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Carbohydrate classification and named mono-/oligosaccharides: NEET MCQs with answers

50 questions on this topic of Biomolecules (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Consider the following statements regarding carbohydrates: Statement I: Maltose is a reducing disaccharide containing two α-D-glucose units. Statement II: The glycosidic linkage in maltose is formed between C1 of one glucose unit and C4 of another glucose unit. In the light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (A) Both Statement I and Statement II are correct

Maltose consists of two α-D-glucose units joined by an α-(1,4)-glycosidic linkage. Since the hemiacetal group at C1 of the second glucose unit remains free, it acts as a reducing sugar, making both statements correct.

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2. Starch consists of two components. Amylose, which forms about 15-20% of starch, is:

  1. The same as cellulose
  2. Water-soluble with an unbranched chain of α-D-glucose units
  3. Water-insoluble and highly branched
  4. Made of fructose
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Answer: (B) Water-soluble with an unbranched chain of α-D-glucose units

Amylose is a water-soluble, long unbranched chain of α-D-glucose (C1-C4 links). Amylopectin (80-85%) is insoluble and branched (C1-C6 links at branch points).

NCERT Class 12

hard

3. Complete acid-catalysed hydrolysis of 1 mole of stachyose, a tetrasaccharide, produces:

  1. 2 moles of D-glucose and 2 moles of D-fructose
  2. 2 moles of D-glucose, 1 mole of D-galactose, and 1 mole of D-fructose
  3. 1 mole of D-glucose, 1 mole of D-fructose, and 2 moles of D-galactose
  4. 4 moles of D-glucose
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Answer: (C) 1 mole of D-glucose, 1 mole of D-fructose, and 2 moles of D-galactose

Stachyose is a tetrasaccharide found in numerous plants. Its complete hydrolysis yields 2 moles of D-galactose, 1 mole of D-glucose, and 1 mole of D-fructose.

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4. What is the maximum number of moles of phenylhydrazine consumed by 1 mole of D-mannose during the formation of its osazone?

  1. 1 mole
  2. 2 moles
  3. 4 moles
  4. 3 moles
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Answer: (D) 3 moles

The formation of an osazone from any aldohexose or ketohexose consumes exactly 3 moles of phenylhydrazine per mole of hexose, releasing aniline, ammonia, and water alongside the osazone.

easy

5. Hydrolysis of sucrose leads to a change in the optical sign of rotation from dextro to laevo. This phenomenon is known as:

  1. Inversion of cane sugar
  2. Mutarotation
  3. Epimerisation
  4. Racemisation
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Answer: (A) Inversion of cane sugar

Sucrose is dextrorotatory, but upon hydrolysis gives an equimolar mixture of D-(+)-glucose (+52.5°) and D-(-)-fructose (-92.4°). The resulting mixture is laevorotatory, so the process is known as the inversion of cane sugar.

hard

6. Trehalose is a naturally occurring disaccharide formed by two α-D-glucose units. It does not reduce Fehling's solution. What type of glycosidic linkage is present in trehalose?

  1. α-1,4-glycosidic linkage
  2. β-1,4-glycosidic linkage
  3. α-1,6-glycosidic linkage
  4. α,α-1,1-glycosidic linkage
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Answer: (D) α,α-1,1-glycosidic linkage

Trehalose is a non-reducing sugar because both anomeric carbon atoms (C1 of both α-D-glucose units) are engaged in the glycosidic bond, forming an α,α-1,1-glycosidic linkage.

easy

7. Consider the structural classification of carbohydrates. Which of the following is a ketotriose?

  1. Glyceraldehyde
  2. Dihydroxyacetone
  3. Erythrulose
  4. Ribulose
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Answer: (B) Dihydroxyacetone

Dihydroxyacetone (CH₂OH-CO-CH₂OH) has three carbon atoms with a ketone group at C2, making it the simplest ketotriose. Glyceraldehyde is an aldotriose, while erythrulose is a ketotetrose and ribulose is a ketopentose.

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8. On complete acetylation with acetic anhydride, one mole of sucrose forms an octaacetate derivative. This proves that sucrose contains:

  1. Eight primary hydroxyl groups
  2. Eight secondary hydroxyl groups
  3. Six hydroxyl groups and two hemiacetal linkages
  4. Eight total hydroxyl groups across its two rings
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Answer: (D) Eight total hydroxyl groups across its two rings

Sucrose (C₁₂H₂₂O₁₁) reacts with acetic anhydride to yield sucrose octaacetate. This confirms the presence of eight free hydroxyl groups distributed over the glucose and fructose rings.

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9. Given below are two statements: Statement I: Maltose contains an α-(1→4) glycosidic linkage between two D-glucopyranose units and shows mutarotation. Statement II: Sucrose does not show mutarotation because both anomeric carbon atoms participate in the glycosidic bond. In the light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (A) Both Statement I and Statement II are correct

Maltose contains a free hemiacetal hydroxyl group at C1 of the second glucose unit, allowing ring opening and mutarotation. In sucrose, the glycosidic bond links C1 of α-D-glucose and C2 of β-D-fructose; both anomeric carbons are locked, preventing mutarotation. Thus, both statements are correct.

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10. Which pair of monosaccharides is classified as ketohexoses?

  1. Glucose and Fructose
  2. Fructose and Sorbose
  3. Mannose and Galactose
  4. Ribulose and Xylulose
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Answer: (B) Fructose and Sorbose

Both fructose and sorbose possess six carbon atoms with a ketone functional group, classifying them as ketohexoses. Ribulose and xylulose are ketopentoses, whereas glucose, mannose, and galactose are aldohexoses.

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11. What is the total number of optical isomers possible for a ketohexose in its open-chain structure?

  1. 8
  2. 4
  3. 16
  4. 32
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Answer: (A) 8

An open-chain ketohexose has three chiral carbon atoms (C3, C4, and C5). The total number of stereoisomers (optical isomers) is given by 2ⁿ = 2³ = 8.

easy

12. Cellobiose, obtained by partial hydrolysis of cellulose, is composed of two D-glucopyranose units connected by which linkage?

  1. α-1,4-glycosidic linkage
  2. α-1,6-glycosidic linkage
  3. β-1,4-glycosidic linkage
  4. β-1,6-glycosidic linkage
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Answer: (C) β-1,4-glycosidic linkage

Cellobiose consists of two β-D-glucose units joined together via a β-1,4-glycosidic linkage, reflecting the repeating unit structure found in cellulose.

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