Carbohydrate structures on MCAT??

[monkeyMD]

The Berkeley Review seems to stress knowing the different CHOs, like galactose for example, in their section on CHOs. And the questions for this chapter also have questions like: "the C2 epimer of galactose is the same as which of the following sugars?" Do we really need to know the different hexose structures, etc??

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[Rabolisk]

I doubt it. I would advise that you know the common ones like glucose, fructose, and galactose, as well as maybe some common disaccharides. You need to understand the concept of epimers and anomers, but they won't trip you up by asking if the C-3 epimer of galactose is allose, gulose, or talose without giving you those structures.

 

[ridethecliche]

And for the record an anomer is a enantiomer about the anomeric carbon and an epimer is a diastereomer about any 1 chiral center on the sugar.

Thus, an anomer is a special case of epimer.

 

[phnerd1105]

Agreed. I would also know that glucose and galactose are 6 carbon molecules and fructose is 5.

 

[Rabolisk]

And for the record an anomer is a enantiomer about the anomeric carbon and an epimer is a diastereomer about any 1 chiral center on the sugar.

Thus, an anomer is a special case of epimer.

And for the record you are wrong. An anomer is a diastereomer about the hemiacetal carbon, the one that is a carbonyl in straight chain form and becomes sp3 hybridized upon ring formation (anomeric).

 

[ridethecliche]

And for the record you are wrong. An anomer is a diastereomer about the hemiacetal carbon, the one that is a carbonyl in straight chain form and becomes sp3 hybridized upon ring formation (anomeric).

Yeah sorry, that's what I meant. I should have written the chirality is opposite at the anomeric carbon. Saying something is a diastereomer about a certain carbon makes little sense in this context...

 

[Etorphine]

when in doubt, learn it if it is included in a text.

 

[Swagster]

I doubt it. I would advise that you know the common ones like glucose, fructose, and galactose, as well as maybe some common disaccharides. You need to understand the concept of epimers and anomers, but they won't trip you up by asking if the C-3 epimer of galactose is allose, gulose, or talose without giving you those structures.

👍

I agree that you should know the common ones by name and for anything else, they'll surely show you structures.

I was so freaked out by the OP's question that I looked up the question they were talking about. It's exactly what you said. They (BR) give you structures for the choices, so all you need to know is the structure of galactose and the definition of an epimer. Seems like a reasonable and likely question.

 

[jsp132]

what course was this taught in ? about the epimers

I was never taught that in my biology and bio ii courses

biochemistry?

 

[ridethecliche]

Some of it is biochem as well.

I dunno.

I just took the last TBR exam (9) and there was a passage about amino acids which had half the questions on structure and such. I.e. stuff that won't be on the MCAT without a passage that explains the stuff. This test did not.

 

[BerkReviewTeach]

Some of it is biochem as well.

I dunno.

I just took the last TBR exam (9) and there was a passage about amino acids which had half the questions on structure and such. I.e. stuff that won't be on the MCAT without a passage that explains the stuff. This test did not.

I hope you take this in the right way, but you need to modify what the phrase "explains the stuff" means to you. The MCAT tests your ability to integrate very basic concepts with the information in the passage. Not everything will be spelled out in the exact form you've seen it before. That passage (I assume you are talking about Passage V on the Strecker Synthesis) is a perfect example of questions you can answer with information from the passage and a basic understanding of the AA structure.

122. All four answer choices shows the same sidechain (leaving room for you to decide whether it's protonated or not), so it wasn't a question of knowing something not given in the passage. The question wasn't asking about structure. You were being tested on the idea that a site will be protonated when pH < pKa and a site will be deprotonated when pH > pKa. All you needed to know was that carboxylic acids have pKas less than 6.3.

123. The idea here is that resolving the mixture means to separate stereoisomers (enantiomers in this synthesis) from one another. This question tested if you knew that the sidechain of glycine was H (which makes glycine achiral). You should have learned this in biochemistry, when glycine was the only AA without a d or l prefix. This is basically asking, "is glycine chiral?"

124. Is asking you about a technique described in the passage.

125. This question is about acid-base extraction, an organic chemistry lab technique listed in the AAMC guide as something you are supposed to know.

126. "What is the isoelectric point of a hydrophobic amino acid?" That's a general question on a very fundamental concept associated with AAs. You should know that pI for an amino acid with a hydrophobic sidechain is an average of the only two pKas, the amino terminal and carboxyl terminal.

127. The question boils down to "What is the starting material necessary to get a 117 g/mole AA?" The passage shows you that the reactant must be an aldehyde, which kills two choices right off the bat. To determine the better answer, all you had to do was figure out that an amino acid's weight is (74 + the R-group). To be 117 g/mole, the R-group must weigh 43 g/mole, which can only be choice A. So this question was asking if you knew that an AA has an amino terminal, a carboxyl terminal, an H, and an R-group bonded to a carbon.

If you break down each question and what it was asking for, you'll see that everything was doable. The MCAT has a puzzle quality to it, and questions can be solved with reasoning and a little background knowledge. Hopefully in retrospect, you can see that this passage is a prime example of that.

Hang in there!

 

[ridethecliche]

I hope you take this in the right way, but you need to modify what the phrase "explains the stuff" means to you. The MCAT tests your ability to integrate very basic concepts with the information in the passage.Not everything will be spelled out in the exact form you've seen it before.
Agreed and your tests are awesome at making me think on the go. If this is what the passages in the books are like, then there's a treasure trove out there!

That passage (I assume you are talking about Passage V on the Strecker Synthesis) is a perfect example of questions you can answer with information from the passage and a basic understanding of the AA structure.

122. All four answer choices shows the same sidechain (leaving room for you to decide whether it's protonated or not), so it wasn't a question of knowing something not given in the passage. The question wasn't asking about structure. You were being tested on the idea that a site will be protonated when pH < pKa and a site will be deprotonated when pH > pKa. All you needed to know was that carboxylic acids have pKas less than 6.3.

Yeup.

123. The idea here is that resolving the mixture means to separate stereoisomers (enantiomers in this synthesis) from one another. This question tested if you knew that the sidechain of glycine was H (which makes glycine achiral). You should have learned this in biochemistry, when glycine was the only AA without a d or l prefix. This is basically asking, "is glycine chiral?"

Right, but I was under the impression that the MCAT doesn't require you to have memorized all the AA structures. In addition, there are a few discretes that I've come across that want you to have memorized structure. I didn't think that was necessary.

Hang in there!

I've taken biochem 😀

However, it's not a required course and I felt that TBR tests push the boundary a bit here. I never said that it's a bad thing. If anything, it forces you to understand the material better. And the practice exams should be more challenging than the AAMC's and of a similar level to the actual test. TBR's shine at that.

And no, not taking this the wrong way at all. Come on, you know that 1) I have thicker skin than that and 2) I know that you're trying to help!

I've added to your quote above.

Also the passage on homeobox-ish genes on the bio section= absolute gold. I had a bunch of fun figuring out. It was an awesome passage that really made me think.

From what many users of the book series have stated, the books go above and beyond what is required. The tests do the same thing. I never made a judgment call on that. If anything, I think it's a good thing ala 'cry in the dojo, laugh on the battlefield'.

Edit: I stand corrected. From the AAMC

B. Amino Acids and Proteins
1. Description
a. a absolute configuration(s)
b. amino acids classified as dipolar ions
c. classification
i. acidic or basic
ii. hydrophobic or hydrophilic
2. Important reactions
a. peptide linkage

 

[sbspftw]

Agreed. I would also know that glucose and galactose are 6 carbon molecules and fructose is 5.

I thought fructose is also a 6-carbon molecule, just in a 5-membered ring. It's a ketose and not an aldose. just for clarification.