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Most of the fatty acids in animal biology consist of even number of carbons in its parent chain. What property of such fatty acids cause the biological systems to prefer them over the odd counterparts? What's so bad about fatty acids containing odd number of carbon atoms in their parent chain?
Why are fatty acids consisting of even number carbons in their parent chain predominant?
In short, it's because fatty acids are made from two-carbon blocks.
The way that most organisms make fatty acids is by the successive addition of two-carbon units (acetyl-CoA). So we usually get even-numbered fatty acids just because the building blocks are also even.
In plants and in synthetic contexts, we can have some reactions that can produce odd-numbered fatty acids (by building with two-carbon units three times to get six, and then breaking that six in half to get three, for example). Such odd fatty acids are seen in some organisms, but humans are usually thought to get them from other sources (e.g. microbiota, diet).
Point 1: Fatty acids are generally built two-carbons at a time.
The European Bioinformatics Institute has a really good explanation of fatty acid synthesis. The quote below is from that (https://www.ebi.ac.uk/interpro/potm/2007_6/Page2.htm):
"Whether existing as one complex or as independent enzymes, the reactions catalysed by types I and II fatty acid synthases are the same. Both systems are primed with acetyl-CoA, and then add 2-carbon units to the growing chain using malonyl-CoA as substrate."
Point 2: Odd-numbered fatty acids can be produced by various organisms, and can be linked to dysfunction in humans.
doi:10.1038/srep44845 talks about the importance of odd chain fatty acids in disease as a preface to its findings about the sources of such fatty acids. They try to narrow down where odd-chain fatty acids come from, but they definitely seem to be building their case on the pretense that odd chain fatty acids are not great in humans.
"According to the literature, the origin of C15:0 and C17:0 has long been attributed to the diet, specifically from ruminant fat as the main contributor in a typical Western diet. This has been explained by the fact that these two OC-FAs are produced by the rumen microbiome and then incorporated into the fat deposits of the host animal destined for human consumption."
Point 3: Plants produce odd-chain fatty acids
An established/old paper (doi:10.1006/abbi.1994.1110) mentions this, and there's a Wikipedia article (https://en.wikipedia.org/wiki/Odd-chain_fatty_acid) that paraphrases in a nice way:
Some plant-based fatty acids, however, have an odd number of carbon atoms, and Phytanic fatty acid absorbed from plant chlorophyll has multiple methyl branch points. As a result, it breaks down into three odd numbered 3C Propionyl segments as well as three even numbered 2C Acetyl segments and one even numbered 4C Isobutynoyl segment.
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with a long aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, from 4 to 28.  Fatty acids are a major component of the lipids (up to 70 wt%) in some species such as microalgae  but in some other organisms are not found in their standalone form, but instead exist as three main classes of esters: triglycerides, phospholipids, and cholesteryl esters. In any of these forms, fatty acids are both important dietary sources of fuel for animals and they are important structural components for cells.
Lipids and Membranes
Fatty acids consist of a carboxylic acid group and a long hydrocarbon chain, which can either be unsaturated or saturated. A saturated fatty acid tail only consists of carbon-carbon single bonds, and an unsaturated fatty acid has at least one carbon-carbon double or triple bond. Fatty acids are distinguished from one another by the lengths of their hydrocarbon tails and degrees of unsaturation. For example, the one depicted above is palmitic acid, and it is identified by its tail consisting of sixteen carbons and its complete lack of carbon-carbon double bonds. Fatty acids are of utmost importance because they are our main source of fuel and serve as primary components of membranes.
Fatty acids have a specific system of nomenclature. For example, palmitic acid would be written as such: C16:0, which means that its hydrocarbon chain is sixteen carbons long and it does not have any double bonds. Similarly, stearic acid would be written as C18:0. Oleic acid, however, would be written as C18:1(cis-&Delta 9 ). The "1" tells us that there is one double bond in the hydrocarn tail, and it is a cis bond that occurs between carbons 9 and 10. Finally, linoleic acid is depicted as C18:2(cis-&Delta 9 , cis-&Delta 12 ), meaning this acid contains two double bonds in its tail and one occurs between carbons 9 and 10, while the other one is located between carbons 12 and 13. Double bonds in fatty acid tails generally tend to be in cis formation and are usually located at carbons 9, 12, and 15.
The behavior of fatty acids depends on their unique hydrocarbon tails. Those with long, unsaturated tails tend to be less soluble and have higher melting points than those with shorter, saturated tails. The cis double bonds present in unsaturated fatty acids cause kinks in their tails, thus interrupting its van der Waals interactions with neighboring fatty acids. This effect leads to an increased solubility and decreased melting points of unsaturated fatty acids. For more information about the importance of the degree and type of unsaturation of fatty acids: [link]
While fatty acids are used as accessible sources of energy, triacylglycerides are used to store energy. Their general structure involves three fatty acid tails, which may or may not be identical to one another, bonded through ester linkage to each carbon of a glycerol molecule.
Triacylglycerols are synthesized and stored in specialized cells called adipocytes, which make up adipose tissue and are generally located under the skin. Adipocytes consist of the necessary organelles along with a large droplet of triacylglycerols that takes up almost the whole cell volume. These lipids are transported throughout the body through the bloodstream.
Membranes consist of three major classes of membrane lipids: phospholipids, glycolipids, and sterols. Membranes are also embedded with proteins, and will be discussed in another section.
Phospholipids are the most important membrane lipids. Phosphoglycerids, like triacylglycerols, consist of a glycerol backbone. The first carbon on the glycerol molecule is attached to a fatty acid tail that is generally saturated, the second carbon is also attached to a fatty acid tail that is normally unsaturated, and the third carbon is attached to the hydrophilic phosphate head group. Below are a few common phospholipids:
Phospholipids that have a shingosine backbone (shown in red) are called sphingolipids. A sphingosine molecule consists of an amino alcohol and a long unsaturated hydrocarbon tail. An example of a common sphingolipid is sphingomyelin:
Glycolipids and Sterols
Glycolipids are characterized by their attached carbohydrate groups. Their function is to serve as markers for the cell so it can be recognized by other cells, molecules, etc. Sterols are characterized by a tetra-ring base. Cholesterol (left), a common sterol in membranes, is distinguished by this specific base, hydrophopic additions, and a hydrophilic hydroxy group.
When phospholipid molecules are in a water solvent, they simply float on the surface with their hydrophilic heads in the water and their hydrophobic tails exposed to the air. However, after passing the "critical micelle concentration", they are spontaneously able to form micells. At even higher concentrations, phospholipids can be submerged in the solvent by forming bilayer leaflets, structures analogous to membranes.
Phopholipid bilayers can transition from being a gel to a liquid crystal state with the addition of heat. The presence of longer chains increases this transition temperature and decrease fluidity of the bilayer. Conversely, presence of double bonds lowers the transition temperature and increases fluidity. Cholesterol tends to make the gel state more fluid and the liquid crystal state less fluid. Protein complexes are also affected by the membrane phase, because they function better in a liquid crystal but stay together better in a gel. All of these phenomena can be explained by the effects of van der Waal's forces discussed above.
Unsaturated Fatty Acids
There are about forty naturally occurring fatty acids. The fatty acids without carbon-carbon double bonds are classified as saturated, and those containing carbon-carbon double bonds are classified as unsaturated. Palmitic and stearic acids are the most common saturated fatty acids, and oleic and linoleic acids are the most common unsaturated fatty acids. Oleic acid is monounsaturated because it has only one carbon-carbon double bond. Linoleic, linolenic, and arachidonic acids are polyunsaturated because they have two, three, and four carbon-carbon double bonds, respectively. A way to measure the relative degree of unsaturation of a fat or an oil is to determine its iodine number. The iodine number is the mass of iodine, in grams, that is consumed by (reacts with) 100 grams of a fat or an oil. Iodine reacts with the carbon-carbon double bonds. Thus the greater the number of double bonds, the higher the iodine number. In general, fats have lower iodine numbers than oils because oils have greater percentages of carbon-carbon bonds that are double bonds. For example, typical iodine numbers for butter are 25 to 40, and for corn oil, 115 to 130.
|Common Saturated Fatty Acids|
|Number of Carbon Atoms||Formula||Common Name||Source|
|4||CH 3 (CH 2 ) 2 COOH||Butyric acid||Butter|
|6||CH 3 (CH 2 ) 4 COOH||Caproic acid||Butter|
|8||CH 3 (CH 2 ) 6 COOH||Caprylic acid||Coconut oil|
|10||CH 3 (CH 2 ) 8 COOH||Capric acid||Coconut oil|
|12||CH 3 (CH 2 ) 10 COOH||Lauric acid||Palm kernel oil|
|14||CH 3 (CH 2 ) 12 COOH||Myristic acid||Oil of nutmeg|
|16||CH 3 (CH 2 ) 14 COOH||Palmitic acid||Palm oil|
|18||CH 3 (CH 2 ) 16 COOH||Stearic acid||Beef tallow|
|18||CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 COOH||Oleic acid||Olive oil|
|18||CH 3 (CH 2 ) 4 CH=CHCH 2 CH(CH 2 ) 7 COOH||Linoleic acid||Soybean oil|
|18||CH 3 CH 2 (CH=CHCH 2 ) 3 (CH 2 ) 6 COOH||Linolenic acid||Fish oils|
|20||CH 3 (CH 2 ) 4 (CH=CHCH 2 ) 4 (CH 2 ) 2 COOH||Arachidonic||acid Liver|
|22||CH 3 (CH 2 ) 20 COOH||Beheric acid||Sesame oil|
|Common Unsaturated Fatty Acids|
|Number of Carbon Atoms||Formula||Common Name||Source|
|16||CH 3 (CH 2 ) 5 CH=CH(CH 2 ) 7 COOH||Palmitoleic acid||Whale oil|
|18||CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 COOH||Oleic acid||Olive oil|
|18||CH 3 (CH 2 ) 4 CH=CHCH 2 CH(CH 2 ) 7 COOH||Linoleic acid||Soybean oil, safflower oil|
|18||CH 3 CH 2 (CH=CHCH 2 ) 3 (CH 2 ) 6 COOH||Linolenic acid||Fish oils, linseed oil|
|20||CH 3 (CH 2 ) 4 (CH=CHCH 2 ) 4 (CH 2 ) 2 COOH||Arachidonic||acid Liver|
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Fatty acid, important component of lipids (fat-soluble components of living cells) in plants, animals, and microorganisms. Generally, a fatty acid consists of a straight chain of an even number of carbon atoms, with hydrogen atoms along the length of the chain and at one end of the chain and a carboxyl group (―COOH) at the other end. It is that carboxyl group that makes it an acid (carboxylic acid). If the carbon-to-carbon bonds are all single, the acid is saturated if any of the bonds is double or triple, the acid is unsaturated and is more reactive. A few fatty acids have branched chains others contain ring structures (e.g., prostaglandins). Fatty acids are not found in a free state in nature commonly they exist in combination with glycerol (an alcohol) in the form of triglyceride.
Among the most widely distributed fatty acids are the 16- and 18-carbon fatty acids, otherwise known as palmitic acid and stearic acid, respectively. Both palmitic and stearic acids occur in the lipids of the majority of organisms. In animals palmitic acid makes up as much as 30 percent of body fat. It accounts for anywhere from 5 to 50 percent of lipids in vegetable fats, being especially abundant in palm oil. Stearic acid is abundant in some vegetable oils (e.g., cocoa butter and shea butter) and makes up a relatively high proportion of the lipids found in ruminant tallow.
Many animals cannot synthesize linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid). Those fatty acids are required, however, for cellular processes and the production of other necessary omega-3 and omega-6 fatty acids. Thus, because they must be taken in through the diet, they are called essential fatty acids. Omega-6 and omega-3 fatty acids derived from linoleic acid and alpha-linolenic acid, respectively, are needed conditionally by many mammals—they are formed in the body from their parent fatty acids but not always at levels needed to maintain optimal health or development. Human infants, for example, are thought to have a conditionally essential need for docosahexaenoic acid (DHA), which is derived from alpha-linolenic acid, and possibly also for arachidonic acid, which is derived from linoleic acid.
Fatty acids have a wide range of commercial applications. For example, they are used not only in the production of numerous food products but also in soaps, detergents, and cosmetics. Soaps are the sodium and potassium salts of fatty acids. Some skin-care products contain fatty acids, which can help maintain healthy skin appearance and function. Fatty acids, particularly omega-3 fatty acids, are also commonly sold as dietary supplements.
Why are fatty acids are insoluble in water in term of the structure of the fatty acids?
The only molecules that are soluble in water are those that are polar. Also, ionic compounds are soluble in water, but they are not true molecules. They are insoluble in water because they are composed primarily of long chains of hydrocarbons. Fatty acids consist of long, unbranched hydrocarbons with a carboxylic acid group at one end. The number of carbon atoms in a fatty acid molecule is usually even (6, 8, 12, 32, 36, etc.), although it is not impossible to find a fatty acid with an odd number of carbon atoms in its structure. While the long, hydrocarbon chain of the fatty acid continues to be strongly hydrophobic, the presence of the carboxylic acid group at one end of the molecule adds some hydrophilic properties. Small fatty acids such as propionic acid (with 3 carbon atoms) mixes with water readily, caproic acid (with 6 carbon atoms) is only 0.4 percent soluble in water. When fatty acids or other nonpolar molecules are put into water, the water repels them because there are no parts of the molecule that have charge, thus no attraction.
Fats and oils store energy
Animals and plants use fats and oils to store energy. As a general rule, fats come from animals and oils come from plants. Because of slight differences in structure, fats are solid at room temperature and oils are liquid at room temperature. However, both fats and oils are called triglycerides because they have three fatty acid chains attached to a glycerol molecule, as shown in Figure 3.
The carbon-hydrogen bonds (abbreviated C-H) found in the long tails of fatty acids are high-energy bonds. Thus, triglycerides make excellent storage forms of energy because they pack many high-energy C-H bonds into a compact structure of three tightly packed fatty acid tails. For this reason, dietary fats and oils are considered "calorie dense." When animals, including humans, consume fats and oils, a relatively small volume can deliver a large number of calories. Animals, particularly carnivores, are drawn to high-fat foods for their high caloric content.
Triglycerides are formed inside plant and animal cells by attaching fatty acids to glycerol molecules, creating an ester linkage. This reaction is called a dehydration synthesis because a water molecule is formed by "pulling out" two hydrogen atoms and an oxygen from the reactants. Because a new water molecule is formed, this new reaction is also called a condensation reaction (see Figure 4).
Figure 4: The dehydration synthesis reaction, where a water molecule is formed by "pulling out" two hydrogen atoms and one oxygen atom.
Fats that we eat are calorie-dense because
Structure of fatty acids
The reason why fats are solid at room temperature while oils are liquid has to do with the shape of the fatty acids these triglycerides contain. Remember that the fatty acids are long chains of carbon molecules that have hydrogen atoms attached. The C-H bonds are where energy is stored. At one end of the tail, fatty acids have a carboxyl group (-COOH), which gives the molecule its acidic properties (Figure 5).
Figure 5: The essential features of a fatty acid showing the long hydrocarbon chain and the carboxylic acid group.
If a fatty acid looks like the molecule above, with only single bonds between the carbons, we say that this fatty acid is saturated. This term is used because every single carbon is surrounded by as many hydrogen atoms as is possible it is saturated with hydrogen.
However, some fatty acids have a double bond between two of the carbons in the chain. Wherever this double bond exists, abbreviated C=C, both of the carbons involved in this double bond have one less hydrogen than the other carbons. This is because carbon can only normally make four bonds. When two carbons form a second bond in between them, they each must "let go" of a hydrogen so that the total number of bonds for each carbon is still four. Because these fatty acids have two fewer hydrogen atoms than they otherwise would have, we call them unsaturated fatty acids (Figure 6). They are unsaturated because they do not contain the maximum number of hydrogen atoms that they could have.
Figure 6: A mono-unsaturated fatty acid.
When a fatty acid has a double bond in its chain, the chain has a "kink" in its shape because there is no free-rotation around a C=C double bond. The kink is "fixed" in the structure of the fatty acid. In contrast, saturated fatty acids have free rotation around all of the single bonds in the chain since saturated fatty acids are long and straight. A comparison is shown in Figure 7.
The kinks found in unsaturated fatty acids make it so that many chains cannot pack together very tightly. Instead, the kinks force the fatty acids to push further apart. For this reason, triglycerides with unsaturated fatty acids are liquid at room temperature. Instead of packing together tightly, the molecules can slide past each other easily. The opposite is true for triglycerides with saturated fatty acids. Because their fatty acid tails are straight with no kinks, they can pack together very tightly. Thus, these molecules are more dense and solid at room temperature.
Figure 7: A comparison of a saturated fatty acid (stearic acid, found in butter) and an unsaturated fatty acid (linoleic acid, found in vegetable oil).
Animal fats are often saturated, which explains why lard, bacon fat, and butter are all solid at room temperature. Plant triglycerides, on the other hand, are typically unsaturated. This is why vegetable oils (such as canola, olive, peanut, etc.) are liquid at room temperature. Most often, unsaturated fats have only one C=C double bond and are thus called monounsaturated. However, some plants make triglycerides with multiple C=C bonds. These kinds of triglycerides are called polyunsaturated. (See Figure 8.)
Figure 8: A comparison of the bonds in a monounsaturated fatty acid (oleic acid) and a polyunsaturated fatty acid (linoleic acid).
Monounsaturated fats appear to be the healthiest triglycerides for humans to consume in their diets because the cells that remove fats from our blood after they are absorbed from our diet do their work most quickly with monounsaturated fats. Because we are slower to remove them from our blood, saturated fats stay in our bloodstream longer and thus have a greater chance to contributing to the formation of plaques and clots. For this reason, doctors and dieticians recommend diets high in monounsaturated fats and low in saturated fats. Polyunsaturated fats are somewhere in between saturated and monounsaturated fats in terms of their healthiness in our diet (Mattson & Grundy, 1985).
Saturated fatty acids have ___________ hydrogen atoms than unsaturated fatty acids.
Another type of fatty acid that has gotten a lot of attention recently is the trans fatty acid. Trans fatty acids have a hydrocarbon tail with a double bond that is in the trans configuration, instead of the more common cis configuration (see Figure 9).
Figure 9: A comparison of the cis double-bond configuration and the trans double-bond configuration.
As discussed above, C=C double bonds are present in the fatty acid tails of unsaturated fats. When these unsaturated fatty acids are made naturally by living cells, most often plant cells, the C=C double bonds are always in the cis configuration, almost never in the trans configuration. However, during industrial production of certain fat-containing products, the trans configuration can be inadvertently formed. This occurs when unsaturated fats, usually vegetable oils, are subjected to the process of hydrogenation in order to turn them into saturated fats (shown in Figure 10).
Figure 10: Unsaturated fats, usually vegetable oils, are subjected to the process of hydrogenation in order to turn them into saturated fats.
The purpose of industrial hydrogenation is to create solid fats, which are more desirable for deep-frying, out of vegetable oils. This is done because vegetable oils are much less expensive than naturally saturated fats such as lard. Crisco™ and margarine are two such chemically-produced saturated fats that are made of hydrogenated vegetable oils. Crisco™, or shortening, is cheaper than lard but can be used similarly and gives similar taste. Margarine, or oleo, was developed as a cheaper substitute for butter, particularly during the era of the World Wars and global depressions that marked the first half of the 20 th century, when rationing and scarcity of staples was common. Today, many packaged desserts and candies also have these kinds of industrially produced saturated fats, which often cost less than natural saturated fats but provide better texture and firmness than unsaturated fats. During hydrogenation, occasionally the chemical reaction does not go to completion and the process of turning a cis unsaturated fat into a saturated fat creates a trans fat instead.
In recent years, trans fats have received a lot of attention from dieticians and the general public because of their association with elevated health risks. Individuals with diets higher in trans fats are more likely to develop coronary heart disease, suffer heart attacks and stroke, and die earlier than those with diets low in trans fats (Mensink & Katan, 1990). It was always known that hydrogenation produces some trans fats, but because they are not acutely toxic, their long-term health dangers are only now being realized.
Scientists have discovered the reason for these elevated risks: Trans fats spend a much longer amount of time in our bloodstream after we consume them, instead of being quickly absorbed into our cells. Unlike saturated fats and cis unsaturated fats, trans fats don't appear in nature in very large amounts – they are an "unnatural" form of fat which humans are not well designed to consume. Because humans only began to eat trans fats in the 20 th century (other than the very tiny amounts that are present in some forms of red meat), we do not have receptor molecules in our blood vessels that seek out these trans fats and remove them from the bloodstream. Thus, when we consume trans fats, they persist in our bloodstream for a very long time, compared to natural forms of fat. The longer these molecules spend in our bloodstream, the more they can contribute to the formation of clots, plaques, and hardened arteries. For this reason, the United States Food and Drug Administration has recently made a preliminary determination that trans fats are “not generally recognized as safe,” a determination that will likely lead to a complete ban on their presence in foodstuffs (Brownell & Pomeranz, 2014).
Trans fats are "not generally recognized as safe" because
Lipid Intake: National Surveys
Actual intakes of various lipids have been estimated in national surveys, but the different surveys fail to agree on trends in actual consumption of fat. Data from the USDA's Nationwide Food Consumption Surveys (NFCS) of 1955, 1965, and 1977-1978 show little change in fat levels used by households, but mean individual intakes were lower during 1977-1978 than in 1965 (USDA, 1984). Furthermore, compared with 1977-1978, a decline in fat intake was indicated in the 1985 and 1986 USDA Continuing Survey of Food Intakes of Individuals (CSFII) (USDA 1986, 1987). On the other hand, data from the National Health and Nutrition Examination Surveys (NHANES) do not support a decline in fat intake. For example, data from the first and second NHANES (19711974 and 1976-1980, respectively) indicate that for women 19 to 50 years of age, mean fat intakes remained stable during the 1970s and early 1980s (see Table 3-4). Systematic biases due to methods used in the surveys appear to explain these differences in estimates of fat intake. For example, in the 1985 and 1986 CSFIIs, interviewers tried to determine whether or not fat was trimmed from meat and the skin removed from poultry before these foods were consumed, but this was not done in the 1977-1978 CSFII.
The 1985 and 1986 CSFIIs indicated that fat provided an average of 36 to 37% of total calories for men and women and 34% for children. The 1977-1978 NFCS reported an average of 41% of total calories as fat for women in this age group, but as noted above, the results of this survey were higher than those of other surveys.
On the basis of 4 nonconsecutive days of intake by women and their children, and on 1 day of intake by men, men and women 19 to 50 years of age consumed a mean of 13% of total calories from SFAs, 14% from MUFAs, and 7% from PUFAs. Children 1 to 5 years old consumed a mean of 14% of calories as SFAs, 13% as MUFAs, and 6% as PUFAs (USDA, 1986, 1987).
The daily intake of cholesterol averaged 280 mg for women 19 to 50 years old (187 mg/1,000 kcal) and 223 mg for children 1 to 5 years old (156 mg/ 1,000 kcal) (USDA, 1987). Intakes for men 19 to 50 years old averaged 439 mg/day (180 mg/1,000 kcal) (USDA, 1986). Cholesterol intake was higher in low-income groups than in high-income groups black women had higher intakes than white women, but white men had higher intakes than other men.
In the 1977-1978 NFCS, people from infancy to 75 years of age and older averaged 385 mg of cholesterol per day (USDA, 1984). Dietary cholesterol levels, in absolute amounts and in mg/ 1,000 kcal, were higher for blacks, for those below the poverty level, for those living in the South and West, and for those living in inner cities.
In the 1985 CSFII, dietary cholesterol came chiefly from meat (48% for men and 45% for women). Eggs provided 18% of the cholesterol intake for men and 15% for women, and grain products furnished 17% of the cholesterol intake for women and 14% for men, but these figures are somewhat misleading in that grain products furnished cholesterol only because they contained milk, butter, and eggs. The milk group provided 14% of the cholesterol intake for men and 16% for women.
Functions of Phospholipids
As membrane components, phospholipids are selectively permeable (also called semi-permeable), meaning that only certain molecules can pass through them to enter or exit the cell. Molecules that dissolve in fat can pass through easily, while molecules that dissolve in water cannot. Oxygen, carbon dioxide, and urea are some molecules that can pass through the cell membrane easily. Large molecules like glucose or ions like sodium and potassium cannot pass through easily. This helps keep the contents of the cell working properly and separates the inside of the cell from the surrounding environment.
Phospholipids can be broken down in the cell and used for energy. They can also be split into smaller molecules called chemokines, which regulate a variety of activities in the cell such as production of certain proteins and migration of cells to different areas of the body. Additionally, they are found in areas such as the lung and in joints, where they help lubricate cells.
In pharmaceuticals, phospholipids are used as part of drug delivery systems, which are systems that help transport a drug throughout the body to the area that it is meant to affect. They have high bioavailability, meaning that they are easy for the body to absorb. Valium is an example of a medication that uses a phospholipid-based drug delivery system.
The brain is highly enriched with fatty acids. These include the polyunsaturated fatty acids (PUFAs) arachidonic acid and docosahexaenoic acid, which are largely esterified to the phospholipid cell membrane. Once PUFAs are released from the membrane, they can participate in signal transduction, either directly or after enzymatic conversion to a variety of bioactive derivatives ('mediators'). PUFAs and their mediators regulate several processes within the brain, such as neurotransmission, cell survival and neuroinflammation, and thereby mood and cognition. PUFA levels and the signalling pathways that they regulate are altered in various neurological disorders, including Alzheimer's disease and major depression. Diet and drugs targeting PUFAs may lead to novel therapeutic approaches for the prevention and treatment of brain disorders.