JEE Advanced 2026 Paper 2 — Full Mock Test (54 Questions, 180 Marks)
Mathematics · Physics · Chemistry
આ MCQ મોડ્યુલ આના પર આધારિત છે: Disaccharides Polysaccharides
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Disaccharides Polysaccharides
🎓 Class 12
Chemistry
CBSE
Theory
Ch 10 – Biomolecules
⏱ ~14 min
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આ મૂલ્યાંકન આના પર આધારિત હશે: Disaccharides Polysaccharides
મૂલ્યાંકન બનાવવામાં તેમની સામગ્રી સામેલ કરવા ચિત્રો, PDF અથવા Word દસ્તાવેજ અપલોડ કરો.
Disaccharides, Polysaccharides and the Importance of Carbohydrates
Part 1 ended with single sugar units. This part joins them together. Everything that follows — sweetness or its absence, reducing power or its absence, whether a polymer stores energy or builds a cell wall — is decided by which carbons are joined and whether the anomeric carbon is left free.
10.1.3 Disaccharides
Disaccharides on hydrolysis with dilute acids or enzymes yield two molecules of either the same or different monosaccharides. The two monosaccharides are joined together by an oxide linkage formed by the loss of a water molecule.
Glycosidic linkage. A linkage between two monosaccharide units through an oxygen atom is called a glycosidic linkage.
The rule that decides reducing power. In disaccharides, if the reducing groups of the monosaccharides — that is, the aldehydic or ketonic groups — are bonded, the sugar is non-reducing (for example sucrose). If these functional groups are free, the sugar is reducing (for example maltose and lactose).
(i) Sucrose
One of the commonest disaccharides is sucrose, which on hydrolysis gives an equimolar mixture of D-(+)-glucose and D-(−)-fructose.
These two monosaccharides are held together by a glycosidic linkage between C1 of α-D-glucose and C2 of β-D-fructose. Since the reducing groups of both glucose and fructose are involved in glycosidic bond formation, sucrose is a non-reducing sugar.
Invert sugar
Sucrose is dextrorotatory, but after hydrolysis it gives dextrorotatory glucose and laevorotatory fructose. Since the laevorotation of fructose (−92.4°) is greater than the dextrorotation of glucose (+52.5°), the mixture is laevorotatory.
Invert sugar. Hydrolysis of sucrose brings about a change in the sign of rotation, from dextro (+) to laevo (−), and the product is therefore named invert sugar.
(ii) Maltose
Maltose is composed of two α-D-glucose units in which C1 of one glucose is linked to C4 of the other glucose unit. The free aldehyde group can be produced at C1 of the second glucose in solution, and it therefore shows reducing properties — maltose is a reducing sugar.
(iii) Lactose
Lactose is more commonly known as milk sugar, since this disaccharide is found in milk. It is composed of β-D-galactose and β-D-glucose, and the linkage is between C1 of galactose and C4 of glucose. A free aldehyde group may be produced at C-1 of the glucose unit, hence lactose is also a reducing sugar.
| Disaccharide | Monosaccharide units | Linkage | Reducing? |
|---|---|---|---|
| Sucrose | α-D-glucose + β-D-fructose | C1 of glucose – C2 of fructose | No — both reducing groups tied up |
| Maltose | two α-D-glucose | C1 – C4 | Yes — free –CHO at C1 of the second unit |
| Lactose | β-D-galactose + β-D-glucose | C1 of galactose – C4 of glucose | Yes — free –CHO at C1 of glucose |
How to answer "is it reducing?" in one step. Ask whether both anomeric carbons are used in the linkage. In sucrose they are (C1 and C2 are both anomeric), so it is non-reducing. In maltose and lactose only one anomeric carbon is used — the other is free to open into an aldehyde — so both are reducing.
10.1.4 Polysaccharides
Polysaccharides contain a large number of monosaccharide units joined together by glycosidic linkages. These are the most commonly encountered carbohydrates in nature, and they mainly act as food storage or structural materials.
(i) Starch
Starch is the main storage polysaccharide of plants and the most important dietary source for human beings. A high content of starch is found in cereals, roots, tubers and some vegetables. It is a polymer of α-glucose and consists of two components.
| Component | Proportion of starch | Water solubility | Structure |
|---|---|---|---|
| Amylose | about 15–20% | water soluble | long unbranched chain of 200–1000 α-D-(+)-glucose units held by C1–C4 glycosidic linkage |
| Amylopectin | about 80–85% | insoluble in water | branched chain polymer of α-D-glucose; chain formed by C1–C4 linkage, branching by C1–C6 linkage |
(ii) Cellulose
Cellulose occurs exclusively in plants and is the most abundant organic substance in the plant kingdom. It is the predominant constituent of the cell wall of plant cells. Cellulose is a straight chain polysaccharide composed only of β-D-glucose units, joined by a glycosidic linkage between C1 of one glucose unit and C4 of the next.
The α versus β difference is everything. Starch and cellulose are both unbranched C1–C4 polymers of glucose. The only difference is that starch uses α-glucose and cellulose uses β-glucose. That single stereochemical change is why we can digest bread but not wood — human enzymes hydrolyse α-linkages but not β-linkages.
(iii) Glycogen
Carbohydrates are stored in the animal body as glycogen. It is also known as animal starch because its structure is similar to amylopectin and is rather more highly branched. It is present in liver, muscles and brain. When the body needs glucose, enzymes break glycogen down to glucose. Glycogen is also found in yeast and fungi.
10.1.5 Importance of Carbohydrates
Carbohydrates are essential for life in both plants and animals, and form a major portion of our food. Honey has been used for a long time as an instant source of energy by Vaids in the ayurvedic system of medicine.
Carbohydrates are used as storage molecules — as starch in plants and glycogen in animals. The cell wall of bacteria and plants is made up of cellulose. We build furniture from cellulose in the form of wood, and clothe ourselves with cellulose in the form of cotton fibre. Carbohydrates provide raw materials for many important industries such as textiles, paper, lacquers and breweries.
Two aldopentoses — D-ribose and 2-deoxy-D-ribose — are present in nucleic acids (taken up in Part 5). Carbohydrates are also found in biosystems in combination with many proteins and lipids.
🧪 Activity 10.2 — Testing three sugars with Tollens' reagentL4 Analyse
One simple test separates sucrose from maltose and lactose, and a second step brings sucrose back into line. This activity connects a visible observation directly to a structural feature.
Predict: Equal amounts of sucrose, maltose and lactose are warmed with Tollens' reagent. Which will deposit a silver mirror? Then predict what happens if the negative sample is first boiled with dilute HCl, neutralised, and retested.
- For each sugar, identify the two monosaccharide units and the carbons joined in the glycosidic linkage.
- Mark whether each anomeric carbon (C1 of an aldose, C2 of a ketose) is used in the linkage or left free.
- Predict the Tollens' result: a free anomeric carbon can open to an aldehyde and reduce Ag⁺.
- Boil the negative sample with dilute HCl, neutralise, and retest. Record the new result and explain it.
Maltose and lactose give a silver mirror; sucrose does not — until it is hydrolysed.
Maltose: two α-D-glucose units, C1–C4. Only the first unit's C1 is used; the second unit's C1 is free and can open to a free aldehyde. Positive.
Lactose: β-D-galactose C1 joined to β-D-glucose C4. The glucose C1 is free. Positive.
Sucrose: C1 of α-D-glucose joined to C2 of β-D-fructose — and C1 and C2 are the anomeric carbons of the two units respectively. Both reducing groups are locked into the linkage, neither ring can open, so there is no aldehyde to reduce Ag⁺. Negative.
After hydrolysis: boiling with dilute HCl cleaves the glycosidic bond and liberates one molecule of glucose and one of fructose. Both are monosaccharides, and all monosaccharides are reducing sugars, so the retested solution now gives a strong positive. The mixture is the invert sugar of Section 10.1.3 — dextrorotatory sucrose has become a net laevorotatory mixture because fructose's −92.4° outweighs glucose's +52.5°. Note carefully that fructose is a ketose and still reduces Tollens' reagent; in the alkaline conditions of the test it isomerises to an aldose form. This is exactly why bromine water, which is mild and does not cause that isomerisation, is the better test for distinguishing an aldose from a ketose.
Intext question
Intext 10.2 — Expected products of hydrolysis of lactose
Lactose is composed of β-D-galactose and β-D-glucose joined through a glycosidic linkage between C1 of galactose and C4 of glucose. On hydrolysis the linkage is cleaved with addition of a water molecule, giving one molecule of D-(+)-galactose and one molecule of D-(+)-glucose.
Competency-Based Questions
A confectionery firm is reformulating a fruit preserve. The current recipe uses sucrose, but the product crystallises on the shelf and tastes less sweet than the competitor's. A food chemist advises adding a small amount of acid and warming the syrup during manufacture. Separately, the firm's nutrition team is asked why the dietary fibre in the fruit pulp passes through the body undigested while the starch in the accompanying biscuit does not.
1. Explain chemically what the acid-and-heat treatment does to the sucrose syrup. L3 Apply
Warming with dilute acid hydrolyses the glycosidic linkage of sucrose, splitting it into an equimolar mixture of D-(+)-glucose and D-(−)-fructose: C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆. This mixture is called invert sugar, because the optical rotation changes from dextro to laevo — the laevorotation of fructose (−92.4°) exceeds the dextrorotation of glucose (+52.5°), so the mixture is net laevorotatory.
2. Why does sucrose give no reaction with Tollens' reagent, whereas both maltose and lactose do? L4 Analyse
In sucrose the linkage joins C1 of α-D-glucose to C2 of β-D-fructose — and those are precisely the anomeric (reducing) carbons of the two units. With both reducing groups bonded, neither ring can open to give a free carbonyl group, so sucrose is non-reducing. In maltose the C1–C4 linkage leaves the C1 of the second glucose free, and in lactose the galactose C1–glucose C4 linkage leaves the glucose C1 free. In each case a free aldehyde group can be produced in solution, so both are reducing sugars.
3. Answer the nutrition team: why is cellulose indigestible to humans while starch is not, given that both are polymers of glucose? L4 Analyse
The basic structural difference is the anomeric configuration of the glucose units. Starch is a polymer of α-glucose, whereas cellulose is a straight-chain polymer composed only of β-D-glucose units. Both use a C1–C4 glycosidic linkage, but the geometry of the α and β linkages differs. Human digestive enzymes are specific and can hydrolyse only the α-linkage, so starch is broken down to glucose while cellulose passes through unchanged and acts as dietary fibre.
4. Fill in the blanks: Starch consists of ______, which is water soluble and constitutes about 15–20%, and ______, which is insoluble and constitutes about 80–85%. Branching in the latter occurs through a ______ glycosidic linkage. L1 Remember
amylose; amylopectin; C1–C6. Amylose is a long unbranched chain of 200–1000 α-D-(+)-glucose units held together by C1–C4 linkages, while amylopectin has a C1–C4 chain with C1–C6 branch points.
5. An athlete's body mobilises stored carbohydrate rapidly during a sprint. Explain why glycogen rather than amylose is the appropriate storage form for an animal, referring to structure. L5 Evaluate
Glycogen is stored in liver, muscles and brain and is structurally similar to amylopectin but rather more highly branched. That branching is the key. Each branch created by a C1–C6 linkage produces an additional chain end, and enzymes release glucose by working inwards from the ends. A highly branched molecule therefore presents very many points of simultaneous attack, so glucose can be liberated at a high rate the moment the body needs it. Amylose, being a long unbranched C1–C4 chain, offers only two ends per molecule and would release glucose far too slowly for a sudden demand. A second advantage is compactness: branching lets a very large mass of glucose be packed into a small granule without the osmotic burden that the equivalent free glucose would impose on the cell. Plants, which are sessile and mobilise reserves slowly, have no need of this and can rely on the less branched amylose–amylopectin mixture of starch.
Assertion–Reason Questions
For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
Assertion (A): Sucrose is a non-reducing sugar.
Reason (R): In sucrose the reducing groups of both glucose and fructose are involved in the glycosidic linkage.
Answer: A. The linkage joins C1 of α-D-glucose to C2 of β-D-fructose, the anomeric carbons of both units, so neither ring can open to a free carbonyl group.
Assertion (A): The mixture obtained on hydrolysis of sucrose is called invert sugar.
Reason (R): The laevorotation of fructose is greater in magnitude than the dextrorotation of glucose.
Answer: A. Since −92.4° outweighs +52.5°, the net rotation of the mixture is laevorotatory, so hydrolysis inverts the sign of rotation from + to − and the product is named invert sugar.
Assertion (A): Cellulose and amylose differ because cellulose is branched while amylose is not.
Reason (R): Cellulose is composed of β-D-glucose units whereas amylose is composed of α-D-glucose units.
Answer: D. The assertion is false — both cellulose and amylose are unbranched straight chains held by C1–C4 linkages; it is amylopectin and glycogen that are branched. The reason is true and is in fact the genuine difference between them: the α versus β configuration of the glucose units, which is why starch is digestible by humans and cellulose is not.
Coming next. Part 3 moves from carbohydrates to Section 10.2 — proteins, the structure and classification of α-amino acids, essential and non-essential amino acids, the zwitterion and amphoteric behaviour, and the peptide linkage.
Frequently Asked Questions
What is a glycosidic linkage?
A glycosidic linkage is the bond between two monosaccharide units through an oxygen atom, formed by the loss of a water molecule. It is the linkage that holds disaccharides such as sucrose, maltose and lactose together, and that joins the many units of polysaccharides such as starch, cellulose and glycogen.
Why is sucrose non-reducing while maltose and lactose are reducing?
In sucrose the glycosidic linkage joins C1 of α-D-glucose to C2 of β-D-fructose, and these are the anomeric or reducing carbons of both units. With both reducing groups bonded, neither ring can open to a free carbonyl group. In maltose the C1–C4 linkage leaves the C1 of the second glucose free, and in lactose the galactose C1 to glucose C4 linkage leaves the glucose C1 free, so in each case a free aldehyde group can be produced and the sugar is reducing.
What is invert sugar and why is it so called?
Invert sugar is the equimolar mixture of D-(+)-glucose and D-(−)-fructose obtained on hydrolysis of sucrose. Sucrose is dextrorotatory, but the laevorotation of fructose (−92.4°) is greater in magnitude than the dextrorotation of glucose (+52.5°), so the mixture is net laevorotatory. Hydrolysis therefore inverts the sign of rotation from dextro to laevo, which is what the name records.
What is the difference between amylose and amylopectin?
Amylose is the water-soluble component, about 15 to 20 percent of starch, and is a long unbranched chain of 200 to 1000 α-D-(+)-glucose units held together by C1–C4 glycosidic linkages. Amylopectin is insoluble in water, constitutes about 80 to 85 percent of starch, and is a branched chain polymer of α-D-glucose in which the chain is formed by C1–C4 linkages while branching occurs through C1–C6 linkages.
What is the basic structural difference between starch and cellulose?
Starch is a polymer of α-glucose while cellulose is a straight chain polysaccharide composed only of β-D-glucose units. Both use C1–C4 glycosidic linkages, but the α and β geometries differ. Human digestive enzymes hydrolyse only the α-linkage, which is why starch is a dietary energy source and cellulose passes through as fibre.
What is glycogen and how does it differ from starch?
Glycogen is the form in which carbohydrates are stored in the animal body, present in liver, muscles and brain, and also found in yeast and fungi. It is called animal starch because its structure resembles amylopectin, but it is rather more highly branched. The extra branching gives many more chain ends for enzymes to attack, allowing glucose to be released rapidly when the body needs it.
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JEE Advanced 2026 Paper 2 — Full Mock Test (54 Questions, 180 Marks)
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