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 of 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 the desired texture was obtained. By reducing the double bonds in an unsaturated fatty acid mixture, the melting point can also be altered. 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. At carbon 17 ring D both sterols as stanols have attached an aliphatic group, consisting in a linear structure of 8, 9 or 10 carbon atoms, depending on whether the sterol is from animal origin 8 carbon atoms or from vegetable origin 9 or 10 carbon atoms [ 59 ].
Figure 9 shows the structure of cyclopentanoperhydrophenanthrene and cholesterol. Often sterols, and less frequent stanols, have esterified the hydroxyl group of carbon 3 ring A with a saturated fatty acid usually palmitic; C or unsaturated fatty acid most frequent oleic; C and less frequent linoleic acid; C The esterification of the hydroxyl group eliminates the anphipaticity of the molecule and converts it into a structure completely non-polar.
Undoubtedly among sterols cholesterol is the most important because it is the precursor of important animal metabolic molecules, such as steroid hormones, bile salts, vitamin D, and oxysterols, which are oxidized derivatives of cholesterol formed by the thermal manipulation of cholesterol and that have been identified as regulators of the metabolism and homeostasis of cholesterol and sterols in general [ 60 ]. Simulation how the structural differences of the fatty acids which comprise phospholipids may affect the physical and chemical behavior of a membrane.
Structure of cyclopentanoperhydrophenanthrene and cholesterol. Lipids are a large and wide group of molecules that are present in all living organism and also in foods and characterized by particular physicochemical properties, such as their non polarity and their solubility in organic solvents. Some lipids, in particular fatty acids and sterols, are essential for animal and plant life. Lipids are key elements in the structure, biochemistry, physiology, and nutritional status of an individual, because are involved in: i the cellular structure; ii the cellular energy reserve, iii the formation of regulatory metabolites, and; iv in the regulation and gene expression, which directly affects the functioning of the body.
Structural and functional characteristics of lipids, discussed in this chapter, will allow you to integrate those metabolic aspects of these important and essential molecules in close relationship of how foods containing these molecules can have a relevant influence in the health or illness of an individual.
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The polar head groups of the phospholipids are represented in red, and their non-polar fatty acid tails are shown as zig-zag lines extending from the polar head group. As we we see in greater detail, cell membranes consist of a bilayer of phospholipids with other molecules inserted into the bilayer. This illustration shows five cholesterol molecules the black structures with four conjoined rings inserted into the lipid bilayer.
Most of the cholesterol molecule in non-polar and therefore associations with the non-polar fatty acid tails of the phospholipids. However, the hydroxyl group -OH on cholesterol carries a negative charge and therefore associates with the polar environment of water either inside the cell or outside.
All Rights Reserved. Date last modified: March 22, Created by Wayne W. Basic Cell Biology. Table of Contents All Modules. Lipids Lipids are a family of compounds whose diversity is also made possible by building complex molecules from multiple units of simpler molecules, and once again one sees characteristic rings and chains. Polar and Non-polar Molecules You have probably heard the expression "oil and water don't mix," and you have observed how salad dressing composed of vinegar which is aqueous, i.
Fatty Acids Fatty acids are chain-like molecules that are important components of several types of lipids. Triglycerides A fat molecule is a type of lipid that consists of three fatty acid molecules connected to a 3 carbon glycerol backbone, as shown on the right. The bonds directed below the rings also alternate in a complementary fashion. Conformational descriptions of cis- decalin are complicated by the fact that two energetically equivalent fusions of chair cyclohexanes are possible, and are in rapid equilibrium as the rings flip from one chair conformation to the other.
In each of these all chair conformations the rings are fused by one axial and one equatorial bond, and the overall structure is bent at the ring fusion. In the conformer on the left, the red ring B is attached to the blue ring A by an axial bond to C-1 and an equatorial bond to C-6 these terms refer to ring A substituents.
In the conformer on the right, the carbon bond to C-1 is equatorial and the bond to C-6 is axial. Each of the angular hydrogens H ae or H ea is oriented axial to one of the rings and equatorial to the other. This relationship reverses when double ring flipping converts one cis-conformer into the other. Cis-decalin is less stable than trans-decalin by about 2. This is due to steric crowding hindrance of the axial hydrogens in the concave region of both cis-conformers, as may be seen in the model display activated by the following button.
This difference is roughly three times the energy of a gauche butane conformer relative to its anti conformer. Indeed three gauche butane interactions may be identified in each of the cis-decalin conformations, as will be displayed by clicking on the above conformational diagram.
These gauche interactions are also shown in the model. Steroids in which rings A and B are fused cis, such as the example on the right, do not have the same conformational mobility exhibited by cis-decalin. The fusion of ring C to ring B in a trans configuration prevents ring B from undergoing a conformational flip to another chair form. This is too great a distance to be bridged by the four carbon atoms making up ring C. Consequently, the steroid molecule is locked in the all chair conformation shown here.
Of course, all these steroids and decalins may have one or more six-membered rings in a boat conformation. However the high energy of boat conformers relative to chairs would make such structures minor components in the overall ensemble of conformations available to these molecules. The essential dietary substances called vitamins are commonly classified as "water soluble" or "fat soluble". Water soluble vitamins, such as vitamin C, are rapidly eliminated from the body and their dietary levels need to be relatively high.
The recommended daily allotment RDA of vitamin C is mg, and amounts as large as 2 to 3 g are taken by many people without adverse effects. The lipid soluble vitamins, shown in the diagram below, are not as easily eliminated and may accumulate to toxic levels if consumed in large quantity. The RDA for these vitamins are:. From this data it is clear that vitamins A and D, while essential to good health in proper amounts, can be very toxic. Vitamin D, for example, is used as a rat poison, and in equal weight is more than times as poisonous as sodium cyanide.
From the structures shown here, it should be clear that these compounds have more than a solubility connection with lipids. Vitamins A is a terpene, and vitamins E and K have long terpene chains attached to an aromatic moiety. The structure of vitamin D can be described as a steroid in which ring B is cut open and the remaining three rings remain unchanged.
The precursors of vitamins A and D have been identified as the tetraterpene beta-carotene and the steroid ergosterol, respectively. Structures for these will be displayed by clicking on the vitamin diagram above.
The complex organic compounds found in living organisms on this planet originate from photosynthesis, an endothermic reductive condensation of carbon dioxide requiring light energy and the pigment chlorophyll.
The products of photosynthesis are a class of compounds called carbohydrates, the most common and important of which is glucose C 6 H 12 O 6.
Subsequent reactions effect an oxidative cleavage of glucose to pyruvic acid CH 3 COCO 2 H , and this in turn is transformed to the two-carbon building block, acetate. The multitude of lipid structures described here are constructed from acetate by enzymatic reactions that in many respects correspond to reactions used by chemists for laboratory syntheses of similar compounds.
However, an important restriction is that the reagents and conditions must be compatible with the aqueous medium, neutral pH and moderate temperatures found in living cells. Consequently, the condensation, alkylation, oxidation and reduction reactions that accomplish the biosynthesis of lipids will not make use of the very strong bases, alkyl halides, chromate oxidants or metal hydride reducing agents that are employed in laboratory work.
Claisen condensation of ethyl acetate or other acetate esters forms an acetoacetate ester, as illustrated by the top equation in the following diagram. Reduction, dehydration and further reduction of this product would yield an ester of butyric acid, the overall effect being the elongation of the acetate starting material by two carbons.
In principle, repetition of this sequence would lead to longer chain acids, made up of an even number of carbon atoms. Since most of the common natural fatty acids have even numbers of carbon atoms, this is an attractive hypothesis for their biosynthesis.
Nature's solution to carrying out a Claisen-like condensation in a living cell is shown in the bottom equation of the diagram. Thioesters are more reactive as acceptor reactants than are ordinary esters, and preliminary conversion of acetate to malonate increases the donor reactivity of this species.
The thiol portion of the thioester is usually a protein of some kind, with efficient acetyl transport occurring by way of acetyl coenzyme A. Depending on the enzymes involved, the condensation product may be reduced and then further elongated so as to produce fatty acids as shown , or elongated by further condensations to polyketone intermediates that are precursors to a variety of natural phenolic compounds.
Click on the diagram to see examples of polyketone condensations. The reduction steps designated by [H] in the equations and the intervening dehydrations needed for fatty acid synthesis require unique coenzymes and phosphorylating reagents.
Partial structures for these important redox reagents are shown on the right. Full structures may be seen by clicking on the partial formulas. As noted earlier , the hydroxyl group is a poor anionic leaving group hydroxide anion is a strong base. Phosphorylation converts a hydroxyl group into a phosphate PO 4 or pyrophosphate P 2 O 7 ester, making it a much better leaving group the pK a s at pH near 7 are 7.
The chief biological phosphorylation reagents are phosphate derivatives of adenosine a ribose compound. The strongest of these is the triphosphate ATP, with the diphosphate and monophosphate being less powerful. The branched chain and cyclic structures of the terpenes and steroids are constructed by sequential alkylation reactions of unsaturated isopentyl pyrophosphate units.
As depicted in the following diagram, these 5-carbon reactants are made from three acetate units by way of an aldol-like addition of a malonate intermediate to acetoacetate. Selective hydrolysis and reduction gives a key intermediate called mevalonic acid. Phosphorylation and elimination of mevalonic acid then generate isopentenyl pyrophosphate, which is in equilibrium with its double bond isomer, dimethylallyl pyrophosphate.
The allylic pyrophosphate group in the latter compound is reactive in enzymatically catalyzed alkylation reactions, such as the one drawn in the green box. This provides support for the empirical isoprene rule. The simplest fashion in which isopentane units combine is termed "head-to-tail". This is the combination displayed in the green box, and these terms are further defined in the upper equation that will appear above on clicking the " Toggle Examples " button.
Non head-to-tail coupling of isopentane units is also observed, as in the chrysanthemic acid construction shown in the second equation. A second click on the diagram displays the series of cation-like cyclizations and rearrangements, known as the Stork - Eschenmoser hypothesis, that have been identified in the biosynthesis of the triterpene lanosterol. Lanosterol is a precursor in the biosynthesis of steroids.
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