Lipids are defined as water-insoluble molecules that are highly soluble in organic. 10 Lipids CHAPTER

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1 ATER 10 Lipids 10.1 Fatty Acids Are a Main Source of Fuel 10.2 Triacylglycerols Are the Storage Form of Fatty Acids 10.3 There Are Three ommon Types of Membrane Lipids live trees. The fruit of a small tree native to the eastern areas of the Mediterranean, olives are unusual in that they are an inedible fruit unless processed. live oil, which has been used in cooking since antiquity, is rich in monounsaturated fatty acids as well as antioxidants. These antioxidants may be one of the reasons for the positive health effects of the Mediterranean diet. [David ick/alamy.] 142 Lipids are defined as water-insoluble molecules that are highly soluble in organic solvents. This important class of molecules has a variety of biochemical roles. For instance, lipids are widely used to store energy. They are also key components of membranes and play a variety of roles in signal-transduction pathways. Unlike the three other classes of biomolecules (carbohydrates, amino acids, and nucleic acids), lipids do not form polymers. Their individual and collective properties as noncovalent assemblies make them extraordinarily important. We will examine five classes of lipids here. 1. Free Fatty Acids (Nonesterified Fatty Acids). This simplest type of lipid is most commonly used as a fuel. 2. Triacylglycerols. This class of lipid is the storage form of fatty acids. 3. hospholipids. These membrane lipids consist of fatty acids attached to a scaffold that also bears a charged phosphoryl group, creating a macromolecule with a polar head and nonpolar tail. 4. Glycolipids. These lipids are bound to carbohydrates and are important membrane constituents. 5. Steroids. These lipids differ from the other lipids in that they are polycyclic hydrocarbons. Steroids function as hormones that control a variety of physiological functions. The most common steroid is cholesterol, another vital membrane component.

2 10.1 Fatty Acids Are a Main Source of Fuel Fats, or fatty acids, are chains of hydrogen-bearing carbon atoms, called hydrocarbons, that terminate with carboxylic acid groups. These hydrocarbon chains are of various lengths and may have one or more double bonds, depending on the fat. Two key roles for fatty acids are as fuels and as building blocks for membrane lipids. Fats are good fuels because they are more reduced than carbohydrates; that is, the carbon atoms are bonded to hydrogen atoms and other carbon atoms rather than to oxygen atoms, as is the case for carbohydrates. Because of this greater reduction, fats yield more energy than carbohydrates when undergoing combustion to carbon dioxide and water. We will return to this discussion in hapter 14. Fats have common names and systematic names. Although we will use the common names for fatty acids, familiarity with the systematic names is important because their use is sometimes required to prevent confusion. A fatty acid s systematic name is derived from the name of its parent hydrocarbon by the substitution of oic for the final e. For example, the 18 saturated fatty acid known familiarly as stearic acid, is called octadecanoic acid because the parent hydrocarbon is octadecane: octadec- for the 18 carbon atoms, and ane because it is composed completely of single bonds. Fatty acids composed of single bonds only are called saturated fatty acids, because every carbon atom is attached to four other atoms. Fatty acids with one or more double or triple bonds are called unsaturated fatty acids. leic acid, a 18 fatty acid with one double bond, is called octadecenoic acid; whereas linoleic acid, with two double bonds, is octadecadienoic acid; and linolenic acid, with three double bonds, is octadecatrienoic acid. The composition and structure of a fatty acid can also be designated by numbers. The notation 18:0 denotes a 18 fatty acid with no double bonds, whereas 18:2 signifies that there are two double bonds. The structure of the ionized forms of two common fatty acids palmitic acid (16:0) and oleic acid (18:1) are shown in Figure When considering fatty acids, we often need to distinguish the individual carbon atoms. Fatty acid carbon atoms are numbered starting at the carboxyl terminus, as shown in the margin. arbon atoms 2 and 3 are often referred to as and, respectively. The last carbon atom in the chain, which is almost always a methyl carbon atom, is called the v-carbon atom ( is the Greek symbol for omega, the last letter of the Greek alphabet). The position of a double bond is represented by the symbol followed by a superscript number. For example, cis- 9 means that there is a cis double bond between carbon atoms 9 and 10; trans- 2 means that there is a trans double bond between carbon atoms 2 and 3. Just as in proteins, cis and trans designate the relative positions of substituents on either side of the double bond. Just like amino acids, fatty acids are ionized at physiological p, and so 3 ω 2 ω-3 double bond β 2 3 n α Fatty Acids 1 ( 2 ) n An -3 fatty acid ω-arbon atom almitate (ionized form of palmitic acid) leate (ionized form of oleic acid) Figure 10.1 Structures of two fatty acids. almitate is a 16-carbon, saturated fatty acid, and oleate is an 18-carbon fatty acid with a single cis double bond.

3 Lipids it is preferable to refer to them according to their carboxylate form: for example, palmitate rather than palmitic acid. almitate (ionized form of palmitic acid) almitic acid 2 Methylene group Fatty Acids Vary in hain Length and Degree of Unsaturation Fatty acids in biological systems usually contain an even number of carbon atoms, typically between 14 and 24 (Table 10.1), with the 16- and 18-carbon fatty acids being the most common. As heretofore described, the hydrocarbon chain may be saturated or it may contain one or more double bonds. The configuration of the double bonds in most unsaturated fatty acids is cis. The double bonds in polyunsaturated fatty acids are separated by at least one methylene group. The properties of fatty acids and of lipids derived from them are markedly dependent on chain length and degree of saturation. Unsaturated fatty acids have lower melting points than those of saturated fatty acids of the same length. For example, the melting point of stearic acid is 69.6, whereas that of oleic acid (which contains one cis double bond) is The melting points of polyunsaturated fatty acids of the 18 series are even lower. The presence of a cis double bond introduces a kink in the fatty acid and makes tight packing between the chains impossible. The lack of tight packing limits the van der Waals interactions between chains, lowering the melting temperature. Stearate Table 10.1 Some naturally occurring fatty acids in animals Number of Number of carbon atoms double bonds ommon name Systematic name Formula 12 0 Laurate n-dodecanoate 3 ( 2 ) Myristate n-tetradecanoate 3 ( 2 ) almitate n-exadecanoate 3 ( 2 ) Stearate n-ctadecanoate 3 ( 2 ) Arachidate n-eicosanoate 3 ( 2 ) Behenate n-docosanoate 3 ( 2 ) Lignocerate n-tetracosanoate 3 ( 2 ) almitoleate cis- 9 -exadecenoate 3 ( 2 ) 5 ( 2 ) leate cis- 9 -ctadecenoate 3 ( 2 ) 7 ( 2 ) Linoleate cis, cis- 9, 12-3 ( 2 ) 4 ( 2 ) 2 () 6 ctadecadienoate 18 3 Linolenate all-cis- 9, 12, ( 2 ) 3 ( 2 ) 6 ctadecatrienoate 20 4 Arachidonate all-cis 5, 8, 11, 14-3 ( 2 ) 4 ( 2 ) 4 ( 2 ) 2 Eicosatetraenoate

4 trans-leate Fatty Acids cis-leate hain length also affects the melting point, as illustrated by the fact that the melting temperature of palmitic acid ( 16 ) is 6.5 degrees lower than that of stearic acid ( 18 ). Thus, short chain length and cis unsaturation enhance the fluidity of fatty acids and of their derivatives. The fat that accumulates in the pan as bacon is fried is composed primarily of saturated fatty acids and solidifies soon after the burner is turned off. live oil, on the other hand, is composed of high concentrations of oleic acid and some polyunsaturated fatty acids and remains liquid at room temperature. The variability of melting points is not merely an arcane chemical insight. Melting temperatures of fatty acids are key elements in the control of the fluidity of cell membranes, and the proper degree of fluidity is essential for membrane function (hapter 11). QUIK QUIZ 1 What factors determine the melting point of fatty acids? The Degree and Type of Unsaturation Are Important to ealth Although fats are crucial biochemicals, too much saturated and trans-unsaturated fats in the diet are correlated with high blood levels of cholesterol and cardiovascular disease. The biochemical basis for this correlation remains to be determined, although trans-unsaturated fats appear to cause inflammation. In contrast, certain cis-polyunsaturated fatty acids are essential in our diets because we cannot synthesize them. Such fatty acids include the -3 fatty acids polyunsaturated fatty acids common in cold-water fish such as salmon, which have been suggested to play a role in protection from cardiovascular disease. The important -3 fatty acids are -linolenic acid, found in vegetable oils, eicosapentaenoic acid (EA) and docosahexaenoic acid (DA), both of which are found in fatty fish and shellfish. -Linolenate Eicosapentaenoate (EA) Docosahexaenoate (DA)

5 Lipids These fatty acids are precursors to important hormones. owever, the nature of their protective effect against heart disease is not known. Studies show that -3 fatty acids, especially those derived from marine organisms, can prevent sudden death from a heart attack, reduce triacylglycerides in the blood, lower blood pressure, and prevent thrombosis by slightly inhibiting blood clotting. Revealing the mechanisms of action of these molecules is an active area of biochemical research Triacylglycerols Are the Storage Form of Fatty Acids Despite the fact that fatty acids are our principal energy source, the concentration of free fatty acids in cells or the blood is low because free fatty acids are strong acids. igh concentrations of free fatty acids would disrupt the p balance of the cells. Fatty acids required for energy generation are stored in the form triacylglycerols, which are formed by the attachment of three fatty acid chains to a glycerol molecule. The fatty acids are attached to the glycerol through ester linkages, in a process known as esterification. ommon soaps are fatty acids generated by treating triacylglyerols with strong bases (Figure 10.2). Figure 10.2 Soaps are the sodium and potassium salts of long chain fatty acids. They are derived from the treatment of triacylglycerides with a strong base, a process called saponification. ommon sources of triacylglycerides are the animal lipids lard (from hogs) or tallow (from beef or sheep). [hotonica/getty Images.] 2 2 ( 2 ) N 3 2 ( 2 ) N ( 2 ) N 3 Glycerol backbone Three fatty acid chains Lipid droplet Nucleus Mitochondria Mitochondrion Figure 10.3 Electron micrograph of an adipocyte. A small band of cytoplasm surrounds the large deposit of triacylglycerols. [Biophoto Associates/hoto Researchers.] When energy is required during a fast (for instance, while sleeping), the fatty acids are cleaved from the triacyglycerol and carried to the cells. The ingestion of food replenishes the triacylglycerol stores. As stated in hapter 9, fatty acids are richer in energy than carbohydrates. Additionally, compared with carbohydrates, fatty acids store energy more efficiently in the form of triacylglycerols, which are hydrophobic and so are stored in a nearly anhydrous form. olar carbohydrates, in contrast, bind to water molecules. Glycogen, for instance, binds to water. In fact, 1 g of dry glycogen binds about 2 g of water. onsequently, a gram of nearly anhydrous fat stores more than six times as much energy as a gram of hydrated glycogen. For this reason, triacylglycerols rather than glycogen were selected in evolution as the major energy reservoir. onsider a typical 70-kg man. Triacylglycerols constitute about 11 kg of his total body weight. If this amount of energy were stored in glycogen, his total body weight would be 55 kg greater. The glycogen and glucose stores provide enough energy to sustain biological function for about 18 to 24 hours, whereas the triacylglycerol stores allow survival for several weeks. In mammals, the major site of accumulation of triacylglycerols is adipose tissue, which is distributed under the skin and elsewhere throughout the body. In adipose cells (fat cells), droplets of triacylglycerol coalesce to form a large globule in the cytoplasm, which may occupy most of the cell volume (Figure 10.3). Adipose cells are specialized for the synthesis and storage of triacylglycerols and for their mobilization

6 Types of Membrane Lipids Figure 10.4 Ruby-throated hummingbird. [William Leaman/Alamy.] into fuel molecules that are transported to other tissues by the blood. Adipose tissue also serves as a thermal insulator to help maintain body temperature. olar bears provide an example of the insulating properties of adipose tissue. During fierce arctic storms, polar bears curl up into tight balls and are so well insulated that they cannot be detected by heat-sensing instruments. The utility of triacylglycerols as an energy source is dramatically illustrated by the abilities of migratory birds, which can fly great distances without eating. Examples are the American golden plover and the ruby-throated hummingbird (Figure 10.4). The golden plover flies from Alaska to the southern tip of South America; a large segment of the flight (3800 km, or 2400 miles) is over open ocean, where the birds cannot feed. The ruby-throated hummingbird can fly nonstop across the Gulf of Mexico. Fatty acids derived from triacylglycerols provide the energy source for these prodigious feats There Are Three ommon Types of Membrane Lipids Thus far, we have considered only one type of complex lipid triacylglycerols, the storage form of fatty acids. owever, lipids can be used for more than just fuel and energy storage: they serve as hormones, messengers in signal-transduction pathways, and components of membranes. The first two of these functions of lipids will be considered in later chapters. We now consider the three major kinds of membrane lipids: phospholipids, glycolipids, and cholesterol. hospholipids Are the Major lass of Membrane Lipids hospholipids are abundant in all biological membranes. A phospholipid molecule is constructed from four components: one or more fatty acids, a platform to which the fatty acids are attached, a phosphate, and an alcohol attached to the phosphate (Figure 10.5). The platform on which phospholipids are built may be glycerol, a threecarbon alcohol, or sphingosine, a more complex alcohol. hospholipids derived from glycerol are called phosphoglycerides and are composed of a glycerol backbone to which are attached two fatty acid chains and a phosphorylated alcohol. In phosphoglycerides, the hydroxyl groups at -1 and -2 of glycerol are esterified to the carboxyl groups of the two fatty acid chains. The -3 hydroxyl group of the glycerol backbone is esterified to phosphoric acid. When no further additions are made, the resulting compound is phosphatidate (diacylglycerol 3-phosphate), Fatty acid Fatty acid G l y c e r o l hosphate Alcohol Figure 10.5 Schematic structure of a phospholipid. The platform is glycerol in this case.

7 Lipids Figure 10.6 Structure of phosphatidate (diacylglycerol 3-phosphate). The absolute configuration of the central carbon atom (-2) is shown. Acyl groups with fatty acid hydrocarbon chains R hosphatidate (Diacylglycerol 3-phosphate) 2 the simplest phosphoglyceride (Figure 10.6). nly small amounts of phosphatidate are present in membranes. owever, the molecule is a key intermediate in the biosynthesis of the other phosphoglycerides as well as triacylglycerides (hapter 28). The major phosphoglycerides are derived from phosphatidate by the formation of an ester linkage between the phosphoryl group of phosphatidate and the hydroxyl group of one of several alcohols. The common alcohol moieties of phosphoglycerides are the amino acid serine, ethanolamine, choline, glycerol, and inositol. The structural formulas of phosphatidylcholine and the other principal phosphoglycerides are given in Figure R N 3 R N hosphatidylserine hosphatidylcholine R hosphatidylethanolamine + N 3 2 R hosphatidylinositol R Diphosphatidylglycerol (cardiolipin) 2 R 3 R 4 Figure 10.7 Some common phosphoglycerides found in membranes. Niemannick disease can result from an accumulation of sphingomyelin owing to the lack of sphingomyelinase, an enzyme that degrades sphingomyelin. Symptoms of Niemannick disease include mental retardation, seizures, eye paralysis, ataxia, and retarded growth. hospholipids built on a sphingosine backbone are called sphingolipids. Sphingosine is an amino alcohol that contains a long, unsaturated hydrocarbon chain (Figure 10.8). Sphingomyelin is a common sphingolipid found in membranes. In sphingomyelin, the amino group of the sphingosine backbone is linked to a fatty acid by an amide bond. In addition, the primary hydroxyl group of sphingosine is attached to phosphorylcholine through an ester linkage. Sphingomyelin is found in the plasma membrane of many cells but is especially rich in the myelin sheath of nerve cells.

8 Sphingosine + 3 N Types of Membrane Lipids 3 ( 2 ) 12 N Sphingomyelin N 3 3 Figure 10.8 Structures of sphingosine and sphingomyelin. The sphingosine moiety of sphingomyelin is highlighted in blue. Membrane Lipids an Include arbohydrates Glycolipids, as their name implies, are sugar-containing lipids. Glycolipids are ubiquitous in all membranes, although their function is unknown. Like sphingomyelin, the glycolipids in animal cells are derived from sphingosine. The amino group of the sphingosine backbone is acylated by a fatty acid, as in sphingomyelin. Glycolipids differ from sphingomyelin in the identity of the unit that is linked to the primary hydroxyl group of the sphingosine backbone. In glycolipids, one or more sugars (rather than phosphorylcholine) are attached to this group. The simplest glycolipid, called a cerebroside, contains a single sugar residue, either glucose or galactose. Fatty acid unit 3 ( 2 ) 12 N erebroside (a glycolipid) 2 Sugar unit glucose or galactose More-complex glycolipids, such as gangliosides, contain a branched chain of as many as seven sugar residues. Glycolipids are oriented in an asymmetric fashion in membranes with the sugar residues always on the extracellular side of the membrane. Steroids Are Lipids That ave a Variety of Roles Steroids, the final class of lipids to be considered in this chapter, function as hormones, facilitate the digestion of lipids in the diet, and are key membrane constituents. Unlike the other classes of lipids, steroids take on a cyclical rather than linear structure. All steroids have a tetracyclic ring structure called the steroid nucleus. The steroid nucleus consists of three cyclohexane rings and a cyclopentane ring joined together. Steroid nucleus All biochemically important steroids are modified versions of this basic structure.

9 Lipids The most common steroid, and the precursor to many biochemically active steroids, is cholesterol holesterol A hydrocarbon tail is linked to the steroid at one end, and a hydroxyl group is attached at the other end. As we will see in hapter 11, cholesterol is important in maintaining proper membrane fluidity. holesterol is absent from prokaryotes but is found to varying degrees in virtually all animal membranes. It constitutes almost 25% of the membrane lipids in certain nerve cells but is essentially absent from some intracellular membranes. Free cholesterol does not exist outside of membranes. Rather, it is esterified to a fatty acid for storage and transport. Biological Insight Membranes of Extremeophiles Are Built from Ether Lipids with Branched hains The membranes of archaea differ in composition from those of eukaryotes or bacteria in two important ways. These differences clearly relate to the hostile living conditions of many archaea. First, the fatty acid chains are joined to a glycerol backbone by ether rather than ester linkages. The ether linkage is more resistant to hydrolysis than the ester linkage is. Second, the alkyl chains are branched rather than linear. They are built up from repeats of a fully saturated five-carbon fragment. These branched, saturated hydrocarbons are more resistant to oxidation N Membrane lipid from the archaeon Methanococcus jannaschii The ability of these lipids to resist hydrolysis and oxidation may help these organisms to withstand the extreme conditions, such as high temperature, low p, or high salt concentration, under which some of these archaea grow. Membrane Lipids ontain a ydrophilic and a ydrophobic Moiety The repertoire of membrane lipids is extensive. owever, these lipids possess a critical common structural theme: membrane lipids are amphipathic molecules (amphiphilic molecules) containing both a hydrophilic and a hydrophobic moiety. Let us look at a model of a phosphoglyceride, such as phosphatidylcholine. Its overall shape is roughly rectangular (Figure 10.9). The two hydrophobic fatty acid chains are approximately parallel to each other, whereas the hydrophilic phosphorylcholine moiety points in the opposite direction. Sphingomyelin has a similar conformation, as does the archaeal lipid depicted. Therefore, the following shorthand has been adopted to represent these membrane lipids: the hydrophilic unit,

10 (A) Types of Membrane Lipids hosphoglyceride Sphingomyelin Archaeal lipid (B) Shorthand depiction Figure 10.9 Representations of membrane lipids. (A) Space-filling models of a phosphoglyceride, sphingomyelin, and an archaeal lipid show their shapes and distribution of hydrophilic and hydrophobic moieties. (B) A shorthand depiction of a membrane lipid. also called the polar head group, is represented by a circle, whereas the hydrocarbon tails are depicted by straight or wavy lines (see Figure 10.9B). QUIK QUIZ 2 What are the three major types of membrane lipids? Some roteins Are Modified by the ovalent Attachment of ydrophobic Groups Just as carbohydrates are attached to proteins to modify the properties of proteins, lipids also can be attached to proteins to provide them with additional biochemical properties. ften, such attachments are necessary for a protein to associate with a hydrophobic environment such as a membrane. These hydrophobic attachments can insert into the hydrophobic interior of the membrane and thus affix, or localize, the protein to the membrane surface. Such localization is required for protein function. Three such attachments are shown in Figure 10.10: (1) a palmitoyl group attached to a cysteine residue by a thioester bond, (2) a farnesyl group attached to a cysteine residue at the carboxyl terminus, and (3) a glycolipid structure termed a glycosylphosphatidylinositol (GI) anchor attached to the carboxyl terminus. N ys S N ys S S-almitoylcysteine 3 -terminal S-farnesylcysteine methyl ester N arboxyl terminus N 2 R R R Glycosyl phosphatidylinositol (GI) anchor R R R Figure Membrane anchors. Membrane anchors are hydrophobic groups that are covalently attached to proteins (in blue) and tether the proteins to the membrane. The green circles and blue square correspond to mannose and GlcNAc, respectively. R groups represent points of additional modification.

11 Lipids (A) Figure uthchinsongilford progeria syndrome (GS). (A) A 15-yearold boy suffering from GS. (B) A normal nucleus. () A nucleus from a GS patient. [(A) A hoto/gerald erbert; (B and ) from. Scaffidi, L. Gordon, and T. Misteli, LoS Biol. 3(11):e395/doi: /journal.pbio , linical Insight remature Aging an Result from the Improper Attachment of a ydrophobic Group to a rotein Farnesyl is a hydrophobic group that is often attached to proteins, usually so that the protein is able to associate with a membrane (see Figure 10.10). Inappropriate farnesylation has been shown to result in utchinsongilford progeria syndrome (GS), a rare disease of premature aging. Early postnatal development is normal, but the children (B) fail to thrive, develop bone abnormalities, and have a small beaked nose, a receding jaw, and a complete loss of hair (Figure 10.11). Affected children usually die at an average age of 13 years of severe atherosclerosis, a cause of death more commonly seen in the elderly. The cause of GS appears to be a mutation in the gene for the nuclear protein lamin, a protein that forms a scaffold for the nucleus and may take part in the regulation of gene expression. The folded () polypeptide that will eventually become lamin is modified and processed many times before the mature protein is produced. ne key processing event is the removal of a farnesyl group that had been added to the nascent protein earlier in processing. In GS patients, the farnesyl group is not removed, owing to a mutation in the lamin. The incorrectly processed lamin results in a deformed nucleus (see Figure 10.11) and aberrant nuclear function that results in GS. Much research remains to determine precisely how the failure to remove the farnesyl group leads to such dramatic consequences. SUMMARY 10.1 Fatty Acids Are a Main Source of Fuel Lipids are defined as water-insoluble molecules that are soluble in organic solvents. Fatty acids are an important lipid in biochemistry. Fatty acids are hydrocarbon chains of various lengths and degrees of unsaturation that terminate with a carboxylic acid group. The fatty acid chains in membranes usually contain between 14 and 24 carbon atoms; they may be saturated or unsaturated. Short chain length and unsaturation enhance the fluidity of fatty acids and their derivatives by lowering the melting temperature Triacylglycerols Are the Storage Form of Fatty Acids Fatty acids are stored as triacylglycerol molecules in adipose cells. Triacylglycerols are composed of three fatty acids esterified to a glycerol backbone. Triacylglycerols are stored in an anhydrous form There Are Three ommon Types of Membrane Lipids The major classes of membrane lipids are phospholipids, glycolipids, and cholesterol. hosphoglycerides, a type of phospholipid, consist of a glycerol backbone, two fatty acid chains, and a phosphorylated alcohol. hosphatidylcholine, phosphatidylserine, and phosphatidylethanolamine are major phosphoglycerides. Sphingomyelin, a different type of phospholipid, contains a sphingosine backbone instead of glycerol. Glycolipids are sugarcontaining lipids derived from sphingosine. holesterol, which modulates

12 membrane fluidity, is constructed from a steroid nucleus. A common feature of these membrane lipids is that they are amphipathic molecules, having one hydrophobic and one hydrophilic end. 153 roblems Key Terms fatty acid (p. 143) triacylglycerol (p. 146) phospholipid (p. 147) sphingosine (p. 147) phosphoglyceride (p. 147) sphingomyelin (p. 148) glycolipid (p. 149) cerebroside (p. 149) ganglioside (p. 149) cholesterol (p. 150) amphipathic molecule (p. 150) Answers to QUIK QUIZZES 1. hain length and the degree of cis unsaturation. 2. hospholipids, sphingolipids (of which glycolipids are a subclass), and cholesterol. roblems 1. Structure and name. Draw the structure of each of the following fatty acids and give the structure its common name: (a) n-dodecanate (b) cis- 9 -exadecenoate (c) cis, cis- 9, 12 -ctadecadienoate 2. Fluidity matters. Triacylglycerols are used for fuel storage in both plants and animals. The triacylglycerols from plants are often liquid at room temperature, whereas those from animals are solid. Suggest some reasons for this difference. 3. ontrast. Distinguish between phosphoglycerides and triacylglycerols. 4. ompare. What structural features differentiate sphingolipids from phosphoglycerides? 5. Depict a lipid. Draw the structure of a triacylglycerol composed of equal amounts of palmitic acid, stearic acid, and oleic acid. 6. Like finds like 1. What structural characteristic of lipids accounts for their solubility in organic solvents? 7. Like finds like 2. Suppose that a small amount of phospholipid were exposed to an aqueous solution. What structure would the phospholipid molecules assume? What would be the driving force for the formation of this structure? 8. Linkages. latelet-activating factor (AF) is a phospholipid that plays a role in allergic and inflammatory responses, as well as in toxic shock syndrome. The structure of AF is shown here. ow does it differ from the structures of the phospholipids discussed in this chapter? 9. oming clean. Draw the structure of the sodium salt of stearic acid. ow might it function to remove grease from your clothes or your hands? 10. ard-water problems. Some metal salts of fatty acids are not as soluble as the sodium or potassium salts. For instance, magnesium or calcium salts of fatty acids are poorly soluble. What would be the effect of taking a bath in water that is rich in magnesium or calcium? 11. Melting point 1. Explain why oleic acid (18 carbons, one cis bond) has a lower melting point than stearic acid, which has the same number of carbon atoms but is saturated. ow would you expect the melting point of trans-oleic acid to compare with that of cis-oleic acid? 12. Melting point 2. Explain why the melting point of palmitic acid ( 16 ) is 6.5 degrees lower than that of stearic acid ( 18 ). 13. A sound diet. Small mammalian hibernators can withstand body temperatures of 0 to 5 without injury. owever, the body fats of most mammals have melting temperatures of approximately 25. redict how the composition of the body fat of hibernators might differ from that of their nonhibernating cousins. Selected readings for this chapter can be found online at 3 ( 2 ) N( 3 ) 3 latelet-activating factor (AF)