2.3 Carbohydrates and lipids Notes

Introduction

In the IB Biology syllabus, Topic 2.3 focuses on Carbohydrates and Lipids. These biomolecules are crucial for various biological functions, including energy storage, structural integrity, and cellular communication. This study note will break down the complex ideas into smaller, digestible sections, covering all the essential concepts and nuances.

Carbohydrates

Structure and Types of Carbohydrates

Carbohydrates are organic molecules composed of carbon (C), hydrogen (H), and oxygen (O) atoms, typically in a ratio of 1:2:1. The general formula for carbohydrates is $C_n(H_2O)_n$.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates consisting of single sugar units. Examples include:

• Glucose (C_6H_{12}O_6): A primary energy source for cells.
• Fructose: Found in fruits.
• Galactose: Part of lactose in milk.

Disaccharides

Disaccharides are formed by the condensation reaction between two monosaccharides, resulting in the formation of a glycosidic bond. Examples include:

• Sucrose (Glucose + Fructose): Common table sugar.
• Lactose (Glucose + Galactose): Found in milk.
• Maltose (Glucose + Glucose): Product of starch digestion.

$$ \text{C}6\text{H}{12}\text{O}6 + \text{C}6\text{H}{12}\text{O}6 \rightarrow \text{C}{12}\text{H}{22}\text{O}_{11} + \text{H}_2\text{O} $$

Polysaccharides

Polysaccharides are long chains of monosaccharides linked by glycosidic bonds. Examples include:

• Starch: Energy storage in plants.
• Glycogen: Energy storage in animals.
• Cellulose: Structural component of plant cell walls.

Functions of Carbohydrates

• Energy Storage: Glycogen in animals and starch in plants.
• Structural: Cellulose in plant cell walls.
• Recognition: Glycoproteins and glycolipids on cell surfaces.

Lipids

Structure of Lipids

Lipids are hydrophobic organic molecules composed mainly of carbon, hydrogen, and a smaller proportion of oxygen compared to carbohydrates. They are not polymers but are composed of smaller units.