v Lipids: Compounds composed largely of reduced carbons and exhibiting low water solubility.
Lipids may be completely hydrophobic or, if they contain polar groups, amphipathic
v Fatty acids: Carboxyl head group and hydrocarbon tail
å Typically even number of carbons (14 to 24) å Saturated fatty acids lack carbon-carbon double bonds å Unsaturated fatty acids contain one or more double bonds in cis configuration
Monosaturated: Single carbon-carbon double bond
Polyunsaturated: cis Double bonds separated by methylene carbon (-CH2-)
Systematic: e.g., octadecanoic acid: 18-carbons long
• 18:0 -18 carbons long with no double bonds
• 18:1(9) -18 carbons long with one double bond starting at carbon 9
å Essential fatty acids: Linoleic and γ-linolenic acid: Plant fatty acids required in diets of
animals: used to synthesize arachidonic acid -precursor to eicosanoids (prostaglandins)
v Triacylglycerols (triacylglycerides): Fats and oils
å Glycerol esterified with three fatty acids å Saponification: Alkali hydrolysis of triacylglycerols: Products: Glycerol and free fatty
v Glycerophospholipids: A class of phospholipids
å 1,2 Diacylglycerol with phosphate ester at carbon 3 å Stereospecific numbering system: Number glycerol carbons based on (R,S) system: C1 is
carbon that would lead to S-configuration if its priority was increased
å Phosphatidic acid: sn-Glycerol-3-phospate with fatty acids esterified to C1 and C2
Phosphatidic acid precursor of glycerophospholipids
å Glycerophospholipids: Polar group esterified to phosphate of phosphatidic acid
å Plasmalogens: Ether glycerophospholipids with cis-α,β-unsaturated alkyl group at C
v Sphingolipids: Backbone of sphingosine: 18-carbon amino alcohol with C-C trans double bond
Chapter 8 . Lipids
å Ceramide: sphingosine with fatty acid in amide linkage å Sphingomyelins: Alcohol esterified to phosphoceramide
å Glycosphingolipids: Ceramide with sugars in β-glycosidic linkage
Cerebroside: Sugar is glucose or galactose
Sulfatide: Sugar is galactose with sulfate esterified at carbon 3 of galactose
Gangliosides: Ceramide with three or more sugars including sialic acid
v Waxes: Esters of long-chain alcohol and long-chain fatty acid v Terpenes: Class of lipids formed from 2-methyl-1,3-butadiene (isoprene)
å Monoterpene: Two isoprene units: 10 carbons å Sesquiterpenes: Three isoprene units: 15 carbons å Diterpenes: Four isoprenes: two monoterpenes: 20 carbons å Triterpenes: Six isoprenes: 30 carbons: Cholesterol precursors squalene and lanosterol å Tetraterpenes: Eight isoprenes: 40 carbons: carotenoids å Polyprenols: Long-chain polyisoprenoid alcohols
Bile salts: Cholic acid and deoxycholic acid
Chapter Objectives Lipids are amphipathic molecules with both polar and nonpolar groups. Understand why this is the case for both simple lipids (like cholesterol) and complex lipids (like phospholipids). Fatty acids are important components of membrane lipids and triacylglycerols (fats and oils). Fatty acids are most commonly composed of an even number of carbons. Saturated fatty acids have only carbon-carbon single bonds (their carbons are saturated with respect to hydrogens), whereas unsaturated fatty acids have one or more (polyunsaturated) carbon-carbon double bonds in cis configuration. When more than one double bond is present, the bonds are not conjugated but rather separated by a -CH2- group. Complex lipids include triacylglycerols, glycerophospholipids and sphingolipids. They are classified as complex because they contain at least one fatty acid group. For example, triacylglycerols have a glycerol backbone to which three fatty acids are esterified.
Glycerophospholipids again have a glycerol backbone but with only two fatty acids (in ester linkage to carbons 1 and 2) and, attached to the third carbon of glycerol, a phosphate group to which a polar alcohol is linked (like ethanolamine, choline, serine or inositol). Sphingolipids are composed of sphingosine, a fatty acid, and a polar head group (like phosphocholine or one or more sugar moieties). The carbons of simple lipids (terpenes or cholesterol and its derivatives) all derive from isoprene. Know the general structure of isoprene and cholesterol and appreciate the fact that important biomolecules such as steroid hormones and bile salts are derivatives of cholesterol. Problems and Solutions
1. Draw the structures of (a) all the possible triacylglycerols that can be formed from glycerol from stearic and arachidonic acid and (b) all the phosphatidylserine isomers that can be formed from palmitic and linolenic acids.
Chapter 8 . Lipids
Answer: Triacylglycerols have a glycerol backbone to which three fatty acids are esterified. With nonidentical fatty acids at carbons 1 and 3, carbon 2 is chiral. Whereas two stereoisomers are possible, biological triacylglycerols have the L- configuration. Stearic acid is an 18-carbon saturated fatty acid. Arachidonic acid is a 20-carbon fatty acid with four cis double bonds at carbons 5, 8, 11 and 14.
b. Palmitic acid is 16:0; linolenic acid is 18:3(•9,12,15). The backbone structure of phosphatidylserine is 3-phosphoglycerol with L-serine in phosphate ester linkage. Fatty acids are esterified at carbons 1 and 2. There is a preference for unsaturated fatty acids at carbon 2. The alpha carbon of serine is chiral; L and D serine are possible; however, the L isomer occurs in phosphatidylserine. The central carbon in the glycerol backbone of phosphatidylserine is prochiral and only one isomer is used, the one based on sn-glycerol-3-phosphate.
Chapter 8 . Lipids
Note: Phosphatidylserine with unsaturated lipids at position 1 are very rare. Unsaturated fatty acids are usually found at position 2. 2. Describe in your own words the structural features of a. a ceramide, and how it differs from a cerebroside. b. a phosphatidylethanolamine, and how it differs from a phosphatidylcholine. c. an ether glycerophospholipid, and how it differs from a plasmalogen. d. a ganglioside, and how it differs from a cerebroside. e. testosterone, and how it differs from estradiol.
Answer: a.) Ceramide (N-acylsphingosine) is derived from sphingosine, a long-chain amino dialcohol synthesized from palmitic acid (fatty acid 16:0) and serine. (During synthesis of sphingosine, the carboxyl group of serine is lost as carbon dioxide, the carboxyl carbon of palmitic acid is attached to serine’s alpha carbon as a ketone and subsequently reduced to an alcohol, and the Cα-Cβ bond of palmitic acid is oxidized to a trans double bond.) Ceramide has a fatty acid attached to sphingosine by an amide bond. Cerebrosides, 1-β-D-galactoceramide and 1-β-D-glucoceramide, have monosaccharides attached in glycosidic linkage to ceramide at what was serine’s side chain. b.) Phosphatidylethanolamine and phosphatidylcholine are both glycerophospholipids synthesized from phosphatidic acid. Phosphatidic acid is sn-glycerol-3-phosphate with fatty acids esterified to carbons 1 and 2. In phosphatidylethanolamine the amino alcohol, ethanolamine, is joined to phosphatidic acid in phosphate ester linkage. The phosphoethanolamine moiety is the head group. In phosphatidylcholine the head group is phosphocholine. Choline is N,N,N-trimethylethanolamine. c.) As the name implies ether glycerophospholipids are glycerophospholipids with an alkyl chain attached to carbon 1 of glycerol by ether linkage. A fatty acid is esterified to carbon 2. Plasmalogens are ether glycerophospholipids with a cis-α,β-double bond on the ether-linked alkyl chain. d.) Cerebrosides, as explained in (a.) are glycolipids with either galactose or glucose attached in glycosidic linkage. Gangliosides are synthesized from 1-β-D-glucoceramide and contain additional sugar moieties including galactose and sialic acid (N-acetylneuraminic acid). e.) Testosterone and estradiol are steroid hormones derived from cholesterol. Testosterone, an androgen, and estradiol, an estrogen, mediate the development of sexual characteristics in animals. Because they are both synthesized from cholesterol they share cholesterol’s basic structure (of three fused six-membered rings a one fused five-membered ring) but lack cholesterol’s alkyl chain. In its place is a hydroxyl group. The oxygen derived from cholesterol’s hydroxyl group is a carbonyl oxygen in testosterone but a hydroxyl group in estradiol (diol implies two hydroxyl groups). 3. From your memory of the structures, name a. the glycerophospholipids that carry a net positive charge. b. the glycerophospholipids that carry a net negative charge. c. the glycerophospholipids that have zero net charge.
Answer: a.) Since all the glycerophospholipids derive from phosphatidic acid, in order to form a positively charged lipid, the head group must carry a positive charge. This is true for ethanolamine and choline and for serine at low pH. However, keeping in mind that the phosphodiester bond is negatively charged, phosphatidylethanolamine, phosphatidylcholine, and phosphatidylserine will be positively charged only at low pH. b.) Negatively charged glycerophospholipids include phosphatidic acid, phosphatidylglycerol, phosphatidylinositol, phosphatidylserine, and diphosphatidylglycerol (cardiolipin). Because glycerol and inositol are uncharged, lipids with these head groups will have -1 charge at all but acidic pH values. The charge on phosphatidic acid will range from 0 to -1 to -2 depending on pH. When phosphotidylserine is negatively charged (at all but acidic values of pH) it ranges from
Chapter 8 . Lipids
-1 to -2 at basic pH when serine’s amino group is not protonated. Cardiolipin typically carries -2 charge. c.) Phosphatidylethanolamine and phosphatidylcholine are the only uncharged glycerophospholipids at neutral pH. 4. Compare and contrast two individuals, one of wh ose diet consist largely of meats containing high levels of cholesterol, and the other of whose diet is rich in plant sterols. Are their risks of cardiovascular disease likely to be similar or different? Explain your reasoning.
Answer: The American Heart Association identifies high blood cholesterol levels as one of the major risk factors for cardiovascular disease. Fortunately, for many individuals, blood cholesterol levels can be maintained at low levels by avoiding diets high in cholesterol and saturated fatty acids. Foods that contain cholesterol include meat, poultry and seafood, and dairy products. (Lean red meats contain similar amounts of cholesterol as poultry and fish. Plants do not contain cholesterol.) High blood cholesterol levels lead to atherosclerosis, a thickening and hardening of arteries that is a consequence of plaque formation on arterial walls. Thus, diets high in cholesterol may contribute to plaque formation. Since plants lack cholesterol, foods derived from plants do not contribute directly to high cholesterol levels. (Diets high in triacylglycerols containing saturated fatty acids will raise blood cholesterol. In addition, trans fatty acids may also contribute to increased blood cholesterol. One source of trans fatty acids is from margarine produced by hydrogenation of vegetable oils.) In addition, there is evidence that plant sterols (phytosterols) may actually lower blood cholesterol by inhibiting cholesterol absorption (See for example P. J. Jones et al. Dietary phytosterols as cholesterol-lowering agents in humans Can. J. Physiol. Pharmacol. 75, 217-227 (1997)) 5. James G. Watt, Secretary of the Interior (1981-1983) in Ronald Reagan’s first term, provoked substantial controversy by stating publicly that trees cause significant amounts of air pollution. Based on your reading of this chapter, evaluate Watt’s remarks.
Answer: During the 1980 presidential campaign Ronald Reagan stated that trees cause more air pollution than do automobiles. Trees in fact do emit large quantities of hydrocarbons principally in the form of isoprenes. These volatile hydrocarbons may react with ozone to form compounds similar to those found in smog. (Trees, however, play a key role in removing pollutants from air.) (The press had a field day with Reagan’s comment, which evoked numerous light-hearted reactions. For example, at Claremont College a tree was draped with a banner that read: “Chop me down before I kill again.” (The Washington Post Oct. 15, 1980)) 6. In a departure from his usual and highly popular western, author Louis L’Amour wrote a novel in 1987, Last of the Breed (Bantom Press), in which a military pilot of Native American ancestry is shot down over the former Soviet Union and is forced to use the survival skills of his ancestral culture to escape his enemies. On the rare occasions when he is able to trap and kill an animal for food, he selectively eats the fat, not the meat. Based on your reading of this chapter, what was his reasoning for doing so?
Answer: Fats and oils are composed of highly reduced carbons and, therefore, they release large amounts of energy when metabolized aerobically into carbon dioxide and water. L’Amour’s hero likely knew of the high caloric content of animal fat. In addition, oxidation of triacylglycerols produces water, which might be of some value in dry climates. 7. Consult a grocery store near you and look for a product in the dairy cooler called Benecol. Examine the package and suggest what the special ingredient is in this Chapter 8 . Lipids product that is credited with blockage of cholesterol uptake in the body. What is the structure of this ingredient, and how does it function? Answer: The first several items in the ingredients list for Benecol are canola oil, water, partially hydrogenated soybean oil, (see the answer to question 11 to understand what “partially hydrogenated” signifies), soybean oil, and plant stanol esters. Stanols are hydrogenated plant sterols and plant sterols are compounds similar in structure to cholesterol. There are several plant sterols and one of the most common is β-sitosterol. (Its structure is shown below and on page 263 of the textbook along with other sterols and one stanol.) The difference between β - sitosterol and β -sitostanol is reduction of the double bond. A stanol ester is a stanol modified by a fatty acid in ester linkage. Esterification increases the lipid solubility of stanol allowing it to dissolve in this oil-based product. When stanol esters are ingested the ester is hydrolyzed in the digestive system. It is known that plant sterols and stanols lower uptake of cholesterol by inhibiting the absorption of cholesterol from the small intestine.
8. If you are still at the grocery store working on problem 7, stop by the rodent poison section and examine a container of warfarin or a related product. From what you can glean from the packaging, how much warfarin would a typical dog (40 lbs) have to consume to risk hemorrhages and/or death?
Answer: The MSDS (Material Safety Data Sheet) for a commercially available product containing 98% warfarin lists the LD50 (the lethal dose for 50% of animals tested) as 3 mg/kg. A 40 lb dog (1 pound = 0.454 kilogram) weighs 18.2 kg and needs about 55 mg of warfarin for a dose equivalent to the LD50. I found the MSDS for a rodenticide in the form of wafarin-coated pellets. The MSDS for rat was listed as 20 g/kg but the product was only 0.025% warfarin by weight. (The LD50 based only on the active ingredient is about 5 mg/kg.) It would take around 360 g (0.8 lb) to be lethal for a 40 lb rat (and presumably for the same sized dog). I found the MSDS for another rodenticide, a “warfarin meal”, which listed the LD50 for rats at 200 g/kg bu t did not state the percentage of the active ingredient. It is likely to be only about 0.0025% by weight. About 8 lbs of this product would be required to do in the dog. Chapter 8 . Lipids 9. Refer to Figure 8.17 and draw each of the structures shown and try to identify the isoprene units in each of the molecules. (Note that there may be more than one correct answer for some of these molecules, unless you have the time and facilities to carry out 14C labeling studies with suitable organisms.)
10. As noted in the Deeper Look box on polar bears, a polar bear may burn as much as 1.5 kg of fat resources per day. What weight of seal blubber would you have to ingest if you were to obtain all your calories from this energy source?
Answer: The amount of seal blubber y ou would have to ingest really depends on who you are and what you do, in terms of physical activity. I searched for on-line calculators that would help me determine my Caloric intake given my age, sex and daily activity. I also looked at “Dietary Guidelines for Americans” (at http://www.health.gov/) and in the end decided to make calculations based on a daily caloric intake of 2,200 Calories (2,200 kcalories). One gram of triacylglycerols yields about 38 kJ of energy. Using the following conversion, 1 kJ = 0.2388 kcalories (0.2388 Calories), 38 kJ represents 9.07 Calories. To get 2,200 Calories from seal blubber you would need to consume
This is about a half-a-pound of USDA prime seal blubber. 11. Just in case you are still at the grocery store working on problems 7 and 8, stop by the cookie shelves and choose your three favorite cookies from the shelves. Estimate how many calories of fat, and how many other calories from other sources, are contained in 100 g of each of these cookies. Survey the ingredients listed on each package, and describe the contents of the package in terms of (a) saturated fat, (b) cholesterol, and (c) trans fatty acids. (Note that food makers are required to list ingredients in order of decreasing amounts in each package.)
Answer: I looked up nutritional information on four popular cookies and easily found most of the information asked for in this question. However, none of the cookies I looked up listed trans fatty acids. As it turns out this information will be required on nutritional labels by 1 January 2006 (21 CFR 101). After a lengthy process, the Food and Drug Administration decided to require this information on a separate line in the nutritional data immediately under the line for saturated fatty acids by 2006. Until then consumers will have to assume that products containing “hydrogenated” or “partially hydrogenated” vegetable oils contain trans fatty acids. I purposefully avoided listing the brand name of the cookies. The data are presented in two charts. The chart below has information taken directly from the nutritional information label. Serving size (# of cookies) Chapter 8 . Lipids
The data in the first chart were used to calculate various values presented below.
Calories per gram
Clearly, there are differences. You may notice that the total Calories does not equal to the sum of the Calories from fat and carbohydrate. In some cases, the data presented by the cookie makers are not consistent and they (the cookie makers) attribute this to rounding errors! The zeros under cholesterol should not be taken as Gospel because they were based on a serving size less than 100 grams and likely rounded to 1 significant figure. Finally, as a gentle reminder nutritional Calories (note the capital C) are actually kilocalories. 12. Describe all of the structural differences between cholesterol and stigmasterol?
Answer: The structures of both compounds are shown below. There are two differences: Stigmasterol has a trans double bond be tween carbons 22 and 23; and, carbon 24 in stigmasterol is modified with an ethyl group. Chapter 8 . Lipids
13. Describe in our own words the functions of androgens, glucocorticoids, and mineralocorticoids.
Answer: Androgens, glucocorticoids and mineralocorticoids are all steroid hormones derived from cholesterol. Androgens are responsible for development of sexual characteristics of males and for sperm production. In addition, androgens control libido and aggressiveness. The principal androgen is testosterone, which is produced by interstitial cells of the testis. Glucocorticoids and mineralocorticoids are steroid hormones p roduced by the adrenal cortex. Glucocorticoids regulate metabolism, specifically of carbohydrate, protein and lipid. The major glucocorticoid is cortisol. It stimulates gluconeogenesis and amino acid uptake by the liver and kidney. In adipocytes (fat cells) it inhibits glucose uptake and stimulates lipolysis.
Glucocorticoids also have anti-inflammatory properties. Mineralocorticoids regulate extracellular fluid volume by modulating potassium uptake in the kidney. The principal mineralocorticoid is aldosterone. 14. Look through your refrigerator, your medicine cabinet, and your cleaning solutions shelf or cabinet, and find at least three commercial products that contain fragrant monoterpenes. Identify each one by its scent and then draw its structure.
Answer: Here are a few. Limonene is found in orange oil and lemon oil, phellandrene in spearmint, pinene in pine and eucalyptus, camphene in firs, sweet fennel and nutmeg, myrcene in coriander, ginger, cinnamon and nutmeg. Chapter 8 . Lipids
15. Gibberellic acid is described in Figure 8.17 as a plant hormone. Look it up on the Internet or in an encyclopedia and describe at least one of its functions in your own words.
Answer: Gibberellic acid is a gibberellin -plant hormones that stimulate plant growth. Gibberellins are diterpenoids. They regulate stem elongation, flowering, dormancy, fruit production. 16. Make a list of the advantages polar bears enjoy from their nonpolar diet. Why wouldn’t juvenile polar bears thrive on an exclusively nonpolar diet?
Answer: Polar bears largely eat seals, which they consume between April and July. It is estimated that they need approximately 2 kg of fat per day to survive. At approximately 9 Calories per gram this amounts to a whopping 18,000 Calories per day! Clearly, polar bears eat to store fat to get them through the summer, oddly enough. When consuming a seal they do eat blubber and muscle but since the body does not store excess amino acids the proteins are largely metabolized. The triacylglycerides, however, are stored for later use. During the summer months they rely of fat metabolism to survive. In addition to being a rich source of calories, this has the advantage of producing water, which allows the polar bear to survive without the need to drink liquid water. In their habitat, water is either solid or salted. The former would require calories simply to melt and bring to body temperature whereas the later is too high in osmolarity to be of use. Juvenile polar bears require, in addition to high calorie diets, diets rich in amino acid because they are growing. (Polar bears need ice from which to hunt seals. So, in the colder months they stock up on seals to get them through the warm months. Global warming may have severe consequences for polar bears bec ause they will have to build up even larger fat stores to survive the ice-free summer periods.) 17. Snake venom phospholipase A2 causes death by generating membrane-soluble anionic fragments from glycerophospholipids. Predict the fatal effects of such molecules on membrane proteins and lipids.
Answer: Phospholipase A2 hydrolyzes of the fatty acid located on carbon 2 of phospholipids to produce a free fatty acid and 1-acylglycerophospholipid also known as 2-lysolecithin. Both fatty acids and lysolecithin are detergents capable of dissolving membr ane components. Thus, phospholipase A2 activity is expected to lead to cell lysis. This results in excessive tissue damage. Questions for Self Study
1. Fill in the blanks. are important biomolecules composed of a long hydrocarbon chain or tail and a carboxyl group. When all of the carbon-carbon bonds are single bonds the compound is said to be . This term also indicates that the carbons in the tail are associated with a maximum number of atoms. Compounds of this type with one carbon-carbon double bond are whereas those with multiple carbon-carbon double bonds are . Usually there are an number of carbons atoms. These compounds are components of fats and oils in which they are joined to a backbone in linkage. The hydrolysis of fats or oils with alkali is called . Chapter 8 . Lipids
a. 2-methyl-1,3-butadiene is also known as isoprene. . b. Cholesterol is a phospholipid. . c. The androgens are a class of terpene-based lipids involved in absorption of dietary lipids in the intestine. . d. Vitamins A, E, and K are highly water-soluble vitamins. . e. Cholesterol is a hydrocarbon composed of three six-membered rings and one five-membered ring in addition to a hydrocarbon tail. .
3. Identify the following from the structures shown below: phosphatidic acid, phosphatidylcholine, phosphatidylserine, phosphatidylinositol, ceramide, phosphatidylethanolamine.
4. Based on your knowledge of lipid and carbohydrate biochemistry identify components of the following compound and state how this compound is chemically similar in structure to triacylglycerols? How does it differ biochemically?
Chapter 8 . Lipids
5. Very often grocery stores sell produce with a waxy coating applied to their outside (cucumbers and turnips are often treated this way). What is the general structure of a wax? For what purpose is the layer of wax applied? Would something like a fatty acid or a triacylglycerol not be a good substitute? Answers
1. Fatty; acids; saturated; hydrogen; monounsaturated; polyunsaturated; even; glycerol; ester; saponification. 2. a./T; b./F; c./F; d./F; e./T. 3. d.; a.; c.; e.; f.; b. 4. You should readily identify the two rings as substituted sugars. The six-membered ring is glucose and the five-membered ring is fructose. The disaccharide they form is sucrose. Each of the hydroxyl groups of sucrose has a fatty acid attached by ester bonds. The compound is sucrose polyester or more commonly known as olestra (Trade name: Olean). Olestra is currently being used as a fat substitute because it has properties identical to fats and oils but in not digested. Triacylglycerols contain fatty acids esterified to glycerol, a three carbon alcohol. Both triacylglycerol and olestra are amphiphilic molecules with uncharged, weakly polar head groups and hydrocarbon tails. 5. Waxes are composed of a long-chain alcohol and a long-chain fatty acid joined in ester linkage. Waxes are often used to make surfaces water impermeable thus a waxy coating will prevent water loss and prolong shelf life. A layer of triacylglycerol might accomplish the same results; however, typical fats and oils have lower melting temperatures and would not be expected to form as stable a layer as wax. Additional Problems
Chapter 8 . Lipids
1. The ancient Romans may have been the first to produce lye soap by mixing animal fat with ash, a rich source of alkaline potassium hydroxide. What is soap and what reaction occurs in this mixture to produce it? 2. In regions with mineral-rich water supplies (hard water) it is often difficult to work up a rich lather using hand soaps. Why? 3. At a romantic candle-light dinner, the conversation turns to properties of waxes and what exactly happens when a candle burns. Contribute to the conversation. 4a. Margarine is made from vegetable oil by a process called hydrogenation in which the oil is reacted with hydrogen gas in the presence of a small amount of nickel that functions as a catalyst. Hydrogenation saturates double bonds. Explain why hydrogenated vegetable oil is a solid. b. Margarines may be purchased in stick-form or in small tubs. What is the important chemical difference between these two kinds of margarines? 5. Venom from honey bees (Apis mellifera) contains a phospholipase and several other components including a small amount of a polypeptide that acts as a detergent. Can you suggest a function for the polypeptide in terms of phospholipase activity? Abbreviated Answers 1. A soap is a salt of a fatty acid. Soaps are formed in a reaction known as saponification in which fatty acids are hydrolyzed from triacylglycerols (fats and oils) in alkaline (basic) solution. The free fatty acids form salts that precipitate. 2. Mineral-rich water contains, among other things, high levels of divalent cations. Divalent cations will interact with and precipitate fatty acids. Water softeners are agents that chelate the offending divalent cations. Water can also be softened by ion exchange with resins that bind divalent cations. 3. Waxes are esters of long-chain alcohols and fatty acids. For example, in beeswax, straight- chain alcohols 24 to 36 carbons in length are esterified to long, straight-chain fatty acids up to 36 carbons in length. The melting temperature of beeswax is around 63°C. When a candle burns, the lit wick produces heat that melts the wax. The liquid wax is drawn up the wick to be consumed in the flame. A good candle will produce very little dripping wax because the wax is all consumed in the flame. Wax is a rich source of oxidizable hydrocarbons and serves the same purpose as oil does in a lamp, or gasoline does in an internal combustion engine. However, waxes and oils do not explode because they have a very low vapor pressures. 4a. Margarines are typically made from vegetable oils such as corn oil and soybean oil. They are oils, liquids at room temperature, because their composition includes greater than 50% unsaturated fatty acids. Hydrogenation is the addition of hydrogen to double-bonds producing a saturated hydrocarbon. For a given chain length, saturated hydrocarbons have a higher melting temperature than do unsaturated hydrocarbons. Thus, saturation of the double-bonds in the fatty acids in corn and soy oil reduces the level of unsaturated fatty acids and as a consequence the melting temperature is increased. b. The difference between tub-margarine and stick-margarine is the degree of saturation. Tub- margarine is distributed in a container because it is softer than stick-margarine due to a lower degree of hydrogenation. 5. The phospholipase of bee venom requires free lipids as substrates and the purpose of the detergent-like polypeptide may be to dissolve some of the victim's membrane to provide substrates. Once the reaction starts, the products, free fatty acids and lysolecithin, are both detergent-like molecules that will aid in dissolving membranes. Chapter 8 . Lipids Summary
Lipids are a large and diverse class of cellular compounds defined by their insolubility in water and solubility in organic solvents. Lipids serve several biological functions. As highly reduced forms of carbon, lipids yield large amounts of energy in the oxidative reactions of metabolism. As hydrophobic molecules, lipids allow membranes to act as effective barriers to polar molecules. The unique bilayer structure of membr anes derives mainly from the amphipathic nature of membrane lipids. Certain lipids also play roles as cell-surface components involved in immunity, cell recognition and species specificity. Other lipids act as intracellular messengers and triggers, which regulate a variety of processes. Most fatty acids found in nature have an even number of carbon atoms. Fatty acids may either be saturated or unsaturated, and double bonds are normally of the cis configuration. “Essential” fatty acids, including linoleic and linolenic acids, are not synthesized by mammals, but are required for growth and life. Triacylglycerols consist of a glycerol molecule with three fatty acids esterified. Triacylglycerols in animals are found primarily in adipose tissue, and serve as a major metabolic reserve for the organism. Glycerophospholipids, a major class of lipids, are composed of an sn-glycerol-3-phosphate with fatty acids esterified at the 1- and 2- positions. Many different “head groups” can be esterified to the phosphate, including choline, ethanolamine, serine, glycerol and inositol. Sphingolipids are lipids based on sphingosine, a long chain fatty alcohol, to which is often attached another fatty acid in an amide linkage to form a ceramide. Sphingomyelin is a phosphate-containing sphingolipid. Glycosphingolipids consist of a ceramide backbone with one or more sugars. Cerebroside is a glycosphingolipid containing either glucose or galactose. Gangliosides have three or more sugars esterified, one of which must be a sialic acid. Glycosphingolipids are present in only small amounts, but serve numerous important cell functions. Terpenes are a class of lipids derived from isoprene units. The steroids, including cholesterol, are an important class of terpene-based lipids. Other steroids in animals, including the androgens and estrogens (male and female hormones, respectively) and the bile acids (used in digestion) are derived from cholesterol. Waxes are esters of long-chain fatty acids and long-chain alcohols. Because of their low water solubility and ability to aggregate, waxes are used to form water-impermeable surfaces.
5. Using Multiple Vocabularies Catalogers of art information require multiple vocabularies because no single vocabulary provides the full set of terminology needed to catalog or index a given set of cultural heritage data; therefore, a combination of vocabularies is necessary for indexing. Furthermore, separate vocabu- laries may be required for retrieval; ideally, retrieval vocabularies are
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