Carbohydrate chemistry: reducing sugars: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Carbohydrate chemistry: reducing sugars

Sample item

A fresh sucrose solution gives a negative Benedict test, but the test becomes positive after the solution is boiled with dilute acid. What is the best explanation?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Its glucose and fructose are joined through both anomeric carbons, so no free reducing group exists until hydrolysis

Sucrose is glucose alpha 1-beta 2 fructose, so the anomeric carbons of both units are locked in the glycosidic bond and neither can open to expose an aldehyde or ketol group. Acid hydrolysis releases free glucose and fructose, which are reducing sugars. Lactose and maltose keep a free anomeric carbon and are reducing.

Option by option

  • A. Sucrose is a monosaccharide with no anomeric carbon

    Sucrose is a disaccharide, and the issue is that its anomeric carbons are bonded together.

  • B. Fructose is unable to reduce cupric ions in alkaline medium

    Fructose reduces in alkali because it tautomerises through an enediol to an aldose.

  • C. Sucrose has a beta 1-4 linkage that acid cannot hydrolyse

    Sucrose has an alpha 1-beta 2 linkage, and dilute acid readily hydrolyses it; beta 1-4 is the lactose linkage.

  • D. Its glucose and fructose are joined through both anomeric carbons, so no free reducing group exists until hydrolysis (correct)

    Correct. Both anomeric carbons are engaged, so sucrose is a non-reducing disaccharide.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, Its glucose and fructose are joined through both anomeric carbons, so no free reducing group exists until hydrolysis. Sucrose is glucose alpha 1-beta 2 fructose, so the anomeric carbons of both units are locked in the glycosidic bond and neither can open to expose an aldehyde or ketol group.

Why is "Sucrose is a monosaccharide with no anomeric carbon" not the answer?

Sucrose is a disaccharide, and the issue is that its anomeric carbons are bonded together.

Why is "Fructose is unable to reduce cupric ions in alkaline medium" not the answer?

Fructose reduces in alkali because it tautomerises through an enediol to an aldose.

Why is "Sucrose has a beta 1-4 linkage that acid cannot hydrolyse" not the answer?

Sucrose has an alpha 1-beta 2 linkage, and dilute acid readily hydrolyses it; beta 1-4 is the lactose linkage.

What concept does this question test?

It tests Carbohydrate chemistry: reducing sugars in Biochemistry. Acid hydrolysis releases free glucose and fructose, which are reducing sugars. Lactose and maltose keep a free anomeric carbon and are reducing.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Carbohydrate chemistry: reducing sugars?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.