MCB354FA08_ExamIIShtAnsKEY (1) - Short Answer(96 points...

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1 Short Answer (96 points total) Answer the following questions in the space provided. Please write legibly! 27. (3 points) Circle the fatty acid in each pair (a-c) that has the higher melting temperature. (a) 18:1 Δ 9 18:2 Δ 9,12 (b) 18:0 18:1 Δ 9 (c) 18:0 16:0 Ans: (a) 18:1 Δ 9 (b) 18:0 (c) 18:0 28. (8 points) Describe the dependence of the melting point of a fatty acid upon (a) chain length and (b) unsaturation; (c) explain these dependencies in molecular terms. Ans: All other things being equal, (a) the longer the acyl chain, the higher the melting temperature; and (b) the more unsaturation, the lower the melting temperature. (c) The melting temperature is a measure of the thermal energy needed to break the intermolecular interactions that stabilize the "solid" form of a lipid, which depends upon how well the individual lipid molecules fit into the nearly crystalline array of lipids. When a shorter acyl chain lies between two longer chains in a nearly crystalline array of lipid molecules, there is a cavity at the end of the short acyl group that allows freer motion to the neighboring acyl chains. A cis double bond introduces a "kink" into the acyl chain, so that it does not pack as easily with its straighter neighbors.
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2 29. (15 points) Draw the structure of phosphatidylserine in the ionic form it would have at pH 7. Ans: For this structure, see Fig. 10-8, p. 350. At neutral pH, there is a charge on the phosphate group, and serine is in the zwitterionic form; it has a protonated amino group and an ionized carboxyl group. 30. (10 points) An enzyme catalyzes a reaction at a velocity of 20 μ mol/min when the concentration of substrate (S) is 0.01 M. The K m for this substrate is 1 x 10 -5 M. Assuming that Michaelis-Menten kinetics are followed, what will the reaction velocity be when the concentration of S is (a) 1 x 10 -5 M and (b) 1 x 10 -6 M? Ans:
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This note was uploaded on 12/09/2010 for the course MCB 354 taught by Professor Orlean during the Spring '09 term at University of Illinois at Urbana–Champaign.

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MCB354FA08_ExamIIShtAnsKEY (1) - Short Answer(96 points...

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