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Chapter 8

Lipids

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Chapter Outline

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.)

Limonene Citronellal Menthol α−Pinene Camphene path 1 path 1 path 1 path 1, 2 path 3 Chapter 8 . Lipids
Chapter 8 . Lipids

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.

Source: http://www.ccunix.ccu.edu.tw/~deptche/08.pdf