Life, in its origin, would not have been possible without the appearance of phospholipids, as these structures are the fundamental components of all cellular membranes. Phospholipids have structural and functional properties that distinguish them from their counterparts, triacylglycerides. In phospholipids positions sn-1 and sn-2 of the glycerol moiety are occupied by fatty acids, more frequently polyunsaturated fatty acids, linked to glycerol by ester bonds.
The sn-3 position of glycerol is linked to orthophosphoric acid [ 50 ]. The structure which is formed, independent of the type of fatty acid that binds at sn-1 and sn-2, is called phosphatidic acid. The presence of phosphate substituent at the sn-3 position of the glycerol gives a great polarity to this part of the molecule, being non-polar the rest of the structure, such as in triacylglycerides. This double feature, a polar extreme and a non-polar domain due the presence of the two fatty acids characterizes phospholipids as amphipathic molecules amphi : both; pathos : sensation [ 51 ].
Positional and geometric isomers of unsaturated fatty acids. The structure of phospholipids is usually simplified representing the polar end as a sphere and the fatty acids as two parallel rods. Figure 6 shows the chemical structure of phosphatidic acid in its simplified representation. The amphipathic character of phosphatidic acid can be increased by joining to the phosphate different basic and polar molecules that increases the polarity to the extreme of the sn-3 position.
When the substituent of the phosphate group is the aminoacid serine it is formed phosphatidylserine; when it is etanolamine it is formed phosphatidylethanolamine frequently known as cephalin ; when choline is the substituent it is formed phosphatidylcholine well known as lecithin ; and when the substituent is the polyalcohol inositol it is formed phosphatidylinositol, a very important molecule involved in cell signaling. Chemical structure of phosphatidic acid and its simplified representation.
These more complex phospholipids are much more common than phosphatidic acid, since this is only the structural precursor of the above molecules. Figure 7 shows the structure of various phospholipids.
A number of other molecules are also classified as phospholipids, but are structurally different. Cardiolipin is a "double" phospholipid in which two phosphatidic acid molecules are attached through their phosphates by a molecule of glycerol. Cardiolipin is a very important in the structure of the inner membrane of mitochondria and due their molecular volume it is the only immunogenic phospholipid which stimulates the formation of antibodies [ 53 ]. Plasmalogens are other lipid molecules related to phospholipids.
In these molecules the substituent at sn-1 position of the glycerol is not a fatty acid, but a fatty alcohol which is linked to glycerol by an ether linkage. Phosphatidalethanolamine different than phosphatidylethanolamine is an abundant plasmalogen in the nervous tissue [ 54 ].
Phosphatidalcholine, the plasmalogen related to phosphatidylcholine, is abundant in the heart muscle. Another structures related to phospholipids are sphingolipids.
In these structures glycerol is replaced by the amino alcohol; sphingosine. When the hydroxyl group alcoholic group of sphingosine is substituted by phosphocholine, it is formed sphingomyelin, which is the only sphingolipid that is present in significant amount in human tissues as a constituent of myelin that forms nerve fibers [ref].
Platelet activating factor PAF is an unusual glycerophospholipid structure. In this molecule position sn-1 of glycerol is linked to a saturated alcohol through an ether bond such as in plasmalogens and at the sn-2 binds an acetyl group instead of a fatty acid. PAF is released by a variety of cells and by binding to membrane receptors produces aggregation and degranulation of platelets, has potent thrombotic and inflammatory effects, and is a mediator of anaphylactic reactions [ 55 ].
Structure of various phospholipids. A fundamental aspect of phospholipids is their participation in the structure of biological membranes, and the structural characteristics of the fatty acids are relevant to determine the behavior and the biological properties of the membrane. As an example, a diet rich in saturated fatty acids result in an increase in the levels of these fatty acids into cell membrane phospholipids, causing a significant decrease in both, membrane fluidity and in the ability of these structure to incorporate ion channels, receptors, enzymes, structural proteins, etc.
At the nutritional and metabolic level this effect is highly relevant because as the fatty acid composition of the diet is directly reflected into the fatty acid composition of phospholipids, changes in the composition of the diet, i.
Figure 8 shows a simulation how the structural differences of the fatty acids which comprise phospholipids may affect the physical and chemical behavior of a membrane. Sterols are derived from a common structural precursor, the sterane or cyclopentanoperhydrophenanthrene, consisting in a main structure formed by four aromatic rings identified as A, B, C and D rings.
All sterols have at carbon 3 of A ring a polar hydroxyl group being the rest of the structure non-polar, which gives them certain amphipathic character, such as phospholipids. Sterols have also a double bound at carbons 5 and 6 of ring B [ 58 ]. This double bond can be saturated reduced which leads to the formation of stanols, which together with plant sterols derivatives are currently used as hypocholesterolemic agents when incorporated into some functional foods.
On the other hand, if the fatty acid contains one or more double bonded carbon atoms, the fatty acid is called unsaturated fatty acid. Fats that contain all saturated fatty acids are called saturated fats. Fats obtained from animal sources, for instance, butter, milk, cheese, and lard, are mostly saturated. Fats from fish or plant sources are often unsaturated, like olive oil, peanut oil, and cod liver oil. The absence of double bonds in the hydrocarbon chains of saturated fatty acids, making them flexible.
The flexible fatty acid chains can pack tightly with each other; hence saturated fats are mostly solid at room temperature.
The presence of cis -double bonds causes a bend in the hydrocarbon chain which makes the long hydrocarbon chain less flexible and difficult to pack. As a consequence, most unsaturated fatty acids are liquid at room temperature. Fats are a long-term energy reservoir in many organisms. If the need arises, the organism breaks down fats to produce energy.
In animals, fat provides cushioning around vital organs, and a subcutaneous layer of fat insulates the body from external temperatures. Phospholipids are critical to the cell as they are major constituents of cell membranes.
Phospholipids are structurally similar to fats but contain only two fatty acids linked to glycerol instead of three. The fatty acid residues can be saturated or unsaturated. In phospholipids, the third hydroxyl group of glycerol is linked to a negatively charged phosphate group. An additional functional group attached to the phosphate group can lead to diverse chemical properties of phospholipids. Most common additives are small polar groups like choline or serine.
Phospholipids are amphipathic molecules, meaning they have parts that are hydrophobic and others that are hydrophilic , or water-loving.
When phospholipids are added to water, they spontaneously form a bilayer, a thin film that is two phospholipid molecules thick. This self-organization takes place because the polar heads are attracted to water, while the hydrophobic fatty acids are buried in the center of the layer to evade contact with water. Such phospholipid bilayer forms the cell membrane in all living organisms. It compartmentalizes the fluids on the interior and exterior of the cell. Embedded in the bilayer are proteins and steroids, another class of lipids.
Additional phospholipid bilayers may further compartmentalize the interior of the eukaryotic cell, for instance, the lysosome and endoplasmic reticulum. Steroids are another biologically important class of lipids. Steroids are composed of four carbon rings that are fused to each other. Steroids vary amongst each other based on the chemical groups attached to the carbon rings.
Although steroids are structurally different, they are hydrophobic and insoluble in water. Steroids reduce the fluidity of the cell membrane. They also function as signaling molecules within the cell. Cholesterol is the most common steroid and is synthesized by the liver. It is present in the cell membrane and is a precursor of sex hormones in animals.
Muro, Eleonora, G. Ekin Atilla-Gokcumen, and Ulrike S. Simons, Kai. Micelles are lipid molecules that arrange themselves in a spherical form in aqueous solution. The formation of a micelle is a response to the amphipathic nature of fatty acids, meaning that they contain both hydrophilic and hydrophobic regions. Steroids, like cholesterol, play roles in reproduction, absorption, metabolism regulation, and brain activity.
Unlike phospholipids and fats, steroids have a fused ring structure. Although they do not resemble the other lipids, they are grouped with them because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings, and many of them, like cholesterol, have a short tail. Many steroids also have the —OH functional group, and these steroids are classified as alcohols called sterols.
Steroid Structures : Steroids, such as cholesterol and cortisol, are composed of four fused hydrocarbon rings. Cholesterol is the most common steroid and is mainly synthesized in the liver; it is the precursor to vitamin D. Cholesterol is also a precursor to many important steroid hormones like estrogen, testosterone, and progesterone, which are secreted by the gonads and endocrine glands.
Cholesterol also plays a role in synthesizing the steroid hormones aldosterone, which is used for osmoregulation, and cortisol, which plays a role in metabolism. Cholesterol is also the precursor to bile salts, which help in the emulsification of fats and their absorption by cells. It is a component of the plasma membrane of animal cells and the phospholipid bilayer. Being the outermost structure in animal cells, the plasma membrane is responsible for the transport of materials and cellular recognition; and it is involved in cell-to-cell communication.
Thus, steroids also play an important role in the structure and function of membranes. It has also been discovered that steroids can be active in the brain where they affect the nervous system, These neurosteroids alter electrical activity in the brain. They can either activate or tone down receptors that communicate messages from neurotransmitters.
Since these neurosteroids can tone down receptors and decrease brain activity, steroids are often used in anesthetic medicines. Privacy Policy. Skip to main content. Search for:. Lipids Lipid Molecules Fats and oils, which may be saturated or unsaturated, can be unhealthy but also serve important functions for plants and animals.
Learning Objectives Differentiate between saturated and unsaturated fatty acids. Key Takeaways Key Points Fats provide energy, insulation, and storage of fatty acids for many organisms. Fats may be saturated having single bonds or unsaturated having double bonds.
Unsaturated fats may be cis hydrogens in same plane or trans hydrogens in two different planes. Olive oil, a monounsaturated fat, has a single double bond whereas canola oil, a polyunsaturated fat, has more than one double bond.
Omega-3 fatty acid and omega-6 fatty acid are essential for human biological processes, but they must be ingested in the diet because they cannot be synthesized. Key Terms hydrogenation : The chemical reaction of hydrogen with another substance, especially with an unsaturated organic compound, and usually under the influence of temperature, pressure and catalysts. It contains the functional group carbon-oxygen double bond joined via carbon to another oxygen atom.
OH ; characteristic of carboxylic acids. Fats also increase the bioavailability of compounds known as phytochemicals, which are plant constituents such as lycopene found in tomatoes and beta-carotene found in carrots.
Phytochemicals are believed to promote health and well-being. As a result, eating tomatoes with olive oil or salad dressing will facilitate lycopene absorption. Other essential nutrients, such as essential fatty acids, are constituents of the fats themselves and serve as building blocks of a cell.
When products such as grain and dairy are processed, these essential nutrients are lost. Manufacturers replace these nutrients through a process called enrichment. Remember, fat-soluble nutrients require fat for effective absorption. For your next snack, look for foods that contain vitamins A, D, E, and K.
Do these foods also contain fat that will help you absorb them? If not, think of ways to add a bit of healthy fat to aid in their absorption. Fat-rich foods naturally have a high caloric density. Foods that are high in fat contain more calories than foods high in protein or carbohydrates.
As a result, high-fat foods are a convenient source of energy. For example, 1 gram of fat or oil provides 9 kilocalories of energy, compared with 4 kilocalories found in 1 gram of carbohydrate or protein.
Depending on the level of physical activity and on nutritional needs, fat requirements vary greatly from person to person. When energy needs are high, the body welcomes the high-caloric density of fats. For instance, infants and growing children require proper amounts of fat to support normal growth and development. If an infant or child is given a low-fat diet for an extended period, growth and development will not progress normally.
Other individuals with high-energy needs are athletes, people who have physically demanding jobs, and those recuperating from illness.
When the body has used all of its calories from carbohydrates this can occur after just twenty minutes of exercise , it initiates fat usage. A professional swimmer must consume large amounts of food energy to meet the demands of swimming long distances, so eating fat-rich foods makes sense. In contrast, if a person who leads a sedentary lifestyle eats the same high-density fat foods, they will intake more fat calories than their body requires within just a few bites.
Use caution—consumption of calories over and beyond energy requirements is a contributing factor to obesity. Fat contains dissolved compounds that contribute to mouth-watering aromas and flavors and increase palatability of food. Fat also adds texture to food.
Baked foods are supple and moist. Frying foods locks in flavor and lessens cooking time. How long does it take you to recall the smell of your favorite food cooking? What would a meal be without that savory aroma to delight your senses and heighten your preparedness for eating a meal?
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