins—namely, Vitamins A, D, E, and K—can only be digested, absorbed, and transported in conjunction with fat molecules. [describe function of these vitamins]
Dietary fats are sources of the essential fatty acids linoleate and linolenate, which cannot be synthesized internally and must be ingested in the diet; they are the starting point for the synthesis of various other unsaturated fatty acids. Polyunsaturated fatty acids are also precursors of the eicosanoids, known as local hormones because they are short-lived, altering the activity of the cell in which they are synthesized and in nearby cells. [function]
Fats in human health and nutrition
In the human body, high levels of triglycerides in the bloodstream have been linked to atherosclerosis, and, by extension, the risk of heart disease and stroke. However, the negative impact of raised levels of triglycerides is lower than that of LDL:HDL ratios. The risk can be partly accounted for by a strong inverse relationship between triglyceride level and HDL-cholesterol level.
ReferencesISBN links support NWE through referral fees
- Donatelle, Rebecca J. 2005. Health: The Basics, 6th edition. San Francisco, CA: Pearson.
- Krogh, David. 2005. Biology: A Guide to the Natural World, 3rd edition. Upper Saddle River, NJ: Pearson.
- Purves, William, David Sadava, Gordon Orians, & H. Craig Heller. 2004. Life: The Science of Biology, 7th edition. Sunderland, MA: Sinauer.
- Stryer, Lubert. 1995. Biochemistry, 4th edition. New York, NY: W.H. Freeman.
External links
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