Aamc 11 bs 109

[Rolling]

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It mentions the structures of (+)-maltose and (+)-cellobiose and asks for why there are structural differences.

I got the question right, but I don't understand how I'm supposed to know this...this is STRAIGHT memorization. I didn't think I had to memorize random disaccharides for the MCAT?

 

[GreenRabbit]

Well you don't need to know the specific structures of these compounds. But you should know that maltose is used in our diet. Cellobiose sounds like cellulose, which is structural.

Fuel sugars (digested by humans) have alpha-glycosidic bonds. Structural sugars like those found in plants have beta-glycosidic bonds that can't be digested by people. So the structural difference is in the stereochemistry of the glycosidic bond.

 

[Rabolisk]

It is sort of out of left field, although it's not a particularly hard question. B and C are just unlikely, so you can eliminate it. I suppose one can think that D could be the right answer, although glucofuranose is relatively unstable compared to pyranose forms. Even without knowing the structures (and I certainly wouldn't know maltose and cellobiose), the best answer that I would select is A, because B, C, and D are simply not as likely.

 

[stefspets]

It's not straight memorization. The question tells you that both are made up of two D-glucose molecules. That means that you can eliminate some of the answer choices--they are not constitutional isomers if both contain D-glucose. They both have the same number of carbons because again, they are both glucose. The answer about boat/chair is not correct because regardless of configuration the structure is the same. The only one that makes any sense is the one that deals with how they are linked together--one has alpha and one has beta linkage. You don't even have to know what alpha and beta are as long as you realize they are different and deal with how the molecules are connected.

 

[dougkaye]

basically with most MCAT PS or BS questions if it seems random and really hard, you're approaching the question wrong and/or the "random" fact is IN THE PASSAGE.