Chapter 3: The Molecules of Cells – General Biology Study Notes

Introduction

The Importance of Biological Molecules

Biological molecules are essential for the daily functions of living organisms. For example, the enzyme lactase is required to digest dairy products. Individuals who lack lactase are lactose intolerant, illustrating the critical role of enzymes and other biological molecules in metabolism and health.

• Lactase: An enzyme that breaks down lactose, a sugar found in milk.

• Lactose intolerance: The inability to digest lactose due to insufficient lactase production.

• Enzymes: Proteins that catalyze biochemical reactions.

Organic Compounds

Life’s Molecular Diversity and the Properties of Carbon

Organic compounds are molecules primarily composed of carbon atoms bonded to other elements. The unique bonding properties of carbon allow for the formation of large and diverse molecules, which are the foundation of life’s molecular diversity.

• Carbon’s bonding ability: Carbon can form four covalent bonds, enabling complex structures.

• Organic compounds: Molecules containing carbon, often with hydrogen, oxygen, nitrogen, and other elements.

• Carbon chains: Serve as the backbone for most organic molecules.

• Isomers: Molecules with the same molecular formula but different structures, leading to different properties.

• Hydrocarbons: Compounds composed only of carbon and hydrogen.

Example: Methamphetamine exists as two isomers: one is a potent illegal drug, the other is used as a sinus medication. The difference in their effects is due to their structural differences.

Structure and Diversity of Organic Molecules

Carbon Skeletons and Functional Groups

The properties of organic molecules depend on the size and shape of their carbon skeletons and the chemical groups attached to them. Functional groups are specific clusters of atoms that impart particular chemical properties to organic molecules.

• Carbon skeletons: Can be straight, branched, or arranged in rings.

• Functional groups: Groups of atoms that affect a molecule’s function (e.g., hydroxyl, carbonyl, carboxyl, amino, phosphate, methyl).

• Hydrophilic groups: Increase solubility in water and reactivity.

Example: Testosterone and estradiol differ only in their functional groups, resulting in distinct biological functions.

Macromolecules and Polymers

Formation and Breakdown of Polymers

Cells construct large molecules called macromolecules from smaller units called monomers. These macromolecules are often polymers, formed by linking monomers in long chains.

• Polymer: A large molecule made of repeating monomer units.

• Monomer: A small molecule that can join with others to form a polymer.

• Dehydration reaction: Joins monomers by removing a water molecule.

• Hydrolysis: Breaks polymers into monomers by adding water.

• Enzymes: Catalyze both dehydration and hydrolysis reactions.

Equation for Dehydration Reaction:

$\text{Monomer}_1 + \text{Monomer}_2 \xrightarrow{\text{Enzyme}} \text{Polymer} + H_2O$

Equation for Hydrolysis:

$\text{Polymer} + H_2O \xrightarrow{\text{Enzyme}} \text{Monomer}_1 + \text{Monomer}_2$

Carbohydrates

Monosaccharides, Disaccharides, and Polysaccharides

Carbohydrates are organic molecules ranging from simple sugars to large polysaccharides. They serve as energy sources and structural components.

• Monosaccharides: Simple sugars (e.g., glucose, fructose) with the general formula $C_nH_{2n}O_n$.

• Disaccharides: Formed by joining two monosaccharides via dehydration (e.g., maltose).

• Polysaccharides: Long chains of monosaccharide units (e.g., starch, glycogen, cellulose, chitin).

Functions:

• Starch and glycogen: Energy storage in plants and animals, respectively.

• Cellulose: Structural component of plant cell walls.

• Chitin: Structural component in fungi and arthropods.

Example: The FDA recommends limiting added sugar to 10% of daily calories due to health risks associated with high sugar intake.

Lipids

Fats, Phospholipids, and Steroids

Lipids are hydrophobic molecules that include fats, phospholipids, and steroids. They play roles in energy storage, membrane structure, and signaling.

• Fats (triglycerides): Composed of glycerol and three fatty acids.

• Saturated fatty acids: No double bonds; solid at room temperature; found in animal fats.

• Unsaturated fatty acids: One or more double bonds; liquid at room temperature; found in plant oils.

• Trans fats: Produced by hydrogenating unsaturated fats; associated with health risks.

• Phospholipids: Major component of cell membranes; have hydrophilic heads and hydrophobic tails.

• Steroids: Include cholesterol and hormones; cholesterol is a precursor for other steroids.

Example: Scientific studies show trans fats pose greater health risks than saturated fats.

Proteins

Structure and Function of Proteins

Proteins are polymers made from 20 different amino acids. They perform a wide variety of functions in cells, including catalysis, transport, defense, signaling, structure, and movement.

• Amino acids: Monomers with a central carbon, amino group, carboxyl group, hydrogen atom, and R group.

• Peptide bond: Covalent bond joining amino acids in a protein.

• Polypeptide: Chain of amino acids.

• Denaturation: Loss of protein shape and function due to environmental changes.

Protein Structure Levels:

• Primary structure: Sequence of amino acids.

• Secondary structure: Coiling or folding stabilized by hydrogen bonds (e.g., alpha helix, beta sheet).

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

• Quaternary structure: Association of multiple polypeptide chains.

Equation for Peptide Bond Formation:

$\text{Amino acid}_1 + \text{Amino acid}_2 \xrightarrow{\text{Dehydration}} \text{Dipeptide} + H_2O$

Example: Hydrolysis breaks peptide bonds during digestion, releasing individual amino acids.

Nucleic Acids

DNA and RNA: Information-Rich Polymers

Nucleic acids are polymers of nucleotides, which store and transmit genetic information. DNA and RNA are the two main types.

• Nucleotide: Monomer consisting of a sugar, phosphate group, and nitrogenous base.

• DNA: Double helix; stores genetic information; molecule of inheritance.

• RNA: Single polynucleotide chain; involved in protein synthesis.

• Gene: Segment of DNA that codes for a protein.

Equation for Nucleotide Polymerization:

$\text{Nucleotide}_1 + \text{Nucleotide}_2 \xrightarrow{\text{Dehydration}} \text{Dinucleotide} + H_2O$

Example: DNA mutations can lead to traits such as lactose tolerance in certain human populations.

Summary Table: Major Classes of Biological Molecules

Class	Monomer	Polymer	Main Functions
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structure
Lipids	Fatty acid, glycerol	Triglyceride, phospholipid, steroid	Energy storage, membranes, signaling
Proteins	Amino acid	Polypeptide	Catalysis, structure, transport, defense
Nucleic Acids	Nucleotide	DNA, RNA	Genetic information, protein synthesis

Evolutionary Connection

Lactose Tolerance in Human Evolution

Lactose tolerance is a recent evolutionary adaptation in some human populations. Mutations in DNA have enabled certain groups to continue producing lactase into adulthood, especially in populations with a history of dairy farming.

• Lactase gene: Remains active due to specific mutations.

• Evolutionary advantage: Ability to digest milk provides nutritional benefits.

Key Concepts to Review

• Importance of carbon in molecular diversity

• Role of functional groups in organic molecules

• Formation and breakdown of macromolecules

• Structure and function of carbohydrates, lipids, proteins, and nucleic acids

• Health implications of dietary fats

• Protein structure and denaturation

• Nucleic acids and inheritance

• Evolution of lactose tolerance

Additional info: Some chemical group details, health recommendations, and evolutionary context were expanded for clarity and completeness.