Disaccharides and Polysaccharides: Starch and Cellulose

When two monosaccharides join through a glycosidic bond, they form a disaccharide, and when hundreds or thousands join, they form a polysaccharide. The glycosidic bond forms by a condensation reaction — a molecule of water is lost between the −OH groups of the two sugars — and whether it links carbon 1 in the α or β configuration completely changes the properties of the resulting molecule.

Starch stores glucose in plants, cellulose gives plants strength, and lactose and sucrose sweeten milk and tea — a favourite topic in boards, JEE and NEET.

Key Concepts

• Glycosidic bond: a C−O−C linkage between sugar units, e.g. α(1→4) means carbon 1 of one unit (α form) joins carbon 4 of the next.
• Reducing vs non-reducing: a disaccharide is reducing only if one anomeric carbon remains free; in sucrose both are locked, so it is non-reducing.
• α vs β linkage: α(1→4) links (starch) coil into digestible helices, while β(1→4) links (cellulose) form straight, rigid chains.
• Monomer unit mass: each glucose unit in a chain has an anhydroglucose mass of 162 g mol⁻¹ (180 − 18 for the lost water).

Key disaccharides: maltose (glucose α(1→4) glucose, reducing), lactose (galactose β(1→4) glucose, reducing) and sucrose (glucose α(1→2)β fructose, non-reducing). Starch has amylose — linear α(1→4), blue-black with iodine — and amylopectin — α(1→4) with α(1→6) branches every 24–30 units. Cellulose is linear β(1→4) glucose hydrogen-bonded into strong microfibrils; glycogen is a more branched animal starch (branches every 8–12 units).

Formulas

• Condensation: n(C₆H₁₂O₆) → (C₆H₁₀O₅)ₙ + nH₂O. Each glycosidic bond releases one water molecule; this formula holds for starch, cellulose and glycogen.
• Sucrose hydrolysis (inversion): C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose). One mole of sucrose (342 g) gives 360 g of products because water is added across the bond.
• Chain length: number of glucose units = molar mass of polymer ÷ 162 g mol⁻¹. Valid for any linear glucan built from glucose.

Worked Example

An amylose sample has a molar mass of 162,000 g mol⁻¹. How many glucose units does each chain contain? Step 1: each glucose unit in the chain contributes an anhydroglucose mass of 162 g mol⁻¹ (180 − 18 for the water lost in the glycosidic bond). Step 2: number of units = 162,000 ÷ 162 = 1000. So the chain contains about 1000 glucose units — right in the typical 200–1000 range for amylose.

Common Misconceptions

• "Starch and cellulose are the same because both are made of glucose." No — starch uses α(1→4) links and coils up for storage, while cellulose uses β(1→4) links and stays straight for strength; humans digest starch but cannot digest cellulose.
• "Sucrose reduces Fehling's solution like glucose does." Wrong — both anomeric carbons of sucrose are tied up in the glycosidic bond, so it is non-reducing and gives a negative test.
• "All polysaccharides are just long chains." Not quite — amylopectin and glycogen are branched (α(1→6) links), which lets many enzyme ends release glucose quickly.

Real-World Applications

• Food and nutrition: starch in rice, wheat and potatoes is the world's main calorie source; amylase in saliva begins its digestion.
• Materials: cellulose makes paper, cotton fabric and rayon; its insolubility comes from the hydrogen-bonded β(1→4) chains.
• Medicine: dextran (a bacterial glucan) is used as a blood-plasma substitute, and starch-based powders are used as tablet binders.

The α or β geometry of the glycosidic bond decides everything — digestible starch versus indigestible cellulose, reducing maltose versus non-reducing sucrose. Learn the three disaccharides, the starch–cellulose–glycogen trio and the 162 g mol⁻¹ trick, and this chapter is yours.