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F09 EXAM 2 comments

F09 EXAM 2 comments - BioG 1101 Cornell University Exam 1...

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BioG 1101 Exam 1 Comments 2009 Cornell University Cole Gilbert Nov. 12, 2009 1 EXAM 2 COMMENTS Average score = 23 out of a possible 35 points (= 66%) Standard deviation = 5 points (14%) This second prelim will count for about 24% (11 lectures) of your final grade in BioG 1101 Answers: (including alternate answers accepted in Bold ) 1) C 1 pt 6) E 2 pts 11) A C 1 pt 16) B 1 pt 21) B 1 pt 26) E 1 pt 31) B 2 pts 2) C 1 pt 7) B 1 pt 12) E 1 pt 17) A 1 pt 22) E 1 pt 27) D 1 pt 32) B 1 pt 3) B 1 pt 8) C 1 pt 13) C 1 pt 18) E 1 pt 23) B 1 pt 28) B 1 pt 4) B 1 pt 9) A 1 pt 14) C 1 pt 19) A 1 pt 24) B 1 pt 29) A E 1 pt 5) D 1 pt 10) D 1 pt 15) D 1 pt 20) C D 2 pts 25) C 1 pt 30) A C 1 pt Comments on selected questions. 1. About half of all students recognized that water is typically moved across membranes by cells using energy to transport a solute molecule and water follows passively by osmosis (choice C). The most popular incorrect answer was choice B, which indicates that the flow of water through aquaporin channels is energy dependent. It is not. Water moves passively through aquaporin channels. They just make it easier for polar water to move through the lipid bilayer membrane. There was an argument in the oral comment session for choice D, based on the idea that contractile vacuoles are mechanical contractions of the plasma membrane that moves water from the inside of a cell to the outside. While this is true, such vacuoles are specialized and occur in a limited number of protozoan species. The question asks about typical energy expenditure of cells. 2. A majority of students answered this one correctly, recognizing that it is the number of reactants with energy greater than the activation energy that determines the rate of the chemical reaction (choice C). Please refer to Fig. 8.14. If only a few reactants have enough energy to reach the unstable transition state the reaction may not move forward very often, whereas if many of the reactants have the requisite energy the reaction will move forward frequently. The most popular incorrect answer was choice A, delta G. This does not really determine the rate, nor does the sign of delta G (choice E). If delta G is negative the reaction can occur spontaneously, but the rate may be exceedingly slow if the reactants do not have the requisite energy. Similarly, choice B, enthalpy and entropy changes, is just a restatement of A, because delta G = delta H – T delta S. 3. This question was answered correctly by 85% of students. Well done. 4. A majority of students recognized that the enzyme substrate does not bind to an allosteric site to inhibit enzyme activity. Some students argued that the material that binds to the allosteric site could be a substrate for some other enzyme, making statement B true. While true, I didn’t find that reasoning compelling. The most popular other incorrect answers, C and E, were discussed in class and the text.
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