Problem 2
Structure of an Omega-3 Fatty Acid The omega-3 fatty acid docosahexaenoic acid (DHA, \(\left.22: 6\left(\Delta^{4,7,10,13,16,19}\right)\right)\) is the most abundant omega-3 fatty acid in the brain and an important component of breast milk. Draw the structure of this fatty acid.
Problem 3
Melting Points of Lipids The melting points of a series of 18-carbon fatty acids are stearic acid, \(69.6^{\circ} \mathrm{C}\); oleic acid, \(13.4^{\circ} \mathrm{C}\); linoleic acid, \(-5{ }^{\circ} \mathrm{C}\); and linolenic acid, \(-11{ }^{\circ} \mathrm{C}\). 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 membrane (that is, give the lipids a lower or a higher melting point)? Why?
Problem 4
Catalytic Hydrogenation of Vegetable Oils Catalytic hydrogenation, used in the food industry, converts double bonds in the fatty acids of the oil triacylglycerols to \(-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\) How does this affect the physical properties of the oils?
Problem 5
Impermeability of Waxes What property of the waxy cuticles that cover plant leaves makes the cuticles impermeable to water?
Problem 7
Chemical Reactivity of Lipids Soaps are salts of fatty acids and can be made by mixing triacylglycerols with a strong base such as \(\mathrm{NaOH}\). This saponification reaction produces glycerol and fatty acid salts. In a lab experiment, students saponify the triacylglycerol tristearin in the presence of \({ }^{18}\) O-labeled water. What saponification reaction products will contain the \({ }^{18} \mathrm{O}\) label?
Problem 9
Deducing Lipid Structure from Composition A biochemist completely digests a glycerophospholipid with a mixture of phospholipases A and D. HPLC and MS analysis reveals the presence of an amino acid of \(105.09 \mathrm{Da}\), a saturated fatty acid of \(256.43 \mathrm{Da}\), and an omega- 3 monounsaturated fatty acid of 282.45 Da. Which amino acid does the glycerophospholipid contain? Draw the most likely structure of this glycerophospholipid.
Problem 10
Deducing Lipid Structure from Molar Ratio of Components Complete hydrolysis of a glycerophospholipid yields glycerol, two fatty acids \(\left(16: 1\left(\Delta^{9}\right)\right.\) and \(\left.16: 0\right)\), phosphoric acid, and serine in the molar ratio \(1: 1: 1: 1: 1\). Name this lipid and draw its structure.
Problem 11
Lipids in Blood Group Determination We note in Figure 10-13 that the structure of glycosphingolipids determines the blood groups A, B, and O in humans. It is also true that glycoproteins determine blood groups. How can both statements be true?
Problem 13
Intracellular Messengers from Phosphatidylinositols The hormone vasopressin is an extracellular signal that activates a specific phospholipase \(\mathrm{C}\) in the membrane. Cleavage of \(\mathrm{PIP}_{2}\) by phospholipase \(\mathrm{C}\) generates two products. What are they? Compare their properties and their solubilities in water, and predict whether either would diffuse readily through the cytosol.
Problem 15
Hydrolysis of Lipids Name the products of mild hydrolysis with dilute \(\mathrm{NaOH}\) of a. 1-stearoyl-2,3-dipalmitoylglycerol b. 1-palmitoyl-2-oleoylphosphatidylcholine.
