Biomolecules: Carbohydrates and Their Biological Importance

Biomolecules Part 1 Recap

Learning Objectives

• Distinguish between dehydration reactions and hydrolysis reactions.

• Compare the structure and function of the three classes of carbohydrates: monosaccharides, disaccharides, and polysaccharides.

• Categorize common sugars (e.g., glucose, galactose, fructose, ribose, deoxyribose, sucrose, lactose, maltose, cellulose, starch, glycogen) into their correct classes.

• Compare the structure and function of major lipid types: triglycerides, phospholipids, and steroids.

Dehydration Synthesis and Hydrolysis

Overview

Cells build and break down macromolecules using two fundamental types of chemical reactions: dehydration synthesis and hydrolysis.

• Dehydration Synthesis (Condensation Reaction): A chemical reaction in which two molecules are covalently bonded to each other with the removal of a water molecule.

• Hydrolysis: A chemical reaction that breaks bonds between two molecules by the addition of water; functions in disassembly of polymers to monomers.

Example: Formation and breakdown of sucrose (table sugar):

• Dehydration synthesis: Glucose + Fructose → Sucrose + H2O

• Hydrolysis: Sucrose + H2O → Glucose + Fructose

Carbohydrates

Definition and General Properties

Carbohydrates are organic molecules that include sugars and starches. They are a primary source of energy for most organisms.

• Composed of carbon (C), hydrogen (H), and oxygen (O).

• Hydrogen and oxygen are typically in a 2:1 ratio (as in water).

• Three main classes based on size and complexity:

  • Monosaccharides: Single sugar units; monomers of carbohydrates.

  • Disaccharides: Two monosaccharides joined together.

  • Polysaccharides: Long chains (polymers) of monosaccharide units.

Monosaccharides

Monosaccharides are the simplest carbohydrates and serve as the building blocks for more complex sugars.

• General formula: (CH2O)n, where n = 3–7.

• Common examples:

  • Glucose (C6H12O6): Main energy source for cells.

  • Fructose: Found in fruits; isomer of glucose.

  • Galactose: Component of milk sugar.

  • Ribose: Component of RNA.

  • Deoxyribose: Component of DNA.

Disaccharides

Disaccharides are formed by joining two monosaccharides via a dehydration synthesis reaction.

• Common examples:

  • Sucrose (glucose + fructose): Table sugar.

  • Lactose (glucose + galactose): Milk sugar.

  • Maltose (glucose + glucose): Product of starch digestion.

• Disaccharides must be broken down into monosaccharides to be absorbed by the body (via hydrolysis).

Polysaccharides

Polysaccharides are large, complex carbohydrates composed of many monosaccharide units.

• Serve as energy storage or structural molecules.

• Examples:

  • Starch: Storage form of glucose in plants.

  • Glycogen: Storage form of glucose in animals (mainly in liver and muscle).

  • Cellulose: Structural component of plant cell walls; not digestible by humans.

Table: Classification of Common Carbohydrates

Class	Examples	Function
Monosaccharides	Glucose, Fructose, Galactose, Ribose, Deoxyribose	Energy source, building blocks for nucleic acids
Disaccharides	Sucrose, Lactose, Maltose	Transport form of sugars, must be digested to monosaccharides
Polysaccharides	Starch, Glycogen, Cellulose	Energy storage (starch, glycogen), structural support (cellulose)

Summary

• Carbohydrates are essential biomolecules for energy and structure in living organisms.

• They are classified by the number of sugar units: monosaccharides, disaccharides, and polysaccharides.

• Dehydration synthesis and hydrolysis are key reactions in the formation and breakdown of carbohydrates.