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Carbohydrates (Saccharides)

Definition, Classification, and Properties.

By chemca Team • Updated Jan 2026

Carbohydrates are optically active polyhydroxy aldehydes or ketones or compounds which produce such units on hydrolysis. They are primarily produced by plants via photosynthesis.

1. Monosaccharides

Simple Sugars

These cannot be hydrolyzed into simpler polyhydroxy aldehydes or ketones.

• General Formula: $(CH_2O)_n$ (usually).
• Examples: Glucose, Fructose, Ribose, Galactose.
• Types:
  • Aldoses (contain Aldehyde group, e.g., Glucose).
  • Ketoses (contain Ketone group, e.g., Fructose).

2. Oligosaccharides

2 to 10 Monosaccharide Units

These yield 2 to 10 monosaccharide units on hydrolysis.

Disaccharides (Yield 2 units):

$$ \text{Sucrose} + H_2O \rightarrow \text{Glucose} + \text{Fructose} $$ $$ \text{Maltose} + H_2O \rightarrow \text{Glucose} + \text{Glucose} $$ $$ \text{Lactose} + H_2O \rightarrow \text{Glucose} + \text{Galactose} $$

Glycosidic Linkage: The oxide linkage formed by the loss of a water molecule between two monosaccharide units through oxygen atom.

3. Polysaccharides

Non-Sugars

Polymers containing a large number of monosaccharide units joined by glycosidic linkages.

• Starch: Storage polysaccharide in plants. Polymer of $\alpha$-glucose. Consists of Amylose (linear, water soluble) and Amylopectin (branched, insoluble).
• Cellulose: Structural polysaccharide in plants (Cell wall). Linear polymer of $\beta$-glucose.
• Glycogen: "Animal Starch". Storage polysaccharide in animals (Liver, Muscles). Highly branched structure similar to amylopectin.

4. Reducing vs Non-Reducing Sugars

Reducing Sugars

Contain free aldehyde or ketone group (or hemiacetal group).

Reduce Tollens' reagent and Fehling's solution.

Examples: All Monosaccharides (Glucose, Fructose), Maltose, Lactose.

Non-Reducing Sugars

No free aldehyde or ketone group (bonded in glycosidic linkage).

Do NOT reduce Tollens' or Fehling's.

Examples: Sucrose, Polysaccharides.

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