Chapter 3: Carbon and the Molecular Diversity of Life

3.1 Carbon Atoms and Molecular Diversity

Carbon is the foundational element of organic molecules, enabling the vast diversity of life due to its unique bonding properties.

• Bonding Capacity: Carbon forms four covalent bonds, allowing for a variety of stable structures (chains, rings, branches).

• Valence: The number of unpaired electrons in the outer shell determines bonding behavior.

• Carbon Skeletons: Can vary in length, branching, and ring formation, contributing to molecular diversity.

• Functional Groups: Specific groups of atoms (e.g., hydroxyl, carboxyl, amino, phosphate) that confer distinct chemical properties.

• ATP (Adenosine Triphosphate): The cell’s energy currency, storing and releasing energy via phosphate group transfer.

Example: The methyl group (-CH3) is nonpolar and affects gene expression when attached to DNA.

3.2 Macromolecules: Polymers and Monomers

Macromolecules are large biological molecules formed by the polymerization of smaller units called monomers.

• Polymers: Long chains of monomers linked by covalent bonds.

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

• Hydrolysis: Breaks polymers into monomers by adding water.

Example: Proteins (polymers) are made of amino acids (monomers).

3.3 Carbohydrates: Fuel and Building Material

Carbohydrates serve as energy sources and structural components in cells.

• Monosaccharides: Simple sugars (e.g., glucose, C6H12O6).

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

• Polysaccharides: Long chains of monosaccharides; can be storage (starch, glycogen) or structural (cellulose, chitin).

• Linkages: Alpha-glycosidic (digestible, e.g., starch), beta-glycosidic (structural, e.g., cellulose).

Example: Humans can digest starch (alpha linkages) but not cellulose (beta linkages).

3.4 Lipids: Hydrophobic Molecules

Lipids are diverse, hydrophobic molecules essential for energy storage, membrane structure, and signaling.

• Fats: Glycerol + 3 fatty acids; saturated (no double bonds, solid) or unsaturated (double bonds, liquid).

• Phospholipids: Hydrophilic head and hydrophobic tails; form bilayers in membranes.

• Steroids: Four fused rings; cholesterol is a key membrane component.

Example: Phospholipid bilayers are the foundation of cell membranes.

3.5 Proteins: Structure and Function

Proteins are polymers of amino acids with diverse structures and functions.

• Functions: Enzymes, transport, structure, signaling.

• Amino Acids: 20 types, differing in side chains (R groups).

• Levels of Structure:

  • Primary: Amino acid sequence

  • Secondary: Alpha helices, beta sheets (hydrogen bonds)

  • Tertiary: 3D folding (disulfide bridges, hydrophobic interactions)

  • Quaternary: Multiple polypeptides

Example: Hemoglobin has quaternary structure with four polypeptide subunits.

3.6 Nucleic Acids: Hereditary Information

Nucleic acids (DNA and RNA) store, transmit, and help express genetic information.

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

• DNA vs. RNA: DNA contains deoxyribose and bases A, T, C, G; RNA contains ribose and bases A, U, C, G.

• Base Pairing: A-T (or A-U in RNA), G-C.

• Directionality: 5’ to 3’ ends determine the direction of synthesis and reading.

Example: mRNA carries genetic instructions from DNA to ribosomes for protein synthesis.

Chapter 4: A Tour of the Cell

4.1 Microscopy and Cell Study

Microscopes and biochemical techniques are essential for studying cell structure and function.

• Light Microscopy: Uses visible light; limited resolution (~200 nm).

• Fluorescence/Confocal Microscopy: Visualizes specific molecules and 3D structures.

• Electron Microscopy: TEM (internal structures), SEM (surface details).

• Key Terms: Magnification (enlargement), resolution (clarity), contrast (brightness difference).

4.2 Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on internal organization.

• Prokaryotes: No nucleus, nucleoid region, cell wall (peptidoglycan), fimbriae.

• Eukaryotes: Nucleus, membrane-bound organelles, larger size.

• Surface-to-Volume Ratio: Limits cell size for efficient exchange.

• Unique Structures: Plant cells (cell wall, chloroplasts, central vacuole); animal cells (lysosomes, centrosomes).

4.3 Nucleus and Ribosomes

The nucleus stores genetic information; ribosomes synthesize proteins.

• Nucleus: Double membrane (nuclear envelope), nuclear pores, chromatin (DNA + proteins), nucleolus (ribosome assembly).

• Ribosomes: rRNA + proteins; free (cytosol) or bound (ER); site of protein synthesis.

4.4 Endomembrane System

The endomembrane system coordinates protein and lipid synthesis, modification, and transport.

• Components: Nuclear envelope, ER (smooth: lipid synthesis; rough: protein synthesis), Golgi apparatus (cis: receiving, trans: shipping), lysosomes (digestion), vacuoles (storage, digestion).

4.5 Energy Conversion: Mitochondria and Chloroplasts

Mitochondria and chloroplasts convert energy into usable forms for the cell.

• Mitochondria: Site of cellular respiration; cristae increase surface area.

• Chloroplasts: Site of photosynthesis; contains thylakoids, stroma, and intermembrane space.

• Peroxisomes: Break down fatty acids, detoxify harmful substances.

4.6 Cytoskeleton

The cytoskeleton provides structural support, motility, and intracellular transport.

• Microtubules: Tubulin; shape, transport, cilia/flagella movement.

• Microfilaments: Actin; cell shape, muscle contraction.

• Intermediate Filaments: Structural support.

• Motor Proteins: Move vesicles along cytoskeleton.

4.7 Extracellular Components and Cell Junctions

Cells interact with their environment and each other via extracellular matrix and junctions.

• Extracellular Matrix (ECM): Collagen (strength), proteoglycans (hydration), fibronectin (adhesion), integrins (connect ECM to cytoskeleton).

• Cell Junctions: Tight junctions (seal), desmosomes (anchor), gap junctions (communication).

Chapter 5: Membrane Transport and Cell Signaling

5.1 Membrane Structure: Fluid Mosaic Model

Cell membranes are dynamic structures composed of lipids, proteins, and carbohydrates.

• Phospholipids: Amphipathic molecules forming bilayers.

• Fluid Mosaic Model: Membrane proteins float in or on the fluid lipid bilayer.

• Cholesterol: Modulates membrane fluidity in animal cells.

• Membrane Proteins: Integral (span membrane) and peripheral (surface-associated); roles in transport, signaling, recognition.

5.2 Selective Permeability

Membranes regulate the passage of substances based on size, polarity, and charge.

• Permeable: Small, nonpolar molecules (e.g., O2, CO2).

• Impermeable: Ions, large polar molecules.

• Aquaporins: Channel proteins for water transport.

5.3 Passive Transport

Passive transport moves substances down their concentration gradients without energy input.

• Diffusion: Movement from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Tonicity: Effect of solution on cell volume (hypotonic, isotonic, hypertonic).

• Facilitated Diffusion: Uses channel or carrier proteins.

5.4 Active Transport

Active transport requires energy (usually ATP) to move substances against their gradients.

• Sodium-Potassium Pump: Maintains electrochemical gradients in animal cells.

• Co-transport: Coupled movement of two substances.

5.5 Bulk Transport

Large molecules and particles are transported via vesicles.

• Exocytosis: Vesicles fuse with membrane to release contents.

• Endocytosis: Includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.

5.6 Cell Signaling

Cells communicate via chemical signals, which are received, transduced, and elicit responses.

• Local Signaling: Paracrine (nearby cells), gap junctions (direct contact).

• Long-Distance Signaling: Endocrine (hormones in bloodstream).

• Signal Transduction Pathway: Reception (ligand binds receptor), transduction (cascade, second messengers), response (gene activation, enzyme activity).

Chapter 7: Cellular Respiration and Fermentation

7.1 Catabolic Pathways and Redox Reactions

Cellular respiration extracts energy from organic molecules through redox reactions.

• Fermentation: Partial degradation of sugars without oxygen.

• Aerobic Respiration: Uses O2 as final electron acceptor.

• Redox: Oxidation (loss of electrons), reduction (gain of electrons).

• Electron Transport Chain (ETC): Transfers electrons, releases energy for ATP synthesis.

7.2 Glycolysis

Glycolysis breaks down glucose into pyruvate, generating ATP and NADH.

• Location: Cytosol.

• Net Yield: 2 ATP, 2 NADH, 2 pyruvate per glucose.

7.3 Citric Acid Cycle

Completes the oxidation of glucose, producing CO2, NADH, FADH2, and ATP.

• Inputs: Acetyl-CoA.

• Outputs: CO2, NADH, FADH2, ATP.

7.4 Oxidative Phosphorylation and Chemiosmosis

ETC and chemiosmosis generate most cellular ATP.

• ETC: Electrons from NADH/FADH2 to O2.

• ATP Synthase: Uses H+ gradient to synthesize ATP.

• Total ATP Yield: ~30–32 per glucose.

Equation:

7.5 Fermentation and Anaerobic Respiration

Cells can generate ATP without oxygen via fermentation or anaerobic respiration.

• Fermentation: Regenerates NAD+; types include alcoholic (ethanol) and lactic acid (lactate).

• Anaerobic Respiration: Uses alternative electron acceptors.

7.6 Metabolic Integration

Catabolic and anabolic pathways intersect at glycolysis and the citric acid cycle.

• Proteins: Deaminated and enter as intermediates.

• Lipids: Glycerol enters glycolysis; fatty acids undergo beta-oxidation to acetyl-CoA.

Chapter 8: Photosynthesis

8.1 Overview and Structures

Photosynthesis converts light energy into chemical energy in chloroplasts of photoautotrophs.

• Equation:

• Key Structures: Mesophyll cells, stomata, thylakoids, grana, chlorophyll.

• Redox: CO2 reduced to glucose; H2O oxidized to O2.

8.2 Light Reactions

Light reactions capture solar energy to produce ATP and NADPH.

• Pigments: Chlorophyll a (primary), chlorophyll b, carotenoids.

• Electron Flow: Excited electrons transferred to primary acceptor, then through ETC to generate ATP (photophosphorylation) and NADPH.

8.3 Calvin Cycle

The Calvin cycle uses ATP and NADPH to fix CO2 into sugars.

• Phases: Carbon fixation (Rubisco), reduction (G3P production), regeneration of RuBP.

• Net Output: For 3 CO2: 1 G3P (requires 9 ATP, 6 NADPH).

• C4 Plants: Adapted for hot, arid climates; spatial separation of steps.

8.4 Importance of Photosynthesis

Photosynthesis sustains life by producing oxygen and organic molecules, forming the base of food chains.

• Integration: Links to protein synthesis, cellular respiration, and membrane transport.

Chapter 9: The Cell Cycle

9.1 Cell Division and Genetic Continuity

Cell division enables reproduction, growth, and tissue repair.

• Genome: Complete genetic material.

• Chromatin: DNA + proteins; condenses into chromosomes during division.

• Somatic Cells: Diploid (2n); Gametes: Haploid (n).

• Mitosis: Produces genetically identical cells; Meiosis: Produces gametes with genetic diversity.

9.2 Cell Cycle Phases

The cell cycle alternates between interphase (growth, DNA replication) and mitotic phase (division).

• Interphase: G1 (growth), S (DNA synthesis), G2 (preparation).

• Mitosis Stages: Prophase, prometaphase, metaphase, anaphase, telophase.

• Cytokinesis: Division of cytoplasm (cleavage furrow in animals, cell plate in plants).

• Binary Fission: Prokaryotic cell division.

9.3 Cell Cycle Regulation

Progression through the cell cycle is controlled by checkpoints and signaling molecules.

• Checkpoints: G1 (restriction point), M (spindle attachment).

• Growth Factors: Stimulate division (e.g., PDGF).

• Cancer: Uncontrolled division due to checkpoint failure; malignant tumors can metastasize.

Chapter 13: Molecular Basis of Inheritance

13.1 DNA as Genetic Material

Experiments established DNA as the hereditary material.

• Griffith’s Transformation: Showed transfer of genetic information between bacteria.

• Hershey-Chase Experiment: Confirmed DNA, not protein, is genetic material using bacteriophages.

13.2 DNA Replication and Repair

DNA replication is semi-conservative, involving multiple enzymes and high fidelity.

• Structure: Double helix, antiparallel strands, complementary base pairing.

• Key Enzymes: Helicase (unwinds), primase (RNA primer), DNA polymerase (synthesizes), ligase (joins fragments), topoisomerase (relieves strain).

• Lagging Strand: Synthesized in Okazaki fragments.

• Repair: Mismatch repair, nucleotide excision repair.

13.3 Chromosome Structure

DNA is packaged with proteins into chromatin, which can be euchromatin (active) or heterochromatin (inactive).

13.4 Genetic Engineering

Understanding DNA structure enables genetic manipulation.

• DNA Cloning: Inserting DNA into plasmids (cloning vectors).

• Restriction Enzymes: Cut DNA at specific sites.

• Gel Electrophoresis: Separates DNA fragments by size.

• PCR: Amplifies DNA (denaturation, annealing, extension).

• CRISPR-Cas9: Genome editing tool guided by RNA.

Chapter 14: Gene Expression

14.1 Genes, Transcription, and Translation

Genes encode proteins via transcription (DNA to RNA) and translation (RNA to protein).

• Beadle & Tatum: One gene–one enzyme hypothesis (now one gene–one polypeptide).

• Central Dogma: DNA → RNA → Protein.

• Genetic Code: Triplet codons specify amino acids; universal across life.

14.2 Transcription

Transcription synthesizes RNA from a DNA template.

• RNA Polymerase: Synthesizes RNA 5’→3’ without a primer.

• Stages: Initiation (promoter, TATA box), elongation, termination.

14.3 RNA Processing (Eukaryotes)

Pre-mRNA is modified before translation.

• 5’ Cap and Poly-A Tail: Protect mRNA, aid export and translation.

• RNA Splicing: Removes introns, joins exons; alternative splicing increases protein diversity.

14.4 Translation

Translation synthesizes polypeptides using mRNA, tRNA, and ribosomes.

• tRNA: Brings amino acids; anticodon pairs with mRNA codon.

• Ribosome Sites: A (aminoacyl), P (peptidyl), E (exit).

• Stages: Initiation (start codon, methionine), elongation (codon recognition, peptide bond, translocation), termination (stop codon, release factor).

• Post-Translation: Folding, modifications, targeting to locations.

14.5 Mutations

Mutations can alter protein structure and function.

• Point Mutations: Substitutions (silent, missense, nonsense), insertions/deletions (frameshift).

• Large-Scale Mutations: Chromosomal changes.

Chapter 15: Regulation of Gene Expression

15.1 Prokaryotic Gene Regulation: Operons

Bacteria regulate gene expression in response to environmental changes using operons.

• trp Operon: Repressible; tryptophan acts as corepressor to inhibit transcription.

• lac Operon: Inducible; allolactose inactivates repressor, allowing lactose metabolism.

• Key Components: Operator, promoter, repressor, regulatory gene, allosteric site, corepressor/inducer.

15.2 Eukaryotic Gene Regulation

Gene expression in eukaryotes is regulated at multiple levels.

• Chromatin Modification: Histone acetylation (activates), DNA methylation (silences).

• Epigenetics: Heritable changes not involving DNA sequence (e.g., agouti mouse study).

• Gene Control Elements: Introns, exons, promoters, enhancers, poly-A signal.

• Alternative Splicing: Generates protein diversity.

• Protein Processing/Degradation: Regulates protein activity and lifespan.

15.3 Noncoding RNAs

Noncoding RNAs (miRNAs, siRNAs) regulate gene expression post-transcriptionally.

• miRNAs: Bind mRNA to degrade or block translation.

• siRNAs: Similar function; often used experimentally.

15.4 Monitoring Gene Expression

Gene expression can be studied using molecular techniques.

• Nucleic Acid Hybridization: Detects specific sequences using labeled probes.

• In Situ Hybridization: Visualizes gene expression in tissues.

• RT-PCR: Converts mRNA to cDNA, amplifies transcripts.

• DNA Microarrays: Measure expression of thousands of genes simultaneously.

Appendix: Sample Table – Comparison of Cell Types

Additional info: This study guide covers foundational topics in General Biology, including molecular structure, cell biology, metabolism, genetics, gene expression, and regulation. Students are encouraged to supplement these notes with diagrams, figures, and further reading from their textbook.