Chapter 3: The Macromolecules of the Cell – Study Notes

Overview of the Molecules of Life

Introduction

All living organisms are composed of four major classes of large biological molecules, known as macromolecules. These macromolecules are essential for cellular structure and function. The four classes are: proteins, nucleic acids, carbohydrates, and lipids.

• Proteins – enzymes and structural molecules

• Nucleic acids – DNA and RNA

• Carbohydrates – sugars

• Lipids – fats and oils

Macromolecules of the Cell

Definition and Formation

Macromolecules are large molecules formed by the polymerization of smaller subunits, which are connected by covalent bonds. Each class of macromolecule is built from specific monomers:

• Proteins: built from amino acids

• Lipids: built from fatty acids and glycerol

• Carbohydrates: built from monosaccharides (sugar residues)

• Nucleic acids: built from nucleotides (nucleobases, sugar, phosphate)

Polymerization Reactions

• Dehydration reaction: Occurs when two monomers bond together through the loss of a water molecule. Example: $A + B \rightarrow A{-}B + H_2O$

• Hydrolysis: Polymers are disassembled to monomers by hydrolysis, which is essentially the reverse of the dehydration reaction. Example: $A{-}B + H_2O \rightarrow A + B$

Proteins

Structure and Function

Proteins are extremely important macromolecules found throughout the cell. They perform a wide variety of functions and are classified into nine major classes:

• Enzymes: Catalyze chemical reactions

• Structural proteins: Provide physical support and shape

• Motility proteins: Responsible for contraction and movement

• Regulatory proteins: Control and coordinate cell function

• Transport proteins: Move substances into and out of cells

• Signaling proteins: Facilitate communication between cells

• Receptor proteins: Enable cells to respond to chemical stimuli

• Defensive proteins: Protect against disease

• Storage proteins: Serve as reservoirs of amino acids

Amino Acid Structure

Proteins are polymers of amino acids, which are connected by peptide bonds (covalent bonds). Each amino acid contains:

• An amino group ($-NH_2$)

• A carboxylic group ($-COOH$)

• A specific side chain (R group)

• A central carbon atom

There are 20 different amino acids, each with a unique R group that determines its properties.

Classification of Amino Acids

• Nonpolar (hydrophobic): 9 amino acids with nonpolar R groups

• Polar (hydrophilic): 11 amino acids with polar or charged R groups

• Acidic amino acids: Negatively charged at cellular pH

• Basic amino acids: Positively charged at cellular pH

Polar amino acids are often found on the surfaces of proteins, interacting with the aqueous environment.

Levels of Protein Structure

• Primary structure: Unique sequence of amino acids

• Secondary structure: Local folding into α-helix and β-sheet conformations

• Tertiary structure: Three-dimensional shape formed by interactions among R groups

• Quaternary structure: Association of multiple polypeptide chains

Types of Bonds in Protein Folding

• Disulfide bonds (covalent)

• Hydrogen bonds

• Van der Waals interactions

• Hydrophobic interactions

Protein Motifs and Domains

• Motifs: Short stretches of α-helices and β-sheets, such as β-α-β, hairpin loop, and helix-turn-helix

• Domains: Discrete, locally folded units of tertiary structure, often associated with specific functions

Nucleic Acids

Structure and Function

Nucleic acids are polymers of nucleotides and exist in two forms: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). DNA stores hereditary information and directs its own replication, while RNA is involved in protein synthesis and gene expression.

• Nucleotide: Composed of a sugar, a phosphate group, and a nitrogenous base

Nitrogenous Bases

• Purines: Double ring structure (Adenine, Guanine)

• Pyrimidines: Single ring structure (Cytosine, Thymine, Uracil)

Sugars in Nucleic Acids

• Deoxyribose: Found in DNA, lacks an OH group at the 2' position

• Ribose: Found in RNA, has an OH group at the 2' position

Base Pairing Rules

• A pairs with T (DNA) or U (RNA) via 2 hydrogen bonds

• G pairs with C via 3 hydrogen bonds

Flow of Genetic Information

The central dogma of molecular biology describes the flow of genetic information:

• DNA → RNA → Protein

Carbohydrates

Structure and Function

Carbohydrates serve as fuel and building material. They include simple sugars (monosaccharides) and polymers (polysaccharides).

• Monosaccharides: Simple sugars, e.g., glucose ($C_6H_{12}O_6$)

• Disaccharides: Two monosaccharides joined by a glycosidic linkage, e.g., maltose

• Polysaccharides: Polymers of sugars, e.g., starch, glycogen, cellulose

Classification of Monosaccharides

• Aldoses: Sugars with an aldehyde group

• Ketoses: Sugars with a ketone group

• Classified by number of carbons: trioses (3C), pentoses (5C), hexoses (6C)

Polysaccharides

• Starch: Storage polysaccharide in plants (α-branching)

• Glycogen: Storage polysaccharide in animals

• Cellulose: Structural polysaccharide in plant cell walls (β-branching)

• Chitin: Structural polysaccharide in exoskeletons of arthropods and fungi cell walls

Lipids

Structure and Function

Lipids are a diverse group of hydrophobic molecules, including fats, oils, phospholipids, and sterols. They are insoluble in water and serve as energy storage, structural components, and signaling molecules.

• Triglycerides: Composed of one glycerol and three fatty acids

• Phospholipids: Composed of glycerol, two fatty acids, and a phosphate group; major component of cell membranes

• Sterols: Lipids with multiple carbon rings, e.g., cholesterol

Fatty Acids

• Saturated fatty acids: No double bonds, straight chains, solid at room temperature

• Unsaturated fatty acids: One or more double bonds, kinked chains, liquid at room temperature

• Essential fatty acids: Cannot be synthesized by the body (e.g., omega-3 and omega-6 fatty acids)

Phospholipids and Steroids

• Phospholipids: Amphipathic molecules, form bilayers in cell membranes

• Steroids: Lipids with four fused carbon rings; cholesterol is a precursor for steroid hormones

Summary Table: Macromolecules of the Cell

Macromolecule	Monomer	Examples	Functions
Proteins	Amino acids	Enzymes, structural proteins	Catalysis, support, movement, regulation, transport, signaling, defense, storage
Nucleic Acids	Nucleotides	DNA, RNA	Genetic information storage, gene expression
Carbohydrates	Monosaccharides	Glucose, starch, glycogen, cellulose	Energy source, structural support
Lipids	Fatty acids, glycerol	Triglycerides, phospholipids, cholesterol	Energy storage, membrane structure, signaling

Key Equations

• Dehydration reaction: $A + B \rightarrow A{-}B + H_2O$

• Hydrolysis reaction: $A{-}B + H_2O \rightarrow A + B$

Conclusion

The macromolecules of the cell—proteins, nucleic acids, carbohydrates, and lipids—are fundamental to cellular structure and function. Understanding their composition, structure, and roles is essential for the study of cell biology.