Chapter 5: Carbohydrates – Structure, Function, and Biological Roles

Carbohydrates: Structure and Function

Introduction to Carbohydrates

Carbohydrates are essential biomolecules that play critical roles in cell structure, identity, and energy storage. They are classified based on the number of sugar units they contain.

• Monosaccharide: Single sugar monomers (e.g., glucose).

• Oligosaccharide: Short chains of sugar monomers ("few-sugars"), typically 2–10 units.

• Polysaccharide: Long chains of sugar monomers ("many-sugars"), often hundreds or thousands of units.

Key roles of carbohydrates:

• Structural support in cells and organisms

• Cellular identity and recognition

• Energy storage and supply

Structure of Monosaccharides

General Features of Monosaccharides

Monosaccharides are the simplest carbohydrates and serve as building blocks for more complex sugars. Their general formula is typically $(CH_2O)_n$.

• Contain at least three carbon atoms

• Have one carbonyl group (C=O)

• Have two or more hydroxyl groups (-OH)

Naming Monosaccharides by Carbonyl Position

Monosaccharides are classified based on the position of the carbonyl group:

• Aldose: Carbonyl group at the end of the carbon chain

• Ketose: Carbonyl group within the carbon chain

Example: Glucose is an aldose; fructose is a ketose.

Naming Monosaccharides by Number of Carbons

The number of carbon atoms in a monosaccharide determines its classification:

• 3C = Triose

• 4C = Tetrose

• 5C = Pentose

• 6C = Hexose

Monosaccharides with the same molecular formula but different arrangements are called isomers.

Examples:

• Dihydroxyacetone (3C, triose)

• Glucose (6C, hexose)

Sugars Exist in Both Linear and Ring Forms

Monosaccharides can exist in a linear form or cyclize to form ring structures, especially in aqueous solutions. The ring formation involves the reaction between the carbonyl group and a hydroxyl group within the same molecule.

• α-Glucose: Hydroxyl group on C1 is below the plane of the ring

• β-Glucose: Hydroxyl group on C1 is above the plane of the ring

This structural variation affects the properties and reactivity of the sugar.

Summary Table: Monosaccharide Classification

Type	Carbonyl Position	Number of Carbons	Example
Aldose	End	3 (Triose)	Glyceraldehyde
Ketose	Internal	3 (Triose)	Dihydroxyacetone
Aldose	End	6 (Hexose)	Glucose
Ketose	Internal	6 (Hexose)	Fructose

Additional info:

• Monosaccharides are the foundation for oligosaccharides and polysaccharides, which have diverse biological functions.

• The ring forms of sugars are predominant in biological systems and are crucial for the formation of glycosidic linkages in larger carbohydrates.