Chemistry · Biomolecules · chapter 10 of 10

Class 12 Chemistry Chapter 10: Biomolecules case study question with answers

One case-based question of the kind the board asks: a passage and a table, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks

A student is given five white carbohydrate samples, P to T. She tests each with Tollens' reagent and then hydrolyses it completely with dilute acid, identifying the monosaccharides formed. Sample T dissolves in hot water. Her teacher adds that T was separated from a natural mixture whose other, water-insoluble component also gives only glucose on hydrolysis but has a branched chain.

Sample	Tollens' reagent	Products of complete hydrolysis
P	silver mirror	not hydrolysed
Q	no change	glucose and fructose, equimolar
R	silver mirror	glucose only, two units per molecule
S	silver mirror	galactose and glucose
T	no change	glucose only, several hundred units in an unbranched chain

Part 1 · 1 mark

Sample Q is

(a) maltose(b) lactose(c) sucrose(d) cellulose

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Answer: (c) sucrose

Q is a non-reducing sugar giving equimolar glucose and fructose: sucrose. Maltose gives only glucose, lactose galactose and glucose, cellulose many glucose units.

Part 2 · 1 mark

In sample R the two units are joined between

(a) C1 of β-D-galactose and C4 of β-D-glucose(b) C1 of α-D-glucose and C2 of β-D-fructose(c) C1 of one α-D-glucose and C4 of the other(d) C1 of one α-D-glucose and C6 of the other

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Answer: (c) C1 of one α-D-glucose and C4 of the other

R is maltose: two α-D-glucose units, C1 of one to C4 of the other. (a) is lactose (sample S), (b) sucrose (Q), (d) a branch point of amylopectin.

Part 3 · 2 marks

Explain why R gives a silver mirror with Tollens' reagent but Q does not.

Answer and marking scheme

In Q, sucrose, the glycosidic linkage joins C1 of α-D-glucose to C2 of β-D-fructose, so the reducing groups of both units are bonded and none is free. In R, maltose, C1 of the first glucose is joined to C4 of the second; C1 of the second unit is free and a free aldehyde group can form there in solution, which reduces Tollens' reagent.

Q: both reducing groups used in the C1–C2 linkage (1)
R: free aldehyde group can form at C1 of the second glucose unit (1)

Identify T and the water-insoluble component the teacher describes, and state how their chains differ.

Answer and marking scheme

T is amylose, the water-soluble component of starch (about 15–20%), an unbranched chain of α-D-glucose units joined by C1–C4 glycosidic linkages. The insoluble component is amylopectin (about 80–85%), a branched polymer of α-D-glucose: its chain is formed by C1–C4 linkages and its branches by C1–C6 linkages.

T amylose, unbranched α C1–C4 chain (1)
amylopectin, branched, with C1–C6 linkages at the branches (1)

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