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 life. They include carbohydrates, lipids, proteins, and nucleic acids, each with unique structures and functions. Understanding these molecules is fundamental to the study of biology.

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 and transmit 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 by dehydration reaction joining two monosaccharides; 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 common.
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 exoskeletons of arthropods and cell walls of fungi.

Table: Types of Polysaccharides

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

Concept 5.3: Lipids are a Diverse Group of Hydrophobic Molecules

Characteristics of Lipids

Not true polymers; hydrophobic due to nonpolar hydrocarbon regions.
Main types: fats, phospholipids, steroids.

Fats

Constructed from glycerol (three-carbon alcohol) and fatty acids (carboxyl group attached to hydrocarbon chain).
Major function: energy storage.
Saturated fats: No double bonds; solid at room temperature; mostly animal fats.
Unsaturated fats: One or more double bonds; liquid at room temperature; 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, making up cell membranes.

Steroids

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

Concept 5.4: Proteins Include a Diversity of Structures and Functions

Functions of Proteins

Enzymatic activity, defense, storage, transport, cellular communication, movement, structural support.
Composed of 20 different amino acids.

Amino Acids and Polypeptides

Amino acids: Organic molecules with amino and carboxyl groups; differ by side chains (R groups).
Polypeptides: Polymers of amino acids linked by peptide bonds; have unique sequences and ends (N-terminus and C-terminus).

Table: Amino Acid Side Chains

Type	Examples
Nonpolar (hydrophobic)	Glycine, Alanine, Valine, Leucine, Isoleucine
Polar (hydrophilic)	Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine
Acidic (negatively charged)	Aspartic acid, Glutamic acid
Basic (positively charged)	Lysine, Arginine, Histidine

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 due to interactions among R groups.
Quaternary structure: Association of multiple polypeptide chains.

Denaturation

Loss of protein's native structure due to changes in pH, salt concentration, temperature, or other environmental factors.

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

Types and Functions of Nucleic Acids

DNA (Deoxyribonucleic acid): Stores genetic information; directs its own replication and synthesis of mRNA.
RNA (Ribonucleic acid): Functions in protein synthesis and gene expression.

Structure of Nucleic Acids

Nucleic acids are polymers called polynucleotides, made of monomers called nucleotides.
Nucleotide components: nitrogenous base (A, T, C, G, U), pentose sugar (deoxyribose in DNA, ribose in RNA), phosphate group.

Table: Nitrogenous Bases

Type	Bases
Pyrimidines	Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA)
Purines	Adenine (A), Guanine (G)

DNA and RNA Structures

DNA: Double helix of two polynucleotide strands held together by hydrogen bonds between complementary bases.
RNA: Single-stranded; various forms (mRNA, tRNA, rRNA) involved in protein synthesis.

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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