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 all living organisms. The four major classes of macromolecules are carbohydrates, lipids, proteins, and nucleic acids. Each class has unique properties and roles in biological systems.

The 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, transport, and structural support.
Nucleic Acids: Store, transmit, and help express hereditary information.

Concept 5.1: Macromolecules are Polymers Built from Monomers

Polymers and Monomers

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.
Carbohydrates, proteins, and nucleic acids are polymers; lipids are not true polymers.

Synthesis and Breakdown of Polymers

Dehydration Reaction: Synthesizes polymers by removing a water molecule, forming a new bond.
Hydrolysis: Breaks down polymers by adding a water molecule, breaking a bond.

Concept 5.2: Carbohydrates Serve as Fuel and Building Material

Types of Carbohydrates

Monosaccharides: Simple sugars with molecular formulas that are multiples of . Example: Glucose .
Disaccharides: Formed when a dehydration reaction joins two monosaccharides. The covalent bond is called a glycosidic linkage.
Polysaccharides: Polymers of sugars with storage (e.g., starch, glycogen) and structural (e.g., cellulose, chitin) roles.

Monosaccharide Structures

Can exist in linear or ring forms; in aqueous solutions, ring forms are predominant.
Serve as major fuel for cells and as raw material for building molecules.

Disaccharides and Glycosidic Linkages

Examples: Maltose (glucose + glucose), Sucrose (glucose + fructose).

Polysaccharides: Storage and Structural Roles

Starch: Storage polysaccharide in plants, composed of α-glucose monomers.
Glycogen: Storage polysaccharide in animals, extensively branched.
Cellulose: Structural polysaccharide in plant cell walls, composed of β-glucose monomers.
Chitin: Structural polysaccharide in the exoskeleton of arthropods and cell walls of fungi.

Polysaccharide	Monomer Type	Function
Starch	α-glucose	Energy storage in plants
Glycogen	α-glucose	Energy storage in animals
Cellulose	β-glucose	Structural support in plants
Chitin	Modified glucose	Structural support in fungi and arthropods

Concept 5.3: Lipids are a Diverse Group of Hydrophobic Molecules

General Properties

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

Fats

Constructed from glycerol (a three-carbon alcohol) and fatty acids (long hydrocarbon chains with a carboxyl group).
Major function: energy storage.
Saturated fats: No double bonds, solid at room temperature (e.g., animal fats).
Unsaturated fats: One or more double bonds, liquid at room temperature (e.g., plant and fish fats).

Phospholipids

Composed of two fatty acids and a phosphate group attached to glycerol.
Fatty acid tails are hydrophobic; phosphate head is hydrophilic.
Form bilayers in water, which are the basis of cell membranes.

Steroids

Characterized by a carbon skeleton with four fused rings.
Cholesterol: Important component of animal cell membranes and precursor for other steroids.

Concept 5.4: Proteins Include a Diversity of Structures and Functions

Functions of Proteins

Enzymatic activity (catalysis)
Defense (immune response)
Storage (e.g., storage of amino acids)
Transport (e.g., hemoglobin)
Cellular communication
Movement (e.g., muscle contraction)
Structural support (e.g., collagen)

Protein Structure

Proteins are polymers of amino acids (20 types).
Polypeptides: Unbranched polymers built from amino acids, linked by peptide bonds.
Each polypeptide has a unique linear sequence of amino acids (N-terminus to C-terminus).

Levels of Protein Structure

Primary structure: Unique sequence of amino acids.
Secondary structure: Coils and folds (α-helix, β-pleated sheet) due to hydrogen bonding.
Tertiary structure: Overall 3D shape determined by interactions among R groups (side chains).
Quaternary structure: Association of multiple polypeptide chains.

Level	Description
Primary	Sequence of amino acids
Secondary	α-helix and β-sheet structures
Tertiary	3D folding due to R group interactions
Quaternary	Multiple polypeptide chains

Protein Denaturation

Loss of native structure due to changes in pH, salt concentration, temperature, or other environmental factors.
Denatured proteins lose their function.

Concept 5.5: Nucleic Acids Store, Transmit, and Help Express Hereditary Information

Types of Nucleic Acids

Deoxyribonucleic acid (DNA)
Ribonucleic acid (RNA)
DNA provides directions for its own replication and directs synthesis of messenger RNA (mRNA), which controls protein synthesis.

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.
Nitrogenous bases: Pyrimidines (Cytosine, Thymine, Uracil), Purines (Adenine, Guanine).
Sugars: Deoxyribose in DNA, Ribose in RNA.

Type	Strands	Sugar	Bases
DNA	Double	Deoxyribose	A, T, C, G
RNA	Single	Ribose	A, U, C, G

Structures of DNA and RNA Molecules

DNA: Two polynucleotides form a double helix, held together by hydrogen bonds between complementary bases.
RNA: Single-stranded, can fold into complex shapes.

Summary Table: Major Biological Macromolecules

Macromolecule	Monomer	Bond Type	Main Function
Carbohydrate	Monosaccharide	Glycosidic linkage	Energy, structure
Lipid	Fatty acid, glycerol	Ester linkage	Energy, membranes
Protein	Amino acid	Peptide bond	Catalysis, structure, transport
Nucleic Acid	Nucleotide	Phosphodiester bond	Genetic information

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