Chapter 3: The Molecules of Cells – Structure and Function of Biological Macromolecules

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Chapter 3: The Molecules of Cells

Introduction

Biological molecules are essential for the structure and function of living organisms. The inability to digest certain molecules, such as lactose due to the absence of the enzyme lactase, highlights the importance of these molecules in daily life. This chapter explores the diversity, structure, and function of the four major classes of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids.

Overview of the four classes of biological macromolecules

Introduction to Organic Compounds

3.1 Life’s Molecular Diversity Is Based on the Properties of Carbon

Organic chemistry is the study of carbon-containing compounds. Carbon's unique ability to form four covalent bonds allows for the construction of a wide variety of complex molecules, forming the backbone of most organic molecules.

Carbon Skeletons: Chains of carbon atoms form the framework for organic molecules.
Isomers: Compounds with the same molecular formula but different structures and properties.
Hydrocarbons: Molecules consisting only of carbon and hydrogen; they are hydrophobic and serve as energy sources.

3.2 A Few Chemical Groups are Key to the Functioning of Biological Molecules

The chemical behavior of organic molecules is largely determined by functional groups attached to the carbon skeleton. These groups confer specific properties, such as polarity and reactivity, to the molecules.

Functional Groups: Common groups include hydroxyl, carbonyl, carboxyl, amino, phosphate, and methyl groups.
Hydrophilic Groups: Most functional groups (except methyl) increase solubility in water.

Table of important chemical groups of organic compounds

Example: Testosterone and estradiol differ only in their functional groups, yet have distinct biological effects.

Comparison of testosterone and estradiol structures

Macromolecules: Polymers and Monomers

3.3 Cells Make Large Molecules from a Limited Set of Small Molecules

Most biological macromolecules are polymers, long chains made from repeating units called monomers. The four major classes are carbohydrates, lipids, proteins, and nucleic acids.

Dehydration Reaction: Monomers are joined by removing a molecule of water.
Hydrolysis: Polymers are broken down into monomers by adding water.
Enzymes: Specialized proteins that catalyze these reactions.

Dehydration and hydrolysis reactions

Carbohydrates

3.4 Monosaccharides are the Simplest Carbohydrates

Carbohydrates serve as fuel and building material. They range from simple sugars (monosaccharides) to complex polysaccharides.

Monosaccharides: Simple sugars with a general formula of (e.g., glucose, fructose).
Functional Groups: Contain hydroxyl and carbonyl groups.

3.5 Two Monosaccharides Are Linked to Form a Disaccharide

Disaccharides are formed by joining two monosaccharides via a dehydration reaction (glycosidic bond).

Examples: Sucrose (glucose + fructose), lactose (glucose + galactose).

3.7 Polysaccharides Are Long Chains of Sugar Units

Polysaccharides are large carbohydrates with storage or structural roles.

Starch: Storage form in plants; composed of glucose monomers.
Glycogen: Storage form in animals; highly branched glucose polymer.
Cellulose: Structural component of plant cell walls; glucose monomers linked differently than in starch.
Chitin: Structural polysaccharide in fungi and arthropods.

Comparison of starch, glycogen, and cellulose structure

Lipids

3.8 Fats Are Lipids That Are Mostly Energy-Storage Molecules

Lipids are hydrophobic molecules, mainly composed of carbon and hydrogen. Fats (triglycerides) are the main energy storage molecules in animals.

Structure: One glycerol molecule joined to three fatty acids.
Saturated Fats: No double bonds; solid at room temperature; found in animal fats.
Unsaturated Fats: One or more double bonds; liquid at room temperature; found in plant oils and fish.
Trans Fats: Produced by hydrogenating unsaturated fats; associated with health risks.

Formation of a fat molecule from glycerol and fatty acids Structure of a triglyceride (fat molecule) Comparison of saturated and unsaturated fats

3.10 Phospholipids and Steroids Are Important Lipids with a Variety of Functions

Phospholipids and steroids are essential for cell structure and signaling.

Phospholipids: Major component of cell membranes; have hydrophilic heads and hydrophobic tails.
Steroids: Lipids with a carbon skeleton of four fused rings; include cholesterol and hormones.

Structure of a phospholipid Phospholipid bilayer in cell membranes Structure of cholesterol, a steroid

Proteins

3.12 Proteins Have a Wide Range of Functions and Structures

Proteins are the most diverse macromolecules, performing many functions in the cell.

Functions: Enzymes, transport, defense, signaling, structure, movement, and storage.
Structure: Made from 20 different amino acids.
Denaturation: Loss of protein structure leads to loss of function.

3.13 Proteins Are Made from Amino Acids Linked by Peptide Bonds

Amino acids are joined by peptide bonds to form polypeptides. The sequence of amino acids determines the protein's structure and function.

Amino Acid Structure: Central carbon bonded to an amino group, carboxyl group, hydrogen, and R group (side chain).
Peptide Bond: Covalent bond formed by dehydration reaction between amino and carboxyl groups.

General structure of an amino acid Examples of hydrophobic and hydrophilic amino acids Formation of a peptide bond between amino acids

3.14 A Protein’s Functional Shape Results from Four Levels of Structure

The function of a protein depends on its three-dimensional shape, which is organized into four levels of structure:

Primary Structure: Sequence of amino acids.
Secondary Structure: Coiling (alpha helix) or folding (beta sheet) stabilized by hydrogen bonds.
Tertiary Structure: Overall 3D shape due to interactions among R groups.
Quaternary Structure: Association of multiple polypeptide chains.

Four levels of protein structure

Nucleic Acids

3.15 The Nucleic Acids DNA and RNA Are Information-Rich Polymers of Nucleotides

Nucleic acids store and transmit genetic information. DNA and RNA are polymers of nucleotides, each consisting of a sugar, a phosphate group, and a nitrogenous base.

DNA: Double helix; stores genetic information; bases are adenine (A), thymine (T), cytosine (C), guanine (G).
RNA: Single-stranded; involved in protein synthesis; bases are adenine (A), uracil (U), cytosine (C), guanine (G).
Complementary Base Pairing: Enables accurate DNA replication and transcription.

Structure of a nucleotide DNA strand with sugar-phosphate backbone and bases DNA double helix with base pairs Flow of genetic information: DNA to RNA to protein

Summary Table: Major Classes of Biological Molecules

Class	Monomer	Polymer	Function	Example
Carbohydrates	Monosaccharide	Polysaccharide	Energy, structure	Glucose, starch, cellulose
Lipids	Fatty acid, glycerol	Triglyceride, phospholipid	Energy storage, membranes	Fats, oils, phospholipids
Proteins	Amino acid	Polypeptide	Catalysis, structure, transport	Enzymes, collagen
Nucleic Acids	Nucleotide	DNA, RNA	Genetic information	DNA, RNA