ch10_selProbs-key - chapter Lipids 1. Operational...

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Unformatted text preview: chapter Lipids 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? Answer The term "lipid" does not specify a particular chemical structure. Whereas one can write a general formula for an amino acid, nucleic acid, or protein, lipids are much more chemically diverse. Compounds are categorized as Lipids based on their greater solubility in organic solvents than in water. 2. Melting Points of Lipids The melting points of a series of 13carbon fatty acids are: stearic acid, 69.5 “’8', oleic acid, 13.4 “C; linoleic acid, —5 “C; and linolenic acid, — 11 °C-. What structural aspect of these lS—carbon fatty acids can be correlated with the melting point? Draw all the possible tIiacylglycerols that can be constructed from glycerol, palmitic acid, and oleic acid. Rank them in order of increasing melting point. Branched—chain fatty acids are found in some bacterial membrane lipids. 1ir‘lfould their presence increase or decrease the fluidity of the membranes {that is, give them a lower or higher melting point}? Why? (a) [bl ('3) Answer (a) [bl ('1') The number of cis double bonds (stearic acid, 13:0, oleic, 13:1; linoleic, 18:2; linolenic, 18:3]. Each cis double bond causes a bend in the hydrocarbon chain, and bent chains are less well packed than straight chains in a. crystal lattice. The lower the extent of packing, the lower the melting temperature. Six different triacylglycerols are possible: one with glycerol and only palmitic acid {PPP}; one with glycerol and only oleic acid (DOD); and four with glycerol and a mixture of oleic and palmitic acids. Four mixed triacylglycerols are possible, because the three carbons of glycerol are not equivalent: thus GDP and CPU are positional isomers, as are POP and OPP. The greater the content of saturated fatty acid {P}, the higher the melting point. Thus, the order of melting points is 000 a: GOP = 0P0 «a: PDP = lDPP a: PPP. See Table 10—1 and Figure 10—3 for information on how to draw the triacylglycerols. Branched-chain fatty acids will increase the fluidity of membranes [i.e., lower their melting point) because they decrease the extent of packing possible within the membrane. The effect of branches is similar to that of bends caused by double bonds. u 9. Catalytic Hydrogenation of 1it"egetable Oils Catalytic hydrogenation, used in the feed mdustm, cen- verts double bonds in the fatty acids of the 01.1 triacylglycerels to —CH,2—CH,2—. How does this affect the physical properties of the oils? Answer it. reduces double bonds, which increases the melting point of lipids containing the fatty acids. 12. Dedncing Lipid Structure from Mnlar Ratio of Components Complete hydrolysis of a glyc- erophosphohpid yields glycerol! two fatty acids {lEzlmg} and 16:0], phosphoric acid, and serine in the molar ratio 1:1:1:1:1_ Name this. Lipid and draw its structure. Answer || CHE—D—P—O—CHg—CH—Itfl-Ig | 4)" COO— Phosphatidylserine 14- The Action of Phospholipases The venom of the Eastern diarnondback rattler and the Indian cobra ' contains phospholipase A3. which catalyzes the hydrolysis of fatty acids at the (3-2 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. dissolying the membranes of erythrocytes and lysing the cells. Extensive hemolysis may be life-ti‘u‘eatening. (a) All detergents are amphipathic. What are the hydrophilic and hydrophobic portions of lysolecithin‘? [bi The pain and inflammation caused by a snake bite can be treated 1s-‘lth certain steroids. What is the basis of this treatment? (I?) Though the high levels of phospholipase A: can be deadly, this enzyme is necessary for a variety of normal metabolic processes. TWhat are these processes? Answer (a) The free —OH group on 0-2 and the phosphocholine head group on 0-3 are the hydrophilic portions; the fatty acid on (3-1 of the lysolecithin is the hydrophobic portion. (b) lCertain steroids such as preerisone inhibit the action of phospholipase A2. the enzyme that releases the fatty acid arachidonate from the {3—2 position of some membrane glycerophospholipids. Aractudonate is converted to a variety of eicosanoids, some of 1a-‘hich cause inflammation and pain. (c) Phospholipase A2 is necessary to release arachidonate from certain membrane glycerophopholipids. Arachidonate is a precursor of other eicosanoids that have vital protective functions in the body. The enzyme is also important in digestion, breaking down dietary glycerophospholipids. 15. Lipids in Blood Group Determination We note in Figure Ill—15 that the structure of glycosphin— golipids determines the blood groups A, B, and 0 in humans. It is also true that glycoproteins deter- mine blood groups. How can both statements be true? Answer The part of the membrane lipid that determines blood type is the oligosaccharide in the head group of the membrane sphingolipids [see Fig. 10—15, p. 355). This same oligosaccha— ride is attached to certain membrane glycoproteins. which also serve as points of recognition by the antibodies that distinguish blood groups. u 17. Storage of Fat-Soluble Vitamins In contrast to water-soluble vitamins, wluch must be part of our daily diet, fatrsoluble vitamins can be stored in the body in amounts sufficient for man}.I months. Sug- gest an explanation for this difference. Answer Unlike water-soluble compounds, lipid-soluble compounds are not readily mobilized— that 151 they do not readily pass into aqueous solution. The body‘s lipids profide a reservoir for storage of lipid-soluble vitamins. Water-soluble vitamins cannot be stored and are rapidly re- moved from the blood by the kidneys. ...
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This note was uploaded on 05/01/2011 for the course CH 53890 taught by Professor Raymonds during the Spring '09 term at University of Texas at Austin.

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ch10_selProbs-key - chapter Lipids 1. Operational...

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