The Structure and Function of Large Biological Molecules (Chapter 5 Study Notes)

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The Structure and Function of Large Biological Molecules

Introduction

Large biological molecules, also known as macromolecules, are essential for the structure and function of living organisms. The four major classes of biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids. Each class has unique structures and functions that contribute to cellular processes and life itself.

Molecules of Life

Main Classes of Biological Molecules

Carbohydrates: Serve as fuel and building material.
Lipids: Diverse group of hydrophobic molecules, important for energy storage and membrane structure.
Proteins: Perform a wide range of functions including catalysis, defense, transport, and structural support.
Nucleic Acids: Store, transmit, and express hereditary information.

Three of these classes (carbohydrates, proteins, nucleic acids) are polymers made from smaller units called monomers. Lipids are not true polymers.

Macromolecules: Polymers Built from Monomers

Polymer and Monomer Definitions

Polymer: A long molecule consisting of many similar or identical building blocks linked by covalent bonds.
Monomer: The repeating units that serve as building blocks of a polymer.

Carbohydrates, proteins, and nucleic acids are examples of polymers in biological systems.

Synthesis and Breakdown of Polymers

Dehydration Reaction: A chemical reaction in which two monomers are covalently bonded to each other with the removal of a water molecule, forming a longer polymer.
Hydrolysis: A chemical reaction that breaks bonds between monomers by adding a water molecule, resulting in shorter polymers or monomers.

Equation for Dehydration Synthesis:

Equation for Hydrolysis:

Carbohydrates: Fuel and Building Material

Types and Functions of Carbohydrates

Carbohydrates include sugars and polymers of sugars.
The simplest carbohydrates are monosaccharides (simple sugars).
Carbohydrate macromolecules are polysaccharides, polymers composed of many sugar building blocks.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates, typically having molecular formulas that are multiples of .
Examples include glucose (), fructose, and galactose.
Monosaccharides serve as major fuel for cells and as raw material for building other molecules.

Disaccharides and Polysaccharides

Disaccharide: Formed when two monosaccharides are joined by a dehydration reaction, creating a covalent bond called a glycosidic linkage.
Polysaccharide: Polymers of sugars that have storage (e.g., starch, glycogen) and structural (e.g., cellulose, chitin) roles in cells.

Comparison Table: Types of Carbohydrates

Type	Structure	Function	Example
Monosaccharide	Single sugar unit	Energy source	Glucose
Disaccharide	Two sugar units	Transport, energy	Sucrose
Polysaccharide	Many sugar units	Storage, structure	Starch, cellulose

Structural Polysaccharides

Cellulose: Major component of plant cell walls; composed of β-glucose monomers.
Chitin: Found in the exoskeleton of arthropods and cell walls of fungi; contains nitrogen-containing appendages.

Lipids: Diverse Hydrophobic Molecules

Characteristics and Types of Lipids

Lipids are hydrophobic molecules, not true polymers.
Main types: fats, phospholipids, and steroids.

Fats

Constructed from glycerol and fatty acids.
Main function: energy storage.
Saturated fats: No double bonds in fatty acid chains; solid at room temperature.
Unsaturated fats: One or more double bonds; liquid at room temperature.

Phospholipids

Composed of two fatty acids, a phosphate group, and glycerol.
Form the phospholipid bilayer of cell membranes, with hydrophobic tails and hydrophilic heads.

Steroids

Characterized by a carbon skeleton consisting of four fused rings.
Cholesterol is a key steroid in animal cell membranes and a precursor for other steroids.

Proteins: Diversity of Structure and Function

Functions of Proteins

Proteins account for more than 50% of the dry mass of most cells.
Functions include catalysis (enzymes), defense, storage, transport, cellular communication, movement, and structural support.

Amino Acids and Polypeptides

Amino acids: Organic molecules with amino and carboxyl groups, differing in their side chains (R groups).
Polypeptide: Polymer of amino acids linked by peptide bonds.
Proteins consist of one or more polypeptides folded into a specific three-dimensional structure.

Levels of Protein Structure

Primary structure: Unique sequence of amino acids.
Secondary structure: Coils and folds resulting from hydrogen bonds (e.g., α-helix, β-pleated sheet).
Tertiary structure: Overall shape determined by interactions among R groups.
Quaternary structure: Association of multiple polypeptide chains.

Protein Denaturation

Loss of a protein's native structure due to changes in pH, salt concentration, temperature, or other environmental factors.
Denatured proteins are biologically inactive.

Nucleic Acids: Storage and Transmission of Hereditary Information

Types and Functions

Two types: Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA).
DNA stores genetic information and directs its own replication.
RNA is involved in protein synthesis and gene expression.

Structure of Nucleic Acids

Nucleic acids are polymers called polynucleotides, made of monomers called nucleotides.
Each nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group.
DNA is double-stranded, forming a double helix; RNA is single-stranded.

Comparison Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Strands	Double	Single
Bases	A, T, C, G	A, U, C, G
Function	Genetic information storage	Protein synthesis, gene expression

Example: DNA provides the instructions for building proteins, which carry out cellular functions.

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