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Lipids
chapter
10
1. Operational Definition of Lipids How is the definition of “lipid” different from the types of definitions
used for other biomolecules that we have considered, such as amino acids, nucleic acids, and proteins?
2. Melting Points of Lipids The melting points of a series of 18-carbon fatty acids are: stearic acid,
69.6 ⬚C; oleic acid, 13.4 ⬚C; linoleic acid, ⫺5 ⬚C; and linolenic acid, ⫺11 ⬚C.
(a) What structural aspect of these 18-carbon fatty acids can be correlated with the melting point?
(b) Draw all the possible triacylglycerols that can be constructed from glycerol, palmitic acid, and
oleic acid. Rank them in order of increasing melting point.
(c) Branched-chain fatty acids are found in some bacterial membrane lipids. Would their presence
increase or decrease the fluidity of the membranes (that is, give them a lower or higher melting
point)? Why?
Answer
3. Preparation of Béarnaise Sauce During the preparation of béarnaise sauce, egg yolks are incorpo-
rated into melted butter to stabilize the sauce and avoid separation. The stabilizing agent in the egg
yolks is lecithin (phosphatidylcholine). Suggest why this works.
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4. Isoprene Units in Isoprenoids Geraniol, farnesol, and squalene are called isoprenoids, because they
are synthesized from five-carbon isoprene units. In each compound, circle the five-carbon units repre-
senting isoprene units (see Fig. 10–22).
Answer
5. Naming Lipid Stereoisomers The two compounds below are stereoisomers of carvone with quite
different properties; the one on the left smells like spearmint, and that on the right, like caraway.
Name the compounds using the RS system.
Geraniol
OH
Farnesol
OH
Squalene
H
CH
2
O
HCH
2
O
Spearmint Caraway
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6. RS Designations for Alanine and Lactate Draw (using wedge-bond notation) and label the (R)
and (S) isomers of 2-aminopropanoic acid (alanine) and 2-hydroxypropanoic acid (lactic acid).
2-Aminopropanoic acid
(alanine)
E
E
E
C
H
2
NCH
3
H
COOH
A
2-Hydroxypropanoic acid
(lactic acid)
E
E
E
C
HO CH
3
H
COOH
A
7. Hydrophobic and Hydrophilic Components of Membrane Lipids A common structural feature of
membrane lipids is their amphipathic nature. For example, in phosphatidylcholine, the two fatty acid
chains are hydrophobic and the phosphocholine head group is hydrophilic. For each of the following
membrane lipids, name the components that serve as the hydrophobic and hydrophilic units: (a) phos-
phatidylethanolamine; (b) sphingomyelin; (c) galactosylcerebroside; (d) ganglioside; (e) cholesterol.
Answer
Hydrophobic unit(s) Hydrophilic unit(s)
(a) 2 Fatty acids Phosphoethanolamine
8. Structure of Omega-6 Fatty Acid Draw the structure of the omega-6 fatty acid 16:1.
Answer
9. Catalytic Hydrogenation of Vegetable Oils Catalytic hydrogenation, used in the food industry, con-
verts double bonds in the fatty acids of the oil triacylglycerols to OCH
2
OCH
2
O. How does this affect the
physical properties of the oils?
10. Alkali Lability of Triacylglycerols A common procedure for cleaning the grease trap in a sink is to
add a product that contains sodium hydroxide. Explain why this works.
11. Deducing Lipid Structure from Composition Compositional analysis of a certain lipid shows
that it has exactly one mole of fatty acid per mole of inorganic phosphate. Could this be a glyc-
erophospholipid? A ganglioside? A sphingomyelin?
12. Deducing Lipid Structure from Molar Ratio of Components Complete hydrolysis of a glyc-
erophospholipid yields glycerol, two fatty acids (16:1(⌬
9
) and 16:0), phosphoric acid, and serine in the
molar ratio 1:1:1:1:1. Name this lipid and draw its structure.
Answer
13. Impermeability of Waxes What property of the waxy cuticles that cover plant leaves makes the cuti-
cles impermeable to water?
14. The Action of Phospholipases The venom of the Eastern diamondback rattler and the Indian cobra
contains phospholipase A
2
, which catalyzes the hydrolysis of fatty acids at the C-2 position of glyc-
erophospholipids. The phospholipid breakdown product of this reaction is lysolecithin (lecithin is phos-
phatidylcholine). At high concentrations, this and other lysophospholipids act as detergents, dissolving
the membranes of erythrocytes and lysing the cells. Extensive hemolysis may be life-threatening.
(a) All detergents are amphipathic. What are the hydrophilic and hydrophobic portions of
lysolecithin?
(b) The pain and inflammation caused by a snake bite can be treated with certain steroids. What is
the basis of this treatment?
(c) Though the high levels of phospholipase A
2
can be deadly, this enzyme is necessary for a variety
of normal metabolic processes. What are these processes?
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15. Lipids in Blood Group Determination We note in Figure 10–15 that the structure of glycosphin-
golipids determines the blood groups A, B, and O in humans. It is also true that glycoproteins deter-
mine blood groups. How can both statements be true?
16. Intracellular Messengers from Phosphatidylinositols When the hormone vasopressin stimulates
cleavage of phosphatidylinositol 4,5-bisphosphate by hormone-sensitive phospholipase C, two products
are formed. What are they? Compare their properties and their solubilities in water, and predict
whether either would diffuse readily through the cytosol.
17. Storage of Fat-Soluble Vitamins In contrast to water-soluble vitamins, which must be part of our
daily diet, fat-soluble vitamins can be stored in the body in amounts sufficient for many months. Sug-
gest an explanation for this difference.
18. Hydrolysis of Lipids Name the products of mild hydrolysis with dilute NaOH of (a) 1-stearoyl-2,
3-dipalmitoylglycerol; (b) 1-palmitoyl-2-oleoylphosphatidylcholine.
19. Effect of Polarity on Solubility Rank the following in order of increasing solubility in water: a
triacylglycerol, a diacylglycerol, and a monoacylglycerol, all containing only palmitic acid.
20. Chromatographic Separation of Lipids A mixture of lipids is applied to a silica gel column, and
the column is then washed with increasingly polar solvents. The mixture consists of phosphatidylser-
ine, phosphatidylethanolamine, phosphatidylcholine, cholesteryl palmitate (a sterol ester), sphin-
gomyelin, palmitate, n-tetradecanol, triacylglycerol, and cholesterol. In what order will the lipids elute
from the column? Explain your reasoning.
21. Identification of Unknown Lipids Johann Thudichum, who practiced medicine in London about
100 years ago, also dabbled in lipid chemistry in his spare time. He isolated a variety of lipids from
neural tissue, and characterized and named many of them. His carefully sealed and labeled vials of
isolated lipids were rediscovered many years later.
(a) How would you confirm, using techniques not available to Thudichum, that the vials labeled
“sphingomyelin” and “cerebroside” actually contain these compounds?
(b) How would you distinguish sphingomyelin from phosphatidylcholine by chemical, physical, or
enzymatic tests?
Answer
22. Ninhydrin to Detect Lipids on TLC Plates Ninhydrin reacts specifically with primary amines to
form a purplish-blue product. A thin-layer chromatogram of rat liver phospholipids is sprayed with
ninhydrin, and the color is allowed to develop. Which phospholipids can be detected in this way?
Data Analysis Problem
23. Determining the Structure of the Abnormal Lipid in Tay-Sachs Disease Box 10–1, Figure 1,
shows the pathway of breakdown of gangliosides in healthy (normal) individuals and individuals with
certain genetic diseases. Some of the data on which the figure is based were presented in a paper by
Lars Svennerholm (1962). Note that the sugar Neu5Ac, N-acetylneuraminic acid, represented in the
Box 10–1 figure as a purple 䉬, is a sialic acid.
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Svennerholm reported that “about 90% of the monosialiogangliosides isolated from normal human
brain” consisted of a compound with ceramide, hexose, N-acetylgalactosamine, and N-acetylneu-
raminic acid in the molar ratio 1:3:1:1.
(a) Which of the gangliosides (GM1 through GM3 and globoside) in Box 10–1, Figure 1, fits this de-
scription? Explain your reasoning.
(b) Svennerholm reported that 90% of the gangliosides from a patient with Tay-Sachs had a molar
ratio (of the same four components given above) of 1:2:1:1. Is this consistent with the Box 10–1
figure? Explain your reasoning.
To determine the structure in more detail, Svennerholm treated the gangliosides with neu-
raminidase to remove the N-acetylneuraminic acid. This resulted in an asialoganglioside that was much
easier to analyze. He hydrolyzed it with acid, collected the ceramide-containing products, and deter-
mined the molar ratio of the sugars in each product. He did this for both the normal and the Tay-Sachs
gangliosides. His results are shown below.
Ganglioside Ceramide Glucose Galactose Galactosamine
Normal
Fragment 1 1 1 0 0
Fragment 2 1 1 1 0
Fragment 3 1 1 1 1
Fragment 4 1 1 2 1
Tay-Sachs
Fragment 1 1 1 0 0
Fragment 2 1 1 1 0
Fragment 3 1 1 1 1
(c) Based on these data, what can you conclude about the structure of the normal ganglioside? Is
this consistent with the structure in Box 10–1? Explain your reasoning.
(d) What can you conclude about the structure of the Tay-Sachs ganglioside? Is this consistent with
the structure in Box 10–1? Explain your reasoning.
Svennerholm also reported the work of other researchers who “permethylated” the normal
asialoganglioside. Permethylation is the same as exhaustive methylation: a methyl group is added to
every free hydroxyl group on a sugar. They found the following permethylated sugars: 2,3,6-trimethyl-
glycopyranose; 2,3,4,6-tetramethylgalactopyranose; 2,4,6-trimethylgalactopyranose; and 4,6-dimethyl-
2-deoxy-2-aminogalactopyranose.
(e) To which sugar of GM1 does each of the permethylated sugars correspond? Explain your
reasoning.
(f) Based on all the data presented so far, what pieces of information about normal ganglioside struc-
ture are missing?
Answer
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