Chapter Questions
What are the three main biochemical functions of lipids?
Why are fats a good source of energy for storage in the body?
Proteins, nucleic acids, and carbohydrates are grouped by common structural features found within their group. What is the basis for grouping substances as lipids?
For the molecules listed below, tell whether they are saturated or unsaturated and give the shorthand nomenclature.(a) Lauric acid(b) Oleic acid(c) Linolenic acid(d) Myristic acid
Palmitic acid and palmitoleic acid both have 16 carbons. Which molecule would have the lower melting point and why?
Draw the line-angle structure for the following:(a) Lauric acid(b) Stearic acid(c) Oleic acid(d) Arachidonic acid
Why does the presence of a double bond change the melting point of a fatty acid?
What is the difference between the $\Delta$ -system and the $\omega$ -system for indicating where a double bond is in a fatty acid?
What is hydrogenation and how is it used in food chemistry?
Draw the structural formula of a fat molecule (triglyceride) made of myristic acid, oleic acid, palmitic acid, and glycerol.
Oleic acid has a melting point of $16^{\circ} \mathrm{C}$. If you converted the cis double bond into a trans double bond, what would happen to the melting point? Explain.
Draw schematic formulas for all possible 1,3-diglycerides made up of glycerol, oleic acid, or stearic acid. How many are there? Draw the structure of one of the diglycerides.
For the diglycerides in Problem $12,$ predict which two will have the highest melting points and which two will have the lowest melting points.
Predict which acid in each pair has the higher melting point and explain why.(a) Palmitic acid or stearic acid(b) Arachidonic acid or arachidic acid
Which has the higher melting point: (a) a triglyceride containing only lauric acid and glycerol or (b) a triglyceride containing only stearic acid and glycerol?
Explain why the melting points of the saturated fatty acids increase as we move from lauric acid to stearic acid.
Predict the order of the melting points of triglycerides containing fatty acids, as follows:(a) Palmitic, palmitic, stearic(b) Oleic, stearic, palmitic(c) Oleic, linoleic, oleic
Look at Table $20.3 .$ Which animal fat has the highest percentage of unsaturated fatty acids?
Rank the following in order of increasing solubility in water (assuming that all are made with the same fatty acids $):(a)$ triglycerides, (b) diglycerides, and (c) monoglycerides. Explain your answer.
How many moles of $\mathrm{H}_{2}$ are used up in the catalytic hydrogenation of one mole of a triglyceride containing glycerol, palmitic acid, oleic acid, and linoleic acid?
What are the main types of complex lipids, and what are the main characteristics of their structures?
Which portion of the phosphatidylinositol molecule contributes to (a) the fluidity of the bilayer and (b) the surface polarity of the bilayer?
How do the unsaturated fatty acids of the complex lipids contribute to the fluidity of a membrane?
Which type of lipid molecule is most likely to be present in membranes?
What is the difference between an integral and a peripheral membrane protein?
Which glycerophospholipid has the most polar groups capable of forming hydrogen bonds with water?
Draw the structure of a phosphatidylinositol that contains oleic acid and arachidonic acid.
Among the glycerophospholipids containing palmitic acid and linolenic acid, which will have the greatest solubility in water: (a) phosphatidylcholine, (b) phosphatidylethanolamine, or (c) phosphatidylserine? Explain.
Name all the groups of complex lipids that contain ceramides.
Are the various phospholipids randomly distributed in membranes? Give an example.
Enumerate the functional groups that contribute to the hydrophilic character of (a) glucocerebroside and (b) sphingomyelin.
Cholesterol has a fused four-ring steroid nucleus and is a part of body membranes. The $-$ OH group on carbon 3 is the polar head, and the rest of the molecule provides the hydrophobic tail that does not fit into the zig-zag packing of the hydrocarbon portion of the saturated fatty acids. Considering this structure, predict whether small amounts of cholesterol that are well dispersed in the membrane will contribute to the stiffening (rigidity) or the fluidity of a membrane. Explain.
Where can pure cholesterol crystals be found in the body?
(a) Find all of the carbon stereocenters in a cholesterol molecule.(b) How many total stereoisomers are possible?(c) How many of these stereoisomers do you think are found in nature?
Look at the structures of cholesterol and the hormones shown in Figure $20.8 .$ Which ring of the steroid structure undergoes the most substitution?
What makes LDL soluble in blood plasma?
How does LDL deliver its cholesterol to the cells??
How does lovastatin reduce the severity of atherosclerosis?
How does VLDL become LDL?
How does HDL deliver its cholesteryl esters to liver cells?
How does the serum cholesterol level control both cholesterol synthesis in the liver and LDL uptake?
What physiological functions are associated with cortisol?
Estradiol in the body is synthesized starting from progesterone. What chemical modifications occur when estradiol is synthesized?
Describe the difference in structure between the male hormone testosterone and the female hormone estradiol.
Considering that $\mathrm{RU} 486$ can bind to the receptors of progesterone as well as to the receptors of cortisone and cortisol, what can you say regarding the importance of the functional group on carbon 11 of the steroid ring in drug and receptor binding?
(a) How does the structure of RU486 resemble that of progesterone?(b) How do the two structures differ?
What are the structural features common to oral contraceptive pills, including mifepristone?
List all of the functional groups that make taurocholate water-soluble.
Explain how the constant elimination of bile salts through the feces can reduce the danger of plaque formation in atherosclerosis.
What is the basic structural difference between:(a) Arachidonic acid and prostaglandin PGE $_{2}$ ?(b) $\mathrm{PGE}_{2}$ and $\mathrm{PGF}_{2 \alpha} ?$
Find and name all of the functional groups in(a) glycocholate,(b) cortisone,(c) prostaglandin $\mathrm{PGE}_{2}$ and(d) leukotriene B4.
What are the chemical and physiological functions of the COX-2 enzyme?
How does aspirin, an anti-inflammatory drug, prevent strokes caused by blood clots in the brain?
Suggest a reason why inorganic ions, such as $\mathrm{K}^{+}, \mathrm{Na}^{+}$ $\mathrm{Ca}^{2+},$ and $\mathrm{Mg}^{2+},$ do not cross biological membranes by simple diffusion.
Which statements are consistent with the known facts about membrane transport?(a) Active transport moves a substance from a region in which its concentration is lower to one in which its concentration is higher.(b) Transport does not involve any pores or channels in membranes.(c) Transport proteins may be involved in bringing substances into cells.
For each of the processes below, tell whether it is based on facilitated diffusion, primary active transport, secondary active transport, or a cell receptor:(a) LDL movement into the cell(b) Movement of glucose into the cell by glucose permease(c) Movement of lactose into the cell by galactoside permease(d) Exchange of $\mathrm{Na}^{+}$ and $\mathrm{K}^{+}$ ions across a membrane by the $\mathrm{Na}^{+} / \mathrm{K}^{+}$ pump
Which of the following explains how an oversupply of cholesterol affects LDL movement into the cell?(a) It blocks the access of LDL to the receptor(b) It prevents the LDL receptor from moving in the membrane(c) It inhibits the synthesis of the LDL receptor protein(d) It prevents the LDL receptor proteins from becoming integrated into the membrane.
(Chemical Connections $20 \mathrm{B}$ ) Compare the complex lipid structures listed for the lipid storage diseases with missing or defective enzymes. Explain why the missing enzyme in Fabry's disease is $\alpha$ -galactosidase and not $\beta$ -galactosidase.
(Chemical Connections $20 \mathrm{B}$ ) Identify the monosaccharides in the accumulating glycolipid of Fabry's disease.
(Chemical Connections $20 \mathrm{C}$ ) How does the oral anabolic steroid methenolone differ structurally from testosterone?
(Chemical Connections $20 \mathrm{C}$ ) In what ways do athletes use steroids to enhance athletic performance?
(Chemical Connections $20 \mathrm{C}$ ) Why were Mark McGwire and Floyd Landis not given the same penalties for taking steroids in their sports?
(Chemical Connections 20 D) What is the role of progesterone and similar compounds in contraceptive pills?
(Chemical Connections $20 \mathrm{E}$ ) How does cortisone prevent inflammation?
(Chemical Connections $20 \mathrm{E}$ ) How does indomethacin act in the body to reduce inflammation?
(Chemical Connections $20 \mathrm{E}$ ) What kind of prostaglandins are synthesized by COX-1 and COX-2 enzymes?
(Chemical Connections $20 \mathrm{E}$ ) Steroids prevent asthma-causing leukotriene synthesis, as well as inflammation-causing prostaglandin synthesis. Nonsteroidal anti-inflammatory agents (NSAIDs) such as aspirin reduce only prostaglandin production. Why do NSAIDs not affect leukotriene production?
(Chemical Connections $20 \mathrm{F}$ ) Define omega- 3 fatty acid.
(Chemical Connections $20 \mathrm{F}$ ) Why is very little heart disease found among people who eat a lot of fish?
What is the role of taurine in lipid digestion?
Draw a schematic diagram of a lipid bilayer. Show how the bilayer prevents the passage by diffusion of a polar molecule such as glucose. Show why nonpolar molecules such as $\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{O}-\mathrm{CH}_{2} \mathrm{CH}_{3}$ can diffuse through the membrane.
How many different triglycerides can you create using three different fatty acids $(\mathrm{A}, \mathrm{B}, \text { and } \mathrm{C}) ?$
Prostaglandins have a five-membered ring closure; thromboxanes have a six-membered ring closure. The synthesis of both groups of compounds is prevented by COX inhibitors; the COX enzymes catalyze ring closure. How can these facts be correlated?
Which lipoprotein is instrumental in removing the cholesterol deposited in the plaque on arteries?
What are coated pits? What is their function?
What are the constituents of sphingomyelin?
What is the difference between a facilitated transport and an active transporter?
Which part of LDL interacts with the LDL receptor?
What is the major difference between aldosterone and the other hormones listed in Figure $20.8 ?$
(Chemical Connections $20 \mathrm{E}$ ) The anti-inflammatory drug Celebrex does not have the usual side effect of stomach upset or ulceration commonly observed with the other NSAIDs. Why?
How many grams of $\mathrm{H}_{2}$ are needed to saturate $100.0 \mathrm{g}$ of a triglyceride made of glycerol and one unit each of lauric, oleic, and linoleic acids?
Prednisolone is the synthetic glucocorticoid medicine most frequently prescribed to combat autoimmune diseases. Compare its structure to the natural glucocorticoid hormone cortisone. What are the similarities and differences in structure?
You have just isolated a pure lipid that contains only sphingosine and a fatty acid. To what class of lipid does it belong?
Suggest a reason why the same protein system moves both sodium and potassium ions into and out of the cell.
Do all proteins associated with membranes span the membrane from one side to the other? Explain.
In the preparation of sauces that involve mixing water and melted butter, egg yolks are added to prevent separation. How do the egg yolks prevent separation? (Hint: Egg yolks are rich in phosphatidylcholine [lecithin].)
Which of the following statements is (are) consistent with what is known about membranes?(a) A membrane consists of a layer of proteins sandwiched between two layers of lipids.(b) The compositions of the inner and outer lipid layers are the same in any individual membrane.(c) Membranes contain glycolipids and glycoproteins.(d) Lipid bilayers are an important component of membranes.(e) Covalent bonding takes place between lipids and proteins in most membranes.
Suggest a reason why animals that live in cold climates tend to have higher proportions of polyunsaturated fatty acid residues in their lipids than do animals that live in warm climates.
Which statements are consistent with the fluid mosaic model of membranes?(a) All membrane proteins are bound to the interior of the membrane.(b) There is much asymmetry in the distribution of proteins and lipids in a membrane.(c) Some proteins and lipids undergo lateral diffusion along the inner or outer surface of the membrane.
Suggest a reason why the cell membranes of bacteria grown at $20^{\circ} \mathrm{C}$ tend to have a higher proportion of unsaturated fatty acids than the membranes of bacteria of the same species grown at $37^{\circ} \mathrm{C}$. In other words, the bacteria grown at $37^{\circ} \mathrm{C}$ have a higher proportion of saturated fatty acids in their cell membranes.
Lipids and carbohydrates are both vehicles for energy storage. How are they similar in terms of molecular structure, and how do they differ? What does the molecular structure of each class of substance imply about the polarity of typical molecules?
To what extent do lipids and carbohydrates play structural roles in living organisms? Do these roles differ in plants and in animals?
Which substances would you expect to consist primarily of carbohydrates and which primarily of lipids: olive oil, butter, cotton, cotton candy?
To what extent would you expect to find the following functional groups in lipids and in carbohydrates: aldehyde groups, carboxyl groups, ester groups, hydroxyl groups?
Sports drinks tend to contain large amounts of sugars, and some contain taurine in small amounts. Would you expect more of the effect of these performance aids to come from dietary carbohydrates or from the role of taurine in breaking down fats?
Which of the following foods consist primarily of carbohydrates and which of fats: soft drinks (not diet drinks), salad dressing, canned fruit, cream cheese?
The ester bonds in lipids do not give rise to macromolecules, but the amide bonds in proteins (Chapter 21 ) do. Comment on the underlying reason for this difference.
Given the structural differences between steroids and other kinds of lipids, would you expect the synthesis of steroids in living organisms to differ from the synthesis of other lipids?
What are stem cells (Hint: See Chapter 30 ), and how are they related to myeloproliferative diseases?
What is the reported link between cholesterol, HDL, and myeloproliferative diseases?
Some of the lipid molecules that occur in membranes are bulkier than others. Are the bulkier molecules more likely to be found on the cytoplasmic side of the cell membrane or on the side facing the exterior of the cell?
What are the functions of a cell membrane? To what extent is a bilayer that consists entirely of lipids able to carry out these functions?
Glycerophospholipids tend to have both a positive charge and a negative charge in their hydrophilic portions. Does this fact help or hinder lipid packing in membranes? Explain.
Leukotrienes differ from prostaglandins and thromboxanes in that they lack a ring closure. They also differ from prostaglandins and thromboxanes (and from all other lipids) in another feature of their structure. What is that structural feature? (Hint: It has to do with the position of their double bonds.)