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Sucrose, maltose, lactose, starch, glycogen and cellulose are all built from simple sugars joined through oxygen bridges. Students often learn them as six separate facts and then mix up the carbon numbers under pressure. This post shows how to read each linkage as a three-part label, works through hydrolysis calculations, and lists the swaps examiners like to test.
What does the syllabus expect?
Sections 10.1.3 and 10.1.4 of the NCERT Class 12 chapter Biomolecules cover disaccharides and polysaccharides. The JEE (Main) 2026 syllabus lists the "constituent monosaccharides of oligosaccharides (sucrose, lactose and maltose)". The JEE (Advanced) 2026 brochure includes "Glycoside formation and hydrolysis of disaccharides (sucrose, maltose, lactose)". Polysaccharides are in the NCERT chapter for board exams. Check your exam's current syllabus.
How do you read a glycosidic linkage?
NCERT defines a glycosidic linkage as the oxygen bridge formed between two sugar units "by the loss of a water molecule". Describe each one with three parts: the sugar units, whether the anomer is α or β, and the carbon numbers joined.
Sucrose: α-D-glucose C1 joined to β-D-fructose C2. Both reducing groups are used, so sucrose is non-reducing.
Maltose: two α-D-glucose units, C1 of one joined to C4 of the next. C1 of the second glucose is free, so maltose is reducing.
Lactose: β-D-galactose C1 joined to C4 of β-D-glucose. C1 of the glucose unit is free, so lactose is reducing.
Amylose (about 15–20% of starch): an unbranched chain of 200–1000 α-D-glucose units joined C1–C4. Water soluble.
Amylopectin (about 80–85% of starch): α-D-glucose with C1–C4 chains and C1–C6 branches. Insoluble in water.
Glycogen: structurally like amylopectin but "rather more highly branched". It is stored in liver, muscles and brain.
Cellulose: a straight chain of β-D-glucose units joined C1–C4.
Why does the reducing question depend on the linkage?
A sugar is reducing if it still has a free aldehyde or ketone group that can open up in solution. In sucrose, the link uses C1 of glucose and C2 of fructose, which are exactly the carbons that carry those groups, so nothing is left free. In maltose and lactose, one unit's C1 is still free. You can work out the answer from the linkage label instead of memorising it. Our post on biomolecules in chemistry covers the reducing-sugar test and invert sugar in more detail.
Worked example 1: hydrolysing sucrose
How much glucose and fructose do you get from 34.2 g of sucrose?
Molar mass of C₁₂H₂₂O₁₁: 12 × 12 + 22 × 1 + 11 × 16 = 342 g mol⁻¹. So 34.2 g is 0.100 mol.
Hydrolysis: C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose). One water is used for each link broken.
Each product has a molar mass of 6 × 12 + 12 × 1 + 6 × 16 = 180 g mol⁻¹, so you get 0.100 × 180 = 18.0 g of each.
Check the mass: 34.2 g sucrose + 1.80 g water = 36.0 g of products.
Worked example 2: starch and the 162 rule
Each glucose unit in a starch chain has lost one water to its neighbours, so a unit has the formula C₆H₁₀O₅ and a mass of 180 − 18 = 162 g mol⁻¹.
Complete hydrolysis of 16.2 g of starch: 16.2 / 162 = 0.100 mol of units, so 0.100 × 180 = 18.0 g of glucose.
An amylose chain of n units has n − 1 links, so its molar mass is 180n − 18(n − 1) = 162n + 18. For NCERT's range of 200 to 1000 units, that is 32,418 to 162,018 g mol⁻¹, or about 3.2 × 10⁴ to 1.6 × 10⁵.
Worked example 3: name the sugar
"A disaccharide gives only glucose on hydrolysis and is a reducing sugar." Lactose gives galactose as well, so it is ruled out. Sucrose gives fructose and is non-reducing. Of the three NCERT disaccharides, the answer is maltose.
Common mistakes and how to avoid them
Writing C1–C4 for sucrose. Sucrose is C1 (glucose) to C2 (fructose).
Making lactose from two glucose units. Lactose is galactose plus glucose.
Calling cellulose an α-polymer. Cellulose is β; starch and glycogen are α.
Giving amylose branches. Amylose is unbranched. The C1–C6 branches belong to amylopectin and glycogen.
Using 180 for a unit inside a chain. Inside a polysaccharide each unit counts 162.
A practice routine
Write all seven linkage labels from memory in the three-part format, then check them against NCERT section 10.1.
For each, say whether it is reducing (for the disaccharides) or branched (for the polysaccharides), and give the reason from the label.
Repeat the two hydrolysis calculations with maltose and with 81 g of starch. (Answers: 0.100 mol of maltose gives 36.0 g of glucose; 81 g of starch gives 90 g of glucose.)
After two days, write five "name the sugar" clues of your own and swap them with a friend.
If the ring structures themselves are what confuses you, Fermi's Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can go back to how glucose forms a ring before you return to linkages.
Today, write the three-part label for sucrose, maltose and cellulose without looking, then check each one.
