The Molecules of Cells: Structure and Function of Biological Macromolecules

Chapter 3: The Molecules of Cells

Big Ideas and Overview

This chapter introduces the major classes of biological molecules essential for cellular structure and function. The focus is on organic compounds, carbohydrates, lipids, proteins, and nucleic acids, which together form the molecular basis of life.

• Organic Compounds: Molecules containing carbon, fundamental to living organisms.

• Carbohydrates: Sugars and polymers of sugars, serving as energy sources and structural materials.

• Lipids: Hydrophobic molecules including fats, phospholipids, and steroids.

• Proteins: Polymers of amino acids with diverse functions.

• Nucleic Acids: DNA and RNA, carriers of genetic information.

Introduction to Organic Compounds

Carbon: The Backbone of Life

Carbon atoms are uniquely suited to form the backbone of organic molecules due to their bonding properties.

• Valence Electrons: Carbon has 4 electrons in its valence shell, allowing it to form up to 4 covalent bonds.

• Diversity: This bonding ability enables the construction of large and diverse organic compounds.

• Structural Versatility: Carbon chains can be straight, branched, or arranged in rings.

Isomers and Hydrocarbons

Organic molecules can exist in different structural forms, known as isomers, and may consist solely of carbon and hydrogen (hydrocarbons).

• Isomers: Compounds with the same molecular formula but different structures and properties.

• Hydrocarbons: Molecules composed only of carbon and hydrogen, such as methane and ethane.

Carbon Skeletons and Molecular Representations

Types of Carbon Skeletons

Carbon skeletons can vary in length, branching, and ring formation, influencing molecular properties.

• Straight Chains: e.g., Propane ($C_3H_8$)

• Branched Chains: e.g., Isobutane

• Rings: e.g., Cyclohexane ($C_6H_{12}$), Benzene ($C_6H_6$)

• Double Bonds: e.g., 1-Butene, 2-Butene (location of double bond affects properties)

Molecule Notation and Representation

Molecules can be represented in various ways to highlight structure and connectivity.

• Structural Formula: Shows all atoms and bonds explicitly.

• Ball-and-Stick Model: Visualizes 3D arrangement.

• Abbreviated Ring Notation: Corners represent carbon atoms; hydrogens are often omitted for simplicity.

Functional Groups

Definition and Importance

Functional groups are specific groups of atoms attached to the carbon skeleton that confer distinct chemical properties to organic molecules.

• Hydroxyl Group (-OH): Found in alcohols; increases solubility in water.

• Carbonyl Group (C=O): Found in aldehydes and ketones.

• Carboxyl Group (-COOH): Characteristic of acids.

• Amino Group (-NH2): Found in amino acids.

• Phosphate Group (-PO4): Important in nucleic acids and energy transfer.

• Methyl Group (-CH3): Affects gene expression and molecular shape.

Chemical Group	Example
Hydroxyl (-OH)	Alcohols
Carbonyl (C=O)	Aldehydes, Ketones
Carboxyl (-COOH)	Acids
Amino (-NH2)	Amino acids
Phosphate (-PO4)	Nucleotides
Methyl (-CH3)	Methylated compounds

Macromolecules and Polymers

Formation and Structure

Most biological macromolecules are polymers, formed by linking smaller units called monomers.

• Polymer: Large molecule made from repeating monomer units.

• Monomer: Small building block molecule.

• Dehydration Reaction: Joins monomers by removing water.

• Hydrolysis: Breaks polymers into monomers by adding water.

Carbohydrates

Monosaccharides

Monosaccharides are simple sugars and the basic units of carbohydrates.

• General Formula: $(CH_2O)_n$

• Hexoses: Six-carbon sugars (e.g., glucose, $C_6H_{12}O_6$)

• Pentoses: Five-carbon sugars (e.g., ribose, important in RNA and DNA)

• Functional Groups: Hydroxyl and carbonyl groups

Disaccharides

Disaccharides are formed by joining two monosaccharides via a dehydration reaction.

• Examples: Sucrose (glucose + fructose), Lactose (glucose + galactose), Maltose (glucose + glucose)

Polysaccharides

Polysaccharides are long chains of sugar units with storage or structural roles.

• Starch: Storage polysaccharide in plants

• Glycogen: Storage polysaccharide in animals

• Cellulose: Structural polysaccharide in plant cell walls

• Chitin: Structural polysaccharide in fungi and exoskeletons of insects and crustaceans

Lipids

Fats and Fatty Acids

Lipids are hydrophobic molecules, primarily composed of carbon and hydrogen.

• Triglycerides: Glycerol linked to three fatty acids; main form of energy storage

• Saturated Fatty Acids: No double bonds; solid at room temperature; common in animal fats

• Unsaturated Fatty Acids: One or more double bonds; liquid at room temperature; typical of plant oils

Phospholipids and Steroids

Phospholipids and steroids have specialized functions in cells.

• Phospholipids: Major component of cell membranes; amphipathic structure

• Steroids: Four fused rings; includes cholesterol and hormones (e.g., estradiol)

Proteins

Amino Acids and Protein Structure

Proteins are polymers of amino acids, each with a unique sequence and structure.

• Amino Acid Structure: Central carbon bonded to amino group, carboxyl group, hydrogen, and R group (side chain)

• Peptide Bond: Covalent bond linking amino acids

• Polypeptide: Chain of amino acids

Levels of Protein Structure

• Primary Structure: Sequence of amino acids

• Secondary Structure: Local folding (alpha helix, beta sheet) stabilized by hydrogen bonds

• Tertiary Structure: Overall 3D shape due to interactions among R groups

• Quaternary Structure: Association of multiple polypeptide chains

Protein Function

• Enzymes: Catalyze biochemical reactions

• Structural Proteins: Provide support (e.g., collagen)

• Transport Proteins: Move substances across membranes

• Defensive Proteins: Antibodies

• Contractile Proteins: Muscle movement

• Signal Proteins: Hormones

Nucleic Acids

DNA and RNA

Nucleic acids are polymers of nucleotides and serve as the genetic material of cells.

• Nucleotide Structure: Composed of a phosphate group, a five-carbon sugar (ribose or deoxyribose), and a nitrogenous base

• DNA: Double helix; stores genetic information

• RNA: Single-stranded; involved in protein synthesis

Function and Importance

• Genetic Blueprint: DNA contains instructions for building proteins

• Inheritance: DNA is the molecule of inheritance, passed from generation to generation

Example:

DNA sequence determines the amino acid sequence of proteins, which in turn determines cellular structure and function.

Additional info: Some explanations and examples have been expanded for clarity and completeness, including the table of functional groups and the summary of macromolecule formation.