AAMC CBT3 and 3R OFFICIAL Q&A

[Vihsadas]

This is the official Q&A thread for AAMC CBT3 and 3R.

Please post ONLY questions pertaining to AAMC CBT3 and 3R.
Out of respect for people who may not have completed the other exams, do not post questions or material from any other AAMC exam.

Please see this thread for the rules of order before you post.

Good luck on your MCAT!

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[jdla]

Why does the fate of developing cells depend on cell position?

 

[Vihsadas]

Why does the fate of developing cells depend on cell position?

Think of the development of the embryo in terms of the neural tube and notochord. How do these specialized cells develop? What causes them to be placed in the position that they are?

Remember that the paracrine signaling functions of various embryon cells often cause the induction of nearby cells to specific functional fates. There are many such examples relevant to the MCAT.

One is the development of the notochord/neural tube.
Another is the development of the eye from the ectoderm.

In both of these examples, it is the positions of the cells relative to other embryonic cells that determines what their differentiated function will be. To reiterate, this is because of paracrine signals from nearby cells inducing other cells to specific fates.

 

[jdla]

What does the 64 cell stage mean?

 

[bluemonkey]

What does the 64 cell stage mean?

This is part of development, occurring early on when the zygote is undergoing cleavage and compaction.

 

[tncekm]

Why does the fate of developing cells depend on cell position?

For MCAT remember that it does, the why isn't as important.

Know that initially in our development (while cleavage is occuring) our cells are totipotent meaning that each cell could give rise to a whole individual. Eventually something happens where each cell is then determined to develop into something distinct from the other cells. While the cells are totipotent if you take one cell and move it to the position of a different cell in the morula you still get a normally developed individual. After determination if you move a cell to a different part you may end up with something like a leg developing out of a back, etc.

 

[jdla]

Can someone explain why co dominance is the answer. I was thinking that the mother was the carrier.

 

[tncekm]

Post the question.

 

[godchaser1016]

I have no idea which part of the passage that I should refer back to.

 

[tncekm]

#67 doesn't have anything to do with the passage, its unrelated. Its "A". Think about it... if you had a spherical balloon and started spinning it very fast, would its shape change, and how? If you have a ball at the end of a string, as you spin the ball-string apparatus faster and faster what happens to the radius of rotation?

#70 also doesn't have any passage based information, but the answer should be "B". Its expecting you to know the formula v=d/t (which can be rearrange to t=d/v) and to know that electromagnetic radiation (radio waves) travel at 3x10^8m/s.

 

[godchaser1016]

THANK YOU for your detailed explanation!

 

[TypeSH07]

Question 114 CBT. Why will a leak in the apparatus increase the pressure? Makes it harder for the oil to travel up the column?

 

[tncekm]

The apparatus is a vacuum, so if there is a leak in a vacuum atmospheric gases will flow in and increase the pressure.

 

[godchaser1016]

I understand why (c) is the answer, but i also think (d) is a correct answer. Any thoughts guys?

 

[godchaser1016]

I thought this question is a negative feedback question and picked (a). What's wrong with my reasoning?

 

[BloodySurgeon]

I don't have passage IX so I'm assuming that it is the same as the CBT version (because I have only the deleted passages and CBT version). So maybe if you explain the question ppl could answer it.

 

[tncekm]

I thought this question is a negative feedback question and picked (a). What's wrong with my reasoning?

#195. All ADH does is increase the collecting ducts permeability to water, increasing the concentration of the urine. So, that won't play a role in clearance, just water reabsorption and urine concentration..

#204 When a person has osteoporosis they are losing bone density (losing calcium). Osteoclasts will remove calcium from the bone and osteoblasts will deposit it. So, a person with osteoporosis will have increased osteoclast activity and decreased osteoblast activity resuling in a net loss of calcium, and net reduction in bone density.

 

[Dr Lyss]

So I just took the AAMC practice test 3 and I wanted to know if anyone else has taken it and how it compares to the other AAMC tests (harder, easier?) I've been doing much less than desired on TPR exams but I got a 8P 10V 9B on the AAMC. Any one else take AAMC 3 (the free one)?

 

[tncekm]

I only took 3 and 5, and maybe 5 was a bit tougher, but I think that most people say 3 was way easier than it actually is b/c its usually the first one they took, and "in hindsight" things always seem much easier.

TPR is much harder in the PS and VR sections for most people, so your AAMC 3 score is a better predictor of how you'll perform on the MCAT.

 

[tourniquet1963]

So I just took the AAMC practice test 3 and I wanted to know if anyone else has taken it and how it compares to the other AAMC tests (harder, easier?) I've been doing much less than desired on TPR exams but I got a 8P 10V 9B on the AAMC. Any one else take AAMC 3 (the free one)?

wow you did very well on aamc 3 verbal... i thought that verbal was brutal.. i managed to walk out with an 8.

 

[Dr Lyss]

thanks. I think its half due to pure luck and half due to strategy. I've been using the TPR process of elimination strategies and things like that, but on the harder passages I just guessed as much as I could! Thanks for the posts

 

[tourniquet1963]

thanks. I think its half due to pure luck and half due to strategy. I've been using the TPR process of elimination strategies and things like that, but on the harder passages I just guessed as much as I could! Thanks for the posts

Interesting, i'll definitely look into that.

 

[Deepa100]

I took it last night and got:

PS: 10

VR: 6

BS: 9

I also did the writing samples. I was pretty tired when I took it which may be why my VR score is so low

 

[RySerr21]

I only took 3 and 5, and maybe 5 was a bit tougher, but I think that most people say 3 was way easier than it actually is b/c its usually the first one they took, and "in hindsight" things always seem much easier.

TPR is much harder in the PS and VR sections for most people, so your AAMC 3 score is a better predictor of how you'll perform on the MCAT.

dude, WTF, i just took AAMC 5 and i literally had no idea on any of the questions. even the biology section which is by far my best section i got an 8.......

i've taken 3, 10, and 9 and improved on each one...but i dunno. somethin about 5 kicked my ***.

on a good note I think i realized what my low low low score is....so i know that even if i walk out feeling like i guessed on every single question (which is how i felt on stupid test 5) i wont be that bad off.

 

[tncekm]

Well, I hope 5 was a hard one, b/c I felt confident in it, did pretty well on it, and most of my mistakes were misreads and calc errors (I love how the MCAT always provides the exact power of 10 error that I make as an answer...) Considering how "all over the place" my VR is, I was super happy to get an 11 on it. AAMC #5 -- PS: 11, VR: 11, BS: 12 Since then I've seen my EK scores go 11 to 8 to 10 to 7, so I'm flipping out.

 

[TypeSH07]

Can anyone explain #4 on the PS. If a compound is more soluble then does that mean it is more likely to form a precipitate? Either way I keep thinking A is right and not B....

 

[tncekm]

Yeah, I originally got that one wrong, too. So, here is my best shot at explaining it.

In short, in the first step Pb(SO4) (Compound A) precipitated after Na2SO4 was added to aqueous Pb(NO3)2. This tells us Pb(SO4) is less soluble then Pb(NO3)2.

The second step involved adding KI and agitating the solution until some Pb(SO4) (Compound A) was dissolved. The result was that PbI precipitated, so PbI2 is less soluble than Pb(SO4).

In the third step PbI2 was then mixed with Na2CO3 and all of the PbI2 was converted to PbCO3, which precipitated out of solution. So, This tells us that PbCO3 is less soluble than PbI2.

Given that, if we now know the following in terms of solubility:

PbCO3 < PbI2 < Pb(SO4) < Pb(NO3)2

So, on to the answers:

(a) SO4 then I --- No, from our trend above you can see that PbI is less soluble than Pb(SO4), so I would precipitate as PbI2 BEFORE SO4 would precipitated as PbSO4
(b) CO3 then I -- Correct order
(c) SO4 then CO3 -- No, see explanation to (a)
(d) I then CO3 -- No, see explanation to (a)

 

[TypeSH07]

Yeah, I originally got that one wrong, too. So, here is my best shot at explaining it.

In short, in the first step Pb(SO4) (Compound A) precipitated after Na2SO4 was added to aqueous Pb(NO3)2. This tells us Pb(SO4) is less soluble then Pb(NO3)2.

The second step involved adding KI and agitating the solution until some Pb(SO4) (Compound A) was dissolved. The result was that PbI precipitated, so PbI2 is less soluble than Pb(SO4).

In the third step PbI2 was then mixed with Na2CO3 and all of the PbI2 was converted to PbCO3, which precipitated out of solution. So, This tells us that PbCO3 is less soluble than PbI2.

Given that, if we now know the following in terms of solubility:

PbCO3 < PbI2 < Pb(SO4) < Pb(NO3)2

So, on to the answers:

(a) SO4 then I --- No, from our trend above you can see that PbI is less soluble than Pb(SO4), so I would precipitate as PbI2 BEFORE SO4 would precipitated as PbSO4
(b) CO3 then I -- Correct order
(c) SO4 then CO3 -- No, see explanation to (a)
(d) I then CO3 -- No, see explanation to (a)

Okay that makes sense. So then they are inversely related? The higher Ksp, the more likely it will dissolve, makes it less likely to precipitate out? THanks.

 

[tncekm]

Well, you didn't need to know Ksp values for this situation, and it would be slightly complicated by the fact that some of these compounds dissociate into three ions while others dissociate into two, but I guess you can generally say that if all things were equal then the higher Ksp value means more solubility, and if its more soluble, then yes its less likely to precipitate.

 

[TypeSH07]

Well, you didn't need to know Ksp values for this situation, and it would be slightly complicated by the fact that some of these compounds dissociate into three ions while others dissociate into two, but I guess you can generally say that if all things were equal then the higher Ksp value means more solubility, and if its more soluble, then yes its less likely to precipitate.

Would you mind explaining #47 also? Thanks.

 

[tncekm]

Would you mind explaining #47 also? Thanks.

Sure, Ka for the first hydrogen is much greater than for the second, so we already know that:

HSO4 > SO4 in concentration just by that fact alone.

Now, we need to know if H2SO4 is more or less concentrated than SO4, how do we do that? Well, I misread this one and got it wrong as well when I took this test, the answer ends up being very simple!

98% H2SO4 is 98% H2SO4 and 2% H2O. Given there is such a low concentration of water for H2SO4 to ionize in, there is likely a VERY HIGH concentration of H2SO4 b/c the proton cannot dissociate if there is no water to solvate it!

So, SO4 would have the very lowest concentration.

 

[DrMattOglesby]

The following comes from AAMC CBT 3:

In which organelle of a eukaryotic cell is the pyrimidine uracil, as part of uridine triphosphate (UTP), incorporated into nucleic acid?

A) The nucleus

The nitrogenous base, uracil, combined with the sugar ribose and phosphate makes up the nucleotide uridine. It is found in RNA, but not in DNA. The corresponding DNA nucleotide is thymine. Uridine is incorporated into RNA in the nucleus where transcription of DNA into RNA takes place. RNA is manufactured in the nucleus from a DNA template. Therefore, the correct answer is answer choice A. RNA is necessary for protein synthesis by free ribosomes and those attached to endoplasmic reticulum, but it is not synthesized there, so answer choices C and D are incorrect. Proteins are packaged for export in the Golgi apparatus, but it too takes no part in RNA synthesis, so answer choice B is incorrect.

B) The Golgi bodies
CThe ribosomes
D) The endoplasmic reticulum

I selected choice C because I reasoned that ribosomes were constructed of ribosomal RNA. rRNA, being a form of RNA, would use uridine triphosphate.
*did i think too much into that question?

 

[iA-MD2013]

Gah I was going to take 3 today...you just ruined the question for me, Matt!!
The fact that it said where is it "incorporated" makes me feel A is the better answer. But, I completely see where you're coming from and it's a good thing you caught that. This question needs to be read question carefully.

 

[DrMattOglesby]

lol my bad. didnt mean to steal your thunder on that.
anyhow, i suppose the key word is "incoporated." in this case, to mean, where it is undergoing the process of becoming part of a larger RNA strand. tricky, tricky!

 

[iA-MD2013]

lol my bad. didnt mean to steal your thunder on that.
anyhow, i suppose the key word is "incoporated." in this case, to mean, where it is undergoing the process of becoming part of a larger RNA strand. tricky, tricky!

haha i don't care...i was just messing with you
yeah, there seem to be a lot of these annoying tricky questions on aamc.

 

[tncekm]

Gah I was going to take 3 today...you just ruined the question for me, Matt!!
The fact that it said where is it "incorporated" makes me feel A is the better answer. But, I completely see where you're coming from and it's a good thing you caught that. This question needs to be read question carefully.

Haven't you already taken #3?

 

[iA-MD2013]

Haven't you already taken #3?

i havent really gone over it in depth, so I thought I'd retake it and 4.

 

[tncekm]

Ah, gotcha... do you normally redo your tests under timed conditions, or just learn from them?

 

[iA-MD2013]

Ah, gotcha... do you normally redo your tests under timed conditions, or just learn from them?

usually, i take them, correct my mistakes, then retake them timed, and feel like a ******* for making the same mistakes again

 

[tncekm]

I selected choice C because I reasoned that ribosomes were constructed of ribosomal RNA. rRNA, being a form of RNA, would use uridine triphosphate.
*did i think too much into that question?

Definitely A is the better answer. Even rRNA is made in the nucleus, and the ribosomal subunits are exported form the nucleus where they combine (outside of the nucleus to form the entire ribosome) upon initiation of translation.

Replication takes place ONLY in the DNA b/c DNA doesn't leave the nucleus. And, because DNA doesn't leave the nucleus transcription must also ONLY occur in the nucleus where the DNA is found. So the nucleotide is incorporated into a nucleic acid during transcription, so its has to be occurring in the nucleus.

 

[DrMattOglesby]

i was more confused because i thought it was asking places it could be found, not where it was used for synthesis of RNA molecules.
so with that in mind...i saw two possible answers; nucleus and ribosome.
and i chose the wrong one.
oh well.

 

[tncekm]

Oh, gotcha! I see where you're coming from.

 

[lisasimpson87]

wow, i just realized this part of the forums existed..sdn is amazing...
can anyone explain these PS questions to me (from 3R)?

#10...why is diethyl ether the most soluble in diethyl ether? i picked KOH bc i figured it would hydrogen bond

#36..i don't understand why A is the right answer...what are the relative amplitudes??

#39...heh? why is it C? looking at table 1, the wavelength is 106 nm..divide this by 3.0X 10^8 to get frequency..no?

#50..about the radium..i just have question marks next to this one

#72, about the zinc..more question marks.

thanks for any help!

 

[bluemonkey]

wow, i just realized this part of the forums existed..sdn is amazing...
can anyone explain these PS questions to me (from 3R)?

#10...why is diethyl ether the most soluble in diethyl ether? i picked KOH bc i figured it would hydrogen bond

thanks for any help!

I don't have the test, so I can only help with this one. Remember the rule that like disolves like. In this case, diethyl ether has a relatively low polarity and thus prefers to dissolve in a low polarity solvent. KOH, on the other hand, is highly polar and dissociates into ions when in water. You also need to remember that diethyl ether is incapable of forming H-bonds. Unless there is a F, O, or N directly attached to a hydrogen, H-bonding will not take place.

 

[tncekm]

Actually, I beieve the oxygen in the R-O-R can participate in hydrogen bonds. Say you have water and diethyl either you can get H-O-H **** O(CH2CH3)2

 

[Chemist0157]

Actually, I beieve the oxygen in the R-O-R can participate in hydrogen bonds. Say you have water and diethyl either you can get H-O-H **** O(CH2CH3)2

That's right. Diethyl ether by itself has no H-bonding to speak of, but it and
water can!

The simple answer to this question comes from "likes dissolve likes." KOH is very polar (ionic really) while ethers are a just somewhat polar. They just don't mix.

 

[bluemonkey]

Actually, I beieve the oxygen in the R-O-R can participate in hydrogen bonds. Say you have water and diethyl either you can get H-O-H **** O(CH2CH3)2

You misread my post. I said that ethers are incapable of forming hydrogen bonds, not that they are incapable of receiving hydrogen bonds. There is a big difference. Molecules like H2O and ROH, on the other hand, are capable of both forming AND receiving hydrogen bonds. This explains why an alcohol will enter into a polar solvent while a ether will not.

 

[tncekm]

You misread my post. I said that ethers are incapable of forming hydrogen bonds, not that they are incapable of receiving hydrogen bonds. There is a big difference. Molecules like H2O and ROH, on the other hand, are capable of both forming AND receiving hydrogen bonds. This explains why an alcohol will enter into a polar solvent while a ether will not.

Gotcha; it was just that, given the context of the post you quoted him on, it came off that way, even if it wasn't your intent. But, it doesn't matter. All that matters is that everyone knows what you said, which is essentiatlly that it can't hydrogen bond with any molecule where hydrogen isn't acting as a "bridge" between two electronegative atoms (F, O, or N).

 

[Snowy]

PS #11: I don't get it at all! The question asks about which peak is more probable based on the intensity vs. wavelength graph. The solution doesn't help me in understanding why you pick D. It seems to be basically an interpretting the graph question. Can anyone help me out? thanks

 

[andafoo]

PS #11: I don't get it at all! The question asks about which peak is more probable based on the intensity vs. wavelength graph. The solution doesn't help me in understanding why you pick D. It seems to be basically an interpretting the graph question. Can anyone help me out? thanks

I take it that you have read the solution and don't understand why it indicates D is the answer?

D states that peak 2 is the most probable event because it has the highest intensity.

The solution says that the probability is measured by the intensity, thus answering the question, which asked about the peak that is more probable.

If that doesn't do it...

is your question: Why does the intensity indicate the probability of the event?

if so, I'll follow up