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Does any one know the answer and a method to arrive at the answer quickly for numbers 8 and 9 below?
Carbohydrate Question
- Thread starter drspacheman
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Problem 8:
My thought process on this problem would be to first identify the molecule in question. From the answer choices, you can conclude immediately that it must be a glucopyranose ring. Also, you should be familiar enough with glucose to identify it as a D-glucopyranose ring, which you can do by looking at the chirality of C5. The hydroxyl on C1 is cis to the C5 substituent, so you now know that it must be a β-D-glucopyranose. Since it actually is β-D-glucopyranose, only one answer makes sense. It's a conformer of β-D-glucopyranose.
Problem 9:
In order to answer this question, you need to know the definition of a glycoside - that is basically the sugar that is forming a glycosidic linkage through its anomeric carbon, in this case the one on the left. You can see from the chirality of C5 that it must also be a D-glycoside, so that eliminates B and D. From there, it's simply a matter of recognizing the difference between mannose and galactose. Galactose is a C4 anomer of glucose, so that is the correct answer.
This is a terrible question and not like anything you will see on the MCAT. If the differences between galactose, fructose, mannose, and all the other monosaccharides are tested on the exam, it will be explained in a passage. I remember seeing this passage in the BR when I was studying this summer and I found it infuriating. Relax. This isn't stuff that you'll be expected to recall or intrinsically know for the MCAT.
Hope this helps.
My thought process on this problem would be to first identify the molecule in question. From the answer choices, you can conclude immediately that it must be a glucopyranose ring. Also, you should be familiar enough with glucose to identify it as a D-glucopyranose ring, which you can do by looking at the chirality of C5. The hydroxyl on C1 is cis to the C5 substituent, so you now know that it must be a β-D-glucopyranose. Since it actually is β-D-glucopyranose, only one answer makes sense. It's a conformer of β-D-glucopyranose.
Problem 9:
In order to answer this question, you need to know the definition of a glycoside - that is basically the sugar that is forming a glycosidic linkage through its anomeric carbon, in this case the one on the left. You can see from the chirality of C5 that it must also be a D-glycoside, so that eliminates B and D. From there, it's simply a matter of recognizing the difference between mannose and galactose. Galactose is a C4 anomer of glucose, so that is the correct answer.
This is a terrible question and not like anything you will see on the MCAT. If the differences between galactose, fructose, mannose, and all the other monosaccharides are tested on the exam, it will be explained in a passage. I remember seeing this passage in the BR when I was studying this summer and I found it infuriating. Relax. This isn't stuff that you'll be expected to recall or intrinsically know for the MCAT.
Hope this helps.
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Yeah this is from the BR ochem book. I agree they get really detailed, we didn't even go in to this much detail on carbohydrates in my college courses lol.
Thanks for your response, it definitely helped!
Thanks for your response, it definitely helped!
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Just wondering. How are you inserting pictures? Are you putting them on a website first? I would like to do the same thing. Please let me know.
Thank you,
Verónica
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