The question that I chose was “Why does this graph level off?” This question appears in Lecture 10, on slide 12. It also has a diagram of a graph with [substrates] as the x axis and initial velocity as the y axis. The concept that is being tested by this question is the students ability to interpret enzyme activity graphs. In order to to answer this question correctly, you need to understand how to interpret what a changing substrate concentration does to an enzymes Vmax. Understanding what initial velocity means is important. You also need to understand what Vmax means, because knowing that will eliminate some of the incorrect answers. Understanding what would cause an enzyme's rate to slow down would enable you to answer this question.
For this question, the correct answer is B, enzyme active sites are filled. This is correct because the enzyme reaches Vmax when the graph is starting to level off. Vmax is reached when the enzymes in the system are all working at maximum rate, which is when all of the enzymes are filled. Answer A, “the substrate is used up,” is incorrect because where the graph levels off, the substrate concentration is not zero. In addition to that, the y axis is initial velocity, which means there would be no time for the substrate to have been used up. Answer C, “the reaction reaches equilibrium,” is incorrect because the graph makes no mention of the reverse reaction, and thus no mention of equilibrium. The graph only illustrates the rate of the enzymes which are performing the forward reaction. Answer D, “the rate of the reverse reaction becomes significant” is incorrect because that would indicate that the graph illustrates a system over time. The y axis shows that the dots mean initial velocity, which means that the reverse reaction would not have time to become significant.
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