alleryStart learning →In a ring form of glucose, what marks the alpha position at carbon 1?
AThe carbonyl group sits above the plane of the ring
BThe hydroxyl group sits below carbon number 1
CThe hydroxyl group sits above the plane of the ring
DThe carbonyl group sits below carbon number 1
Answer & Solution
Correct answer: B. The hydroxyl group sits below carbon number 1
1. In aqueous solution monosaccharides are usually found as rings rather than linear chains.
2. When the ring closes, carbon 1 becomes asymmetric and is called the anomeric carbon.
3. The group whose position defines alpha or beta is the hydroxyl group on that carbon.
4. A hydroxyl below carbon number 1 is the alpha position.
5. A hydroxyl above the plane is the beta position, so that option names the other anomer.
6. The two options built on a carbonyl group are wrong because the carbonyl is consumed when the ring closes.
7. The distinction matters downstream, since starch uses alpha linkages while cellulose uses beta linkages.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 3 "Biological Macromolecules", section 3.2 Carbohydrates_
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