JEE Mains Chemistry · Biomolecules
Disaccharides, Polysaccharides and Reducing Sugars
A glycosidic link joins the anomeric carbon of one sugar to an OH of the next; a sugar still reduces Tollens' or Fehling's reagent only if some anomeric carbon keeps its free OH, which is why maltose and lactose reduce and sucrose does not; starch, glycogen and cellulose differ in the anomer and branching of their glucose chains.
Why this matters
Twenty-six PYQs, twenty-five multiple choice and one asking for a number, four from 2026. Thirteen name the units and the glycosidic link of sucrose, maltose or lactose, or explain why hydrolysed sucrose turns laevorotatory. Eight ask whether a sugar reduces Tollens', Fehling's or Benedict's reagent, often from a drawn structure. Five compare amylose, amylopectin, glycogen and cellulose by linkage, branching and source.
Concept 1 of 3: Reducing and non-reducing sugars
A sugar reduces Tollens' or Fehling's reagent through its open chain, and a ring can open only at a hemiacetal: an anomeric carbon that still carries an OH. When a glycosidic link uses the anomeric carbon, that ring is locked as an acetal. So the test is one question: is any anomeric carbon still free?
Definition
- Monosaccharides (glucose, fructose, galactose, ribose, 2-deoxyribose) are all reducing.
- Maltose and lactose use the C-1 of one unit and the C-4 of the other. The second unit's C-1 is still a hemiacetal, so both are reducing.
- Sucrose links C-1 of glucose to C-2 of fructose. Both anomeric carbons are used, so sucrose is non-reducing.
- A methyl glycoside has its anomeric OH replaced by ; it is an acetal and non-reducing.
- Starch, amylose, glycogen and cellulose have one reducing end in a chain of hundreds of units, too little to show: they give no Fehling's or Benedict's test.
- To judge a drawn structure, find each ring's anomeric carbon, the ring carbon bonded to two oxygens. If one of those oxygens is an OH, the sugar is reducing.
Worked example
How many of these reduce Tollens' reagent: fructose, sucrose, lactose, cellulose, methyl α-D-glucoside, maltose, galactose?
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
JEE Mains · 2023 · 11 April 2023 · Q141Moderate
Example 1 · Biomolecules · Disaccharides, Polysaccharides and Reducing Sugars
Number of compounds from the following which will not produce orange red precipitate with Benedict solution is Glucose, maltose, sucrose, ribose, 2-deoxyribose, amylose, lactose.
A C-1 to C-4 link leaves one C-1 free
In maltose the first glucose uses its C-1, but the second glucose uses only its C-4. Its own C-1 stays a hemiacetal, so maltose is reducing. A statement calling maltose non-reducing is false.
Polysaccharides count as non-reducing
Starch and amylose have a single reducing end on a very long chain, so they give no Benedict's or Fehling's precipitate. Count them with sucrose, not with glucose.
Concept 2 of 3: Glycosidic linkages in sucrose, maltose and lactose
A glycosidic bond forms when the anomeric OH of one sugar condenses with an OH of another and water is lost. It is named by the anomer that used its C-1 and by the two carbons joined: maltose is α1–4, lactose β1–4, and sucrose α1–β2 because it joins two anomeric carbons.
Definition
- Hydrolysis of sucrose (cane sugar) gives equal amounts of D-(+)-glucose and D-(−)-fructose.
- Sucrose is dextrorotatory (about +66.5°). The hydrolysed mixture is laevorotatory, because the laevorotation of fructose (−92.4°) is larger than the dextrorotation of glucose (+52.5°). The sign changes, so the mixture is called invert sugar; the enzyme invertase does the same hydrolysis.
- Sucrose is made in plants from α-D-glucose and β-D-fructose; fructose is D-(−), never D-(+).
- Maltose (malt sugar) gives two α-D-glucose; lactose (milk sugar) gives β-D-galactose and β-D-glucose.
- A sugar that gives saccharic acid with nitric acid is glucose; its laevorotatory partner from a sugar hydrolysis is fructose, so the parent is sucrose.
| Disaccharide | Units on hydrolysis | Glycosidic link | Reducing? |
|---|---|---|---|
| Sucrose (cane sugar) | α-D-(+)-glucose and β-D-(−)-fructose | C-1 of glucose to C-2 of fructose, α1–β2 | No |
| Maltose (malt sugar) | Two α-D-glucose units | C-1 of one glucose to C-4 of the next, α1–4 | Yes |
| Lactose (milk sugar) | β-D-galactose and β-D-glucose | C-1 of galactose to C-4 of glucose, β1–4 | Yes |
Only sucrose joins two anomeric carbons, so only sucrose is non-reducing.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
JEE Mains · 2025 · 28 January 2025 · Q103Moderate
Example 2 · Biomolecules · Disaccharides, Polysaccharides and Reducing Sugars
Match List-I with List-II.
Choose the correct answer from the options given below:
| List-I (Saccharides) | List-II (Glycosidic linkages found) | ||
|---|---|---|---|
| (A) | Sucrose | (I) | |
| (B) | Maltose | (II) | and |
| (C) | Lactose | (III) | |
| (D) | Amylopectin | (IV) |
Fructose, not glucose, is laevorotatory
Invert sugar is laevorotatory because fructose (−92.4°) rotates more strongly than glucose (+52.5°). A reason that gives the laevorotation to glucose is false.
Sucrose joins α-glucose to β-fructose
The link is C-1 of α-D-glucose to C-2 of β-D-fructose. A statement with β-glucose and α-fructose has the anomers swapped and is false.
Lactose uses C-1 of galactose
In lactose the galactose unit gives its anomeric C-1 and the glucose unit gives C-4. The reverse, C-1 of glucose to C-4 of galactose, is a common wrong option.
Concept 3 of 3: Starch, glycogen and cellulose: linkages and sources
All three are made of glucose only. Two choices tell them apart: is the glucose α or β, and does the chain branch? Starch and glycogen use α-glucose, which animals can digest; cellulose uses β-glucose, which humans cannot. Branching at C-6 makes amylopectin, and even more branching makes glycogen.
Definition
- Starch is 15–20% amylose and 80–85% amylopectin. It is the main storage polysaccharide of plants.
- Boiling starch or cellulose with dilute at 393 K under 2 to 3 atm hydrolyses it to glucose; bromine water then gives gluconic acid.
- Glycogen is called animal starch. Its structure is like amylopectin but more highly branched. It is stored in liver, muscles and brain; yeast and fungi also make it.
- Cellulose is the main polysaccharide of plant cell walls; humans have no enzyme to break its β links.
- Biopolymers and their monomers: starch from α-glucose, cellulose from β-glucose, nucleic acids from nucleotides, proteins from α-amino acids.
| Polysaccharide | Unit and linkage | Shape and solubility | Found in |
|---|---|---|---|
| Amylose | α-D-glucose, C-1 to C-4 only | Unbranched chain of 200 to 1000 units; soluble in water | Starch of plants, 15–20% |
| Amylopectin | α-D-glucose, C-1 to C-4 in chains, C-1 to C-6 at branches | Branched; insoluble in water | Starch of plants, 80–85% |
| Glycogen | α-D-glucose, C-1 to C-4 with C-1 to C-6 branches | More highly branched than amylopectin | Liver, muscles and brain of animals; yeast and fungi |
| Cellulose | β-D-glucose, C-1 to C-4 only | Straight chains packed side by side; insoluble in water | Plant cell walls; not digested by humans |
Only cellulose has β links; only amylopectin and glycogen branch.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
JEE Mains · 2025 · 29 Jan 2025 · Q43Moderate
Example 3 · Biomolecules · Disaccharides, Polysaccharides and Reducing Sugars
Match List-I with List-II.
Choose the correct answer from the options given below:
| List-I (Carbohydrate) | List-II (Linkage, Source) | ||
|---|---|---|---|
| (A) | Amylose | (I) | , plant |
| (B) | Cellulose | (II) | , animal |
| (C) | Glycogen | (III) | , , plant |
| (D) | Amylopectin | (IV) | , plant |
Amylose is the water-soluble fraction
NCERT calls amylose water soluble and amylopectin insoluble. An assertion that amylose is insoluble in water is false, even though amylose is indeed a long chain of 200 to 1000 glucose units.
Cellulose and starch differ in the anomer, not the carbons
Both join C-1 to C-4. Starch uses α-glucose and cellulose uses β-glucose. A polysaccharide described as having only β-glycosidic links is cellulose.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Glycosidic linkages in sucrose, maltose and lactose3 rows
| Disaccharide | Units on hydrolysis | Glycosidic link | Reducing? |
|---|---|---|---|
| Sucrose (cane sugar) | α-D-(+)-glucose and β-D-(−)-fructose | C-1 of glucose to C-2 of fructose, α1–β2 | No |
| Maltose (malt sugar) | Two α-D-glucose units | C-1 of one glucose to C-4 of the next, α1–4 | Yes |
| Lactose (milk sugar) | β-D-galactose and β-D-glucose | C-1 of galactose to C-4 of glucose, β1–4 | Yes |
Only sucrose joins two anomeric carbons, so only sucrose is non-reducing.
Starch, glycogen and cellulose: linkages and sources4 rows
| Polysaccharide | Unit and linkage | Shape and solubility | Found in |
|---|---|---|---|
| Amylose | α-D-glucose, C-1 to C-4 only | Unbranched chain of 200 to 1000 units; soluble in water | Starch of plants, 15–20% |
| Amylopectin | α-D-glucose, C-1 to C-4 in chains, C-1 to C-6 at branches | Branched; insoluble in water | Starch of plants, 80–85% |
| Glycogen | α-D-glucose, C-1 to C-4 with C-1 to C-6 branches | More highly branched than amylopectin | Liver, muscles and brain of animals; yeast and fungi |
| Cellulose | β-D-glucose, C-1 to C-4 only | Straight chains packed side by side; insoluble in water | Plant cell walls; not digested by humans |
Only cellulose has β links; only amylopectin and glycogen branch.
Watch out for (7)
- A C-1 to C-4 link leaves one C-1 free→ Reducing and non-reducing sugars
- Polysaccharides count as non-reducing→ Reducing and non-reducing sugars
- Fructose, not glucose, is laevorotatory→ Glycosidic linkages in sucrose, maltose and lactose
- Sucrose joins α-glucose to β-fructose→ Glycosidic linkages in sucrose, maltose and lactose
- Lactose uses C-1 of galactose→ Glycosidic linkages in sucrose, maltose and lactose
- Amylose is the water-soluble fraction→ Starch, glycogen and cellulose: linkages and sources
- Cellulose and starch differ in the anomer, not the carbons→ Starch, glycogen and cellulose: linkages and sources
Test yourself on Biomolecules
20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.