Cell and Molecular Biology

Introduction

This study guide covers foundational concepts in cell and molecular biology, focusing on the chemical basis of life, the unique properties of water, the role of carbon in organic molecules, and the structure and function of biological macromolecules. These topics are essential for understanding the molecular mechanisms underlying cellular processes.

Properties of Water

Hydrophobic and Hydrophilic Interactions

The ability of organisms, such as raft spiders (Dolomedes), to interact with water is determined by the molecular properties of their surfaces.

• Hydrophobic hairs on raft spiders repel water, allowing them to walk on its surface.

• If the hairs were hydrophilic, water would adhere to them, causing the spider to sink.

• Emergent property: Water's surface tension, resulting from cohesive forces between water molecules, enables small objects or organisms to remain on the surface.

• Atomic-level explanation: Water molecules form hydrogen bonds (intermolecular) due to their polarity. Intra-molecular polar covalent bonds within each molecule create partial charges, leading to strong cohesion.

Key bonds:

• Intramolecular: Polar covalent bonds within water molecules.

• Intermolecular: Hydrogen bonds between water molecules.

Example: Water striders and raft spiders exploit surface tension to move across water without sinking.

Carbon: The Chemical Backbone of Life

Why Carbon?

Carbon is the primary element in organic molecules due to its unique chemical properties.

• Versatility: Carbon can form four covalent bonds, allowing for diverse molecular structures.

• Molecular diversity: Carbon's ability to bond with itself and other elements (H, O, N, S, P) enables the formation of chains, rings, and complex branching structures.

• Prevalence: Carbon constitutes a significant portion of biological matter (see pie chart below).

Example: Carbon forms the backbone of carbohydrates, proteins, lipids, and nucleic acids.

Organic Molecules and Isomerism

Structural Diversity

Organic molecules exhibit diversity due to variations in carbon skeletons and functional groups.

• Chain length: Carbon chains can vary in length.

• Branching: Chains may be branched or unbranched.

• Double bond position: Location of double bonds affects molecular properties.

• Ring structures: Carbon atoms can form rings (e.g., benzene).

Isomers

Isomers are molecules with the same chemical formula but different structures.

• Structural isomers: Differ in covalent arrangement of atoms.

• Cis-trans isomers: Differ in spatial arrangement around double bonds.

• Enantiomers: Mirror-image isomers due to asymmetric (chiral) carbon atoms. Only one enantiomer is usually biologically active.

Example: Thalidomide enantiomers—one is therapeutic, the other teratogenic.

Functional Groups in Organic Molecules

Role and Types

Functional groups are specific groups of atoms that confer distinct chemical properties to organic molecules.

• Hydroxyl (-OH): Alcohols; polar, forms hydrogen bonds.

• Carbonyl (C=O): Aldehydes and ketones; polar.

• Carboxyl (-COOH): Acids; can donate H+.

• Amino (-NH2): Amines; acts as a base.

• Sulfhydryl (-SH): Thiols; forms disulfide bonds.

• Phosphate (-PO4): Organic phosphates; involved in energy transfer.

• Methyl (-CH3): Nonpolar; affects gene expression.

Example: Estradiol and testosterone differ only in functional groups, leading to distinct biological effects.

Biologically Important Macromolecules

Overview

Macromolecules are large molecules essential for life, often formed by polymerization of smaller units (monomers).

• Carbohydrates: Polymers of monosaccharides; energy storage and structure.

• Proteins: Polymers of amino acids; diverse functions including catalysis, structure, and signaling.

• Nucleic acids: Polymers of nucleotides; store and transmit genetic information.

• Lipids: Not true polymers; include fats, phospholipids, and steroids.

Carbohydrates

Structure and Function

Carbohydrates are classified by size and complexity.

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides joined by glycosidic bonds (e.g., sucrose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

Glycosidic bond: Covalent bond formed between C1 of one sugar and -OH of another via dehydration synthesis.

Polysaccharide Comparison

Example: Cotton towels (cellulose) absorb water due to hydrogen bonding; starch towels would not, as starch is less hydrophilic.

Proteins

Structure and Levels of Organization

Proteins are polymers of amino acids linked by peptide bonds. Their structure determines their function.

• Primary structure: Sequence of amino acids; covalent peptide bonds.

• Secondary structure: Regular patterns (α-helix, β-sheet) stabilized by hydrogen bonds between backbone atoms.

• Tertiary structure: Overall 3D shape; interactions among side chains (hydrogen, ionic, covalent, van der Waals).

• Quaternary structure: Arrangement of multiple polypeptides; same types of interactions as tertiary.

Example: Hemoglobin has quaternary structure, composed of multiple polypeptide subunits.

Protein Folding and Denaturation

• Chaperonins: Cellular components that assist in proper protein folding.

• Denaturation: Loss of protein shape and function due to heat, pH, or other factors.

Equation for peptide bond formation:

Nucleic Acids

Structure and Function

Nucleic acids (DNA and RNA) are polymers of nucleotides, which consist of a nitrogenous base, a pentose sugar, and a phosphate group.

• DNA: Double-stranded; stores genetic information.

• RNA: Single-stranded; involved in protein synthesis and regulation.

Phosphodiester bond: Covalent bond joining nucleotides in a chain via dehydration synthesis.

Equation for phosphodiester bond formation:

Lipids

Types and Functions

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids.

• Fats (triacylglycerols): Composed of glycerol and three fatty acids; energy storage.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails; major component of cell membranes.

• Steroids: Four fused carbon rings; involved in cell signaling (e.g., cholesterol, hormones).

Example: Phospholipid bilayer forms the structural basis of biological membranes.

Summary Table: Macromolecules

Additional info: Some explanations and examples have been expanded for clarity and completeness, including molecular details and academic context.