Oligosaccharides and Polysaccharides; Carbohydrates are classified into monosaccharides, oligosaccharides, and polysaccharides based on the number of sugar units they contain.

• Oligosaccharides

Definition: Oligosaccharides are carbohydrates that consist of 2 to 10 monosaccharide units linked by glycosidic bonds. They are soluble in water, sweet in taste, and crystalline in nature.

General Formula: (C₆H₁₀O₅)n where n = 2 to 10.

Classification of Oligosaccharides: Oligosaccharides are classified based on the number of monosaccharide units:

• Disaccharides (2 sugar units) – e.g., Sucrose, Lactose, Maltose
• Trisaccharides (3 sugar units) – e.g., Raffinose
• Tetrasaccharides (4 sugar units) – e.g., Stachyose

Types of Oligosaccharides

• Disaccharides (n = 2): Disaccharides are the most common oligosaccharides. They are formed by the condensation of two monosaccharides through a glycosidic bond (removal of water).

• Sucrose: Non-reducing sugar (No free aldehyde or ketone group).
• Lactose and Maltose: Reducing sugars (Free aldehyde/ketone group).

• Trisaccharides (n = 3); Trisaccharides contain three monosaccharide units. Example: Raffinose → Galactose + Glucose + Fructose (found in beans, cabbage).
• Tetrasaccharides (n = 4): Tetrasaccharides contain four monosaccharide units. Example: Stachyose → Two Galactose + Glucose + Fructose (found in legumes).

Functions of Oligosaccharides

1. Energy Source – Easily hydrolyzed into monosaccharides for energy production.
2. Prebiotics – Some oligosaccharides (e.g., raffinose) act as prebiotics, supporting gut health.
3. Glycoproteins & Glycolipids – Found in cell membranes, involved in cell recognition and signaling.
4. Milk Oligosaccharides – Important for infant gut health (e.g., human milk oligosaccharides).

2. Polysaccharides

Definition: Polysaccharides are complex carbohydrates made up of more than 10 monosaccharide units, linked by glycosidic bonds. They are insoluble in water, tasteless, and non-reducing.

General Formula: (C₆H₁₀O₅)n  where n = hundreds to thousands.

Classification of Polysaccharides: Polysaccharides can be classified based on their composition and function.

i) Based on Composition

• Homopolysaccharides – Contain only one type of monosaccharide. Example: Starch, Glycogen, Cellulose, Chitin.
• Heteropolysaccharides – Contain two or more types of monosaccharides. Example: Hyaluronic acid, Heparin, Agar, Peptidoglycan.

ii) Based on Function

• Storage Polysaccharides – Energy storage in plants and animals. Examples: Starch (plants), Glycogen (animals).
• Structural Polysaccharides – Provide structural support to plants and animals. Examples: Cellulose (plants), Chitin (exoskeleton of insects, fungi).

Types of Polysaccharides

1. Storage Polysaccharides

(a) Starch (Plant Storage Polysaccharide)

• Made of: Amylose + Amylopectin.
• Source: Cereals, tubers, grains.
• Function: Stores energy in plants.

• Iodine Test for Starch → Blue-black color indicates the presence of starch.

(b) Glycogen (Animal Storage Polysaccharide)

• Structure: Highly branched polymer of glucose (like amylopectin but more branched).
• Glycosidic Bond: α(1→4) in chains, α(1→6) at branching points.
• Location: Liver and muscles.
• Function: Energy storage in animals.

2. Structural Polysaccharides

(a) Cellulose (Plant Structural Polysaccharide)

• Structure: Linear polymer of β-D-glucose.
• Glycosidic Bond: β(1→4) (forms rigid fibers).
• Function: Provides mechanical strength to plant cell walls.
• Digestion: Humans cannot digest cellulose due to lack of cellulase enzyme.
• Uses: Paper, textiles, cotton.

(b) Chitin (Animal Structural Polysaccharide)

• Structure: Linear polymer of N-acetylglucosamine (NAG).
• Glycosidic Bond: β(1→4).
• Location: Exoskeleton of insects, fungi cell walls.
• Function: Provides strength and protection.
• Uses: Surgical threads, biodegradable plastics.

3. Heteropolysaccharides (Mixed Monosaccharides)

(a) Hyaluronic Acid

• Composition: Glucuronic acid + N-acetylglucosamine.
• Function: Present in connective tissue, synovial fluid, maintains lubrication.

(b) Heparin

• Composition: Glucosamine + Iduronic acid + Sulfate groups.
• Function: Anticoagulant, prevents blood clotting.

(c) Peptidoglycan (Murein)

• Composition: N-acetylglucosamine (NAG) + N-acetylmuramic acid (NAM).
• Function: Bacterial cell wall component, provides rigidity.

Summary Table