The Structure and Function of Large Biological Molecules

Chapter 5: The Structure and Function of Large Biological Molecules

Overview: The Molecules of Life

All living organisms are composed of four major classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids. These molecules, known as macromolecules, are formed by thousands of covalently connected atoms. The structure of these molecules is closely linked to their function in biological systems.

• Macromolecules: Large molecules essential for life, composed of many atoms.

• Structure and Function: The specific arrangement of atoms determines the molecule's role in the cell.

Concept 5.1: Macromolecules are Polymers Built from Monomers

Most biological macromolecules are polymers, which are long chains made from repeating units called monomers. Three of the four major classes—carbohydrates, proteins, and nucleic acids—are polymers.

• Polymer: A long molecule consisting of many similar or identical building blocks linked by covalent bonds.

• Monomer: The repeating unit that serves as a building block for a polymer.

• Examples of Biological Polymers:

  • Carbohydrates (e.g., starch, cellulose)

  • Proteins (e.g., enzymes, structural proteins)

  • Nucleic acids (e.g., DNA, RNA)

Synthesis and Breakdown of Polymers

Polymers are assembled and disassembled by two key chemical reactions: dehydration synthesis and hydrolysis.

• Dehydration Reaction: Two monomers are joined together by removing a water molecule, forming a new covalent bond.

• Hydrolysis: Polymers are broken down into monomers by adding a water molecule, which breaks the covalent bond.

Equation for Dehydration Synthesis:

$\text{Monomer}_1 + \text{Monomer}_2 \xrightarrow{\text{Dehydration}} \text{Polymer} + H_2O$

Equation for Hydrolysis:

$\text{Polymer} + H_2O \xrightarrow{\text{Hydrolysis}} \text{Monomer}_1 + \text{Monomer}_2$

Diversity of Polymers

Cells contain thousands of different macromolecules, which vary among cells, species, and individuals. This diversity arises from the arrangement of a small set of monomers into countless unique polymers.

• Variation: Macromolecules differ between cells, species, and individuals.

• Immense Variety: A small set of monomers can create a vast array of polymers.

Concept 5.2: Carbohydrates Serve as Fuel and Building Material

Carbohydrates are organic molecules that include sugars and their polymers. They serve as energy sources and structural components in cells.

• Monosaccharides: The simplest carbohydrates, also known as single sugars (e.g., glucose).

• Polysaccharides: Carbohydrate macromolecules composed of many sugar building blocks.

Monosaccharides

Monosaccharides have molecular formulas that are usually multiples of $CH_2O$. Glucose ($C_6H_{12}O_6$) is the most common monosaccharide. They are classified by:

• The location of the carbonyl group (as aldose or ketose).

• The number of carbons in the carbon skeleton (triose, pentose, hexose, etc.).

Examples:

Type	Example	Structure
Aldose (Aldehyde Sugar)	Glucose, Ribose	Carbonyl group at end of carbon chain
Ketose (Ketone Sugar)	Fructose, Ribulose	Carbonyl group within carbon chain

Monosaccharides are often drawn as linear skeletons, but in aqueous solutions, many form ring structures.

Functions of Monosaccharides

• Serve as fuel for cellular respiration.

• Act as raw material for building other molecules.

Additional info:

Monosaccharides can be joined by glycosidic linkages to form disaccharides and polysaccharides. The structure and function of polysaccharides depend on the type of monomers and the positions of glycosidic bonds.