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Chap. 10. Problem 2. Part (a). The melting points of fatty acids are strongly influenced by the number of cis double bonds present in the fatty acid acyl chain. Cis double bonds cause acyl chains to adopt a kinked (bent) conformation in a crystal lattice, and the poorer packing (relative to straight acyl chains) decreases melting point. This explains the decline in the melting points of the 18-carbon fatty acids series listed above (# of double bonds: stearic acid, 18:0; oleic acid, 18:1; linoleic acid, 18:2; linolenic acid, 18:3). Part (b). There are six possible triacylglycerols that can occur. These can be represented as OOO, OOP, OPO, POP, OPP, & PPP, based on the fatty acids attached to the glycerol backbone. Because the melting point of oleic acid (O) is less than palmitic acid (P), the order of their melting points (from low-to-high) is OOO < OOP = OPO < POP = OPP < PPP. Part (c). Branched-chain fatty acids do not pack as well as straight-chain species and thus have lower melting points and impart greater fluidity to membranes.
Chap. 10. Problem 4. The five-carbon isoprene units in geraniol, farnesol, and squalene are circled in the diagram.
Chap. 10. Problem 10. Grease produced by cooking is predominantly animal fat consisting of triacylglycerols. The ester linkages in triacylglycerols are readily hydrolyzed by treatment with strong base (e.g., sodium hydroxide). Treatment with strong base produces glycerol and 3 fatty acids from a triacylglycerol. These hydrolysis products are much more soluble in water than triacylglycerols themselves. The cleavage products can be washed away with water.
Chap. 10. Problem 16. The two products produced by phospholipase C cleavage of phosphatidyl 4,5-bisphosphate are diacylglycerol and inositol 1,4,5-trisphosphate (IP3). Both of these cleavage products are second messengers that activate intracellular pathways used in vasopressin signaling (Chap. 12). Diacylglycerol is a hydrophobic lipid that remains in the cytoplasmic membrane in which the cleavage event occurs. IP3, on the other hand, is very polar and is released into the cytoplasm of the cell.
Chap. 10. Problem 17. Water-soluble vitamins are subject to excretion by the kidneys on a daily basis. Fat-soluble vitamins (A, D, E, K) are lipid-soluble and thus partition into fat stores in body tissues. These tissues therefore provide a reservoir of fat-soluble vitamins that can serve the individual for months.
Chap. 10. Problem 20. Silica gel is a polar matrix than binds polar lipids more strongly than nonpolar lipids. Thus nonpolar lipids will elute from a silica gel column earlier than polar lipids. Based on consideration of the hydrophobic-hydrophilic balance for the mentioned lipids, cholesteryl palmitate and triacylglycerol should elute from the column first. Cholesterol and n-tetradecanol being somewhat more polar due to their free hydroxyl groups would elute next. The neutral phospholipids phosphatidylcholine and phosphatidylethanolamine would then elute, followed by sphingomyelin, which has a slightly greater content of polar functional groups. Finally, the negatively charged phosphatidylserine and palmitate would elute from the column. Of these two lipids phosphatidylserine would elute first because it has a lower charge-to-mass ratio than palmitate.
