Question
Classify carbohydrates into monosaccharides, disaccharides, and polysaccharides with examples. What are reducing and non-reducing sugars?
(NEET and CBSE 12 — a high-frequency biomolecules topic)
Solution — Step by Step
Simplest sugars that cannot be hydrolysed further. Classified by number of carbon atoms:
- Triose (C3): Glyceraldehyde, dihydroxyacetone
- Pentose (C5): Ribose (in RNA), deoxyribose (in DNA)
- Hexose (C6): Glucose (grape sugar), fructose (fruit sugar), galactose
Glucose () is the most important monosaccharide — it is the primary energy currency of cells.
Formed by condensation of two monosaccharides via a glycosidic bond:
| Disaccharide | Components | Linkage | Source |
|---|---|---|---|
| Sucrose | Glucose + Fructose | alpha-1,2 | Sugarcane, sugar beet |
| Maltose | Glucose + Glucose | alpha-1,4 | Malt, starch hydrolysis |
| Lactose | Glucose + Galactose | beta-1,4 | Milk |
Reducing sugars have a free anomeric carbon (hemiacetal/hemiketal) that can reduce Tollen’s/Fehling’s reagent. Maltose and lactose are reducing; sucrose is non-reducing (both anomeric carbons are involved in the glycosidic bond).
| Polysaccharide | Monomer | Linkage | Function |
|---|---|---|---|
| Starch (amylose + amylopectin) | Glucose | alpha-1,4 (+ alpha-1,6 branching) | Energy storage in plants |
| Glycogen | Glucose | alpha-1,4 + alpha-1,6 (more branched) | Energy storage in animals |
| Cellulose | Glucose | beta-1,4 | Structural (plant cell wall) |
The alpha vs beta linkage makes all the difference: we can digest starch (alpha linkage) but not cellulose (beta linkage) — humans lack the enzyme cellulase.
graph TD
A[Carbohydrates] --> B["Monosaccharides"]
A --> C["Disaccharides"]
A --> D["Polysaccharides"]
B --> E["Glucose, Fructose, Galactose"]
C --> F["Sucrose = Glu + Fru"]
C --> G["Maltose = Glu + Glu"]
C --> H["Lactose = Glu + Gal"]
D --> I["Starch: alpha-1,4"]
D --> J["Cellulose: beta-1,4"]
D --> K["Glycogen: alpha-1,4 + 1,6"]Why This Works
Carbohydrate classification is based on hydrolysis — how many sugar units you get. Monosaccharides are the endpoint. The glycosidic bond type (alpha or beta) determines digestibility and function. Alpha bonds create helical, compact structures (starch, glycogen) suited for storage. Beta bonds create extended, straight chains (cellulose) that form strong fibres for structural support.
The reducing sugar concept is about chemistry: a free aldehyde or ketone group can act as a reducing agent. In sucrose, both the aldehyde of glucose and the ketone of fructose are locked in the glycosidic bond — neither is free to reduce Cu2+ or Ag+.
Alternative Method
For NEET, the key fact is: sucrose is the only common non-reducing disaccharide. If a question asks “which sugar does not reduce Fehling’s solution?” among glucose, fructose, maltose, lactose, and sucrose — the answer is sucrose. All monosaccharides are reducing sugars, and among common disaccharides, only sucrose is non-reducing.
Common Mistake
Students often say “fructose is not a reducing sugar because it is a ketose (not aldose).” This is wrong — fructose IS a reducing sugar. In alkaline conditions (Tollen’s, Fehling’s), fructose tautomerises to glucose via an enediol intermediate and can reduce the reagent. The reducing ability of a sugar depends on a free anomeric carbon, not just on being an aldose.
Also, do not confuse starch and cellulose — both are glucose polymers but with different linkages. Starch has alpha-1,4 (and alpha-1,6 in amylopectin), cellulose has beta-1,4. The iodine test gives blue colour with starch but not with cellulose.