AAMC Section Bank Question

irenebees
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AAMC Section Bank Question

Postby irenebees » Mon Apr 10, 2017 6:02 pm

In the Chemistry section #4, the passage gives a figure with wavelength, power, and pulse duration and the question asks "which laser is suitable for the MALDI technique after it's frequency is doubled?" I was between C and D

C - Laser C: wavelength 650 nm, power 1.5 mW
D - Laser D: wavelength 532 nm, power 1.5 mW

I'm not sure how to arrive at what power it is supposed to be and why D is correct over C.
NS_Tutor_Andrew
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Re: AAMC Section Bank Question

Postby NS_Tutor_Andrew » Tue Apr 11, 2017 11:01 am

Hi irenebees,

Excellent question. This is one where understanding what the question is asking is probably harder than the underlying science. The first step -- and one that may be so automatic that you don't even think about -- is to realize that the MCAT doesn't expect you to have any preexisting knowledge about MALDI, so your information is limited to what's presented in the passage, and in particular Table 1.

This question is essentially saying: double the frequency of lasers A-D and see which corresponds to an entry in Table 1. Since we don't get any specific information about frequency, we have to translate this into terms of wavelength. Doubling frequency halves the wavelength, so the way we'll search for a correct answer here is to halve the wavelength of each laser and see if it corresponds to something in Table 1. Let's go through the answer choices 1 by 1:

Laser A: --> halve wavelength = wavelength 413 nm, power 1.2 mW, doesn't equal anything in Table 1--> ELIMINATE.

Laser B: halve wavelength = wavelength 357 nm, power 1.2 mW, doesn't equal anything in Table 1 --> ELIMINATE.

Laser C: halve wavelength = wavelength 325 nm, power 1.5 mW, doesn't equal anything in Table 1 --> ELIMINATE.

Laser D: halve wavelength = wavelength 266 nm, power 1.5 mW, this one does equal the first laser given in Table 1 --> CORRECT.

Note that the only science knowledge you have to use here is the knowledge that frequency and wavelength are inversely related (v = λf). You definitely don't have to recognize some non-obvious point about which power thresholds are most appropriate. The hard thing about this question is recognizing what they're looking for, and in particular that as far as we're concerned (since we can only operate within the constraints of the passage), the only two lasers in the world suitable for MALDI are those given in Table 2.

Hope this helps and best of luck studying!
Andrew D.
Content Manager, Next Step Test Prep.
irenebees
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Re: AAMC Section Bank Question

Postby irenebees » Fri Apr 14, 2017 8:37 pm

Hi Andrew,

Thanks for your help on that one. I actually have another one as well: C/P MCAT Section Bank # 52. It asks about the how the students monitor the rate of reaction and I saw that it was yellow and know that ROYGBIV is 700-400 so I chose B because it was the only one close to those numbers but I would have chosen something around 500-600 if given the option. How come in the answer it talks about the complementary color? Why would I need to look at the complementary color?
NS_Tutor_Andrew
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Re: AAMC Section Bank Question

Postby NS_Tutor_Andrew » Fri Apr 14, 2017 10:24 pm

Hi irenebees,

First off, good work getting the most out of what you do know -- that's a hugely important skill for the MCAT!

Your question touches on a really important principle, though. The way that color works is that we see the wavelengths of light that an object doesn't absorb. These wavelengths of light get bounced off of the object and make it to our eyes. The wavelengths that do get absorbed, on the other hand, will never make it to our eyes. The idea in this question is that if the solution turns yellow, then it must be absorbing the wavelengths of light that are complementary to yellow -- that is, purple, which has a wavelength of ~360 nm.

This site has a good review of absorption, reflection, and transmission of light: http://www.physicsclassroom.com/class/light/Lesson-2/Light-Absorption,-Reflection,-and-Transmission, and this page has a good discussion of the same principles that talks a little bit more about how complementary colors fit into all of this: https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/uv-vis/spectrum.htm.

Hope this helps and good luck!
Andrew D.
Content Manager, Next Step Test Prep.
bklynite
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Re: AAMC Section Bank Question

Postby bklynite » Fri May 12, 2017 3:26 pm

Regarding question 39 in the CP section bank:

The question premises that binding is fast relative to subsequent catalytic steps. Im going to induce (perhaps wrongly) that had the opposite been true - binding is slow relative to consequent steps - that the answer would also be the inverse - substitution effects binding more that catalytic turnover. I dont understand why that would be true though; the Kcat is still decreasing by many more orders of magnitude
NS_Tutor_Andrew
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Re: AAMC Section Bank Question

Postby NS_Tutor_Andrew » Fri May 12, 2017 5:10 pm

Hi bklynite,

My sense of this is that assuming that binding is much faster than subsequent catalytic steps is just an assumption that helps you analyze the data straightforwardly, because otherwise you could build up more complicated scenarios around the question -- for instance, you could say, oh, well, if binding takes a long time, then changing from a charged to a hydrophobic residue (E>A) could dramatically affect Km independently of the data they give you, which could make it harder to interpret the data. In contrast, if you're told essentially not to worry about binding time, that allows you to just focus on the data directly.
Andrew D.
Content Manager, Next Step Test Prep.
Squiter
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Re: AAMC Section Bank Question

Postby Squiter » Mon Dec 04, 2017 12:00 pm

That was actually interesting. I thought the answer is C. How wrong was i...

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