How can the answer to this GRE question be wrong???

Discussion in 'Science' started by ryobi, Mar 25, 2020.

  1. ryobi

    ryobi Well-Known Member

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    Is the answer to this GRE Biology question wrong???

    In the presence of alcohol dehydrogenase, the rate of reduction of acetaldehyde to ethanol increases as the concentration of acetaldehyde is increased. Eventually, the rate of the reaction reaches a maximum, after which point further increases in the concentration of acetaldehyde have no effect. Which of the following is true at the maximal rate of reaction?

    The test says A is the right answer (A) Nearly all of the enzyme molecules are interacting with acetaldehyde molecules, but shouldn't it be C

    ( C ) The change in free energy of the reaction decreases

    The binding energy is the free energy released when enzyme combinds with substrate. This energy is used both to bind substrate and for catalysis


    These are the the multiple choice questions and why I ruled them out

    (A) Nearly all of the enzyme molecules are interacting with acetaldehyde molecules.

    Except for the very earliest moment in the reaction when acetaldehyde and enzyme molecues were finding each other all the enzyme molecules were reacting with the acetaldehyde

    ( B ) The activation energy of the reaction decreases


    There’s an inverse relationship between the activation energy and the rate of the RxN as the activation energy goes down the rate of the reaction goes up Therefore, I can eliminate B

    ( C ) The change in free energy of the reaction decreases

    The binding energy is the free energy released when enzyme combinds with substrate. This energy is used both to bind substrate and for catalysis



    ( D ) The enzyme is no longer specific for acetaldehyde

    Enzymes are very specific for their substrates so I can eliminate D


    ( E ) The Enzyme has mostly been used up in earlier phases of the reaction


    No, because enzymes increase the rate of the reaction without being changed therefore the enzyme was not, ‘used up’ in earlier phases of the RxN Therefore, I can eliminate E
     
    Last edited: Mar 25, 2020
  2. Grey Matter

    Grey Matter Active Member

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    I think A is correct. Since the question is about the kinetics of the reaction then C, I believe is independent of species concentration. Free energy and heats of reaction are both independent of concentration and do not effect the kinetics of a reaction. I agree with your reasons for eliminating the others.

    It is kinda a bogus question though - reaction kinetics, at least when I finished up my studies back in 95 lacked any theoretical basis. Still fascinates me a bit I suppose. Thermo/PChem can tell us exactly where the species will distribute upon reaching equilibrium, but we had no theoretical basis 25y ago to really explain how fast a reaction would go. Catalytic reactions maybe even less.

    I could be missing a lot here also because I'm equaling a biochem enzyme reaction to a catalytic reaction - they're pretty much the same, yes?
     
    Last edited: Mar 29, 2020 at 1:19 AM
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  3. ryobi

    ryobi Well-Known Member

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    I believe so. Enzymes weaken and help break the bonds between reactants (a catalytic reaction) and enzymes also help form bonds creating products. So enzymes catalyze both sides of the reaction.
     

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