Carbon and the Molecular Diversity of Life

Carbon Atoms and Functional Groups

Carbon is the foundational element in biological molecules due to its ability to form four covalent bonds, resulting in diverse molecular structures. Functional groups attached to carbon skeletons determine the chemical properties and biological functions of molecules.

• Bonding Capacity: Carbon forms four covalent bonds, allowing for varied structures such as chains, branches, and rings.

• Functional Groups: Key groups include hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate, and methyl.

• ATP: Adenosine triphosphate is the cell's energy currency, storing energy in its phosphate bonds.

• Example: ATP hydrolysis releases energy for cellular processes.

Macromolecules: Polymers and Monomers

Macromolecules are large biological polymers constructed from repeating monomer units. Their synthesis and breakdown involve dehydration and hydrolysis reactions.

• Polymers: Long chains of monomers (e.g., proteins, nucleic acids, polysaccharides).

• Dehydration Reaction: Joins monomers by removing water.

• Hydrolysis: Breaks polymers by adding water.

• Example: Protein synthesis involves dehydration; digestion involves hydrolysis.

Carbohydrates: Structure and Function

Carbohydrates serve as energy sources and structural materials. Their properties depend on the type of sugar and glycosidic linkage.

• Monosaccharides: Simple sugars like glucose.

• Disaccharides: Two monosaccharides linked (e.g., sucrose).

• Polysaccharides: Storage (starch, glycogen) and structural (cellulose, chitin).

• Glycosidic Linkages: Alpha (digestible) in starch/glycogen; beta (indigestible) in cellulose/chitin.

• Example: Humans digest starch but not cellulose due to linkage type.

Lipids: Hydrophobic Molecules

Lipids are diverse hydrophobic molecules, including fats, phospholipids, and steroids. Their structure determines their function in membranes and energy storage.

• Fats: Glycerol + 3 fatty acids; saturated (solid) vs. unsaturated (liquid).

• Phospholipids: Hydrophilic head and hydrophobic tails form bilayers.

• Steroids: Four fused rings; cholesterol modulates membrane fluidity.

• Example: Phospholipid bilayer forms the basis of cell membranes.

Proteins: Structure and Function

Proteins are polymers of amino acids with diverse functions. Their structure is hierarchical and determines their activity.

• Amino Acids: 20 types, classified by side chain properties.

• Levels of Structure:

  • Primary: Sequence of amino acids.

  • Secondary: Alpha helices and beta sheets (hydrogen bonds).

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

  • Quaternary: Multiple polypeptides.

• Functions: Enzymes, transport, structure, signaling.

• Example: Hemoglobin (quaternary structure) transports oxygen.

Nucleic Acids: Hereditary Information

Nucleic acids (DNA and RNA) store, transmit, and express genetic information. Their structure is key to function.

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

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

• Base Pairing: Purines (A, G) pair with pyrimidines (T, C, U).

• Directionality: 5’ to 3’ ends are important for replication and transcription.

• Example: mRNA carries genetic information from DNA to ribosomes.

A Tour of the Cell

Microscopy and Cell Study

Cells are studied using various types of microscopy and biochemical techniques, each with unique advantages.

• Light Microscopy: Uses visible light; limited resolution.

• Fluorescence/Confocal Microscopy: Allows visualization of specific molecules and 3D structures.

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

• Key Concepts: Magnification, resolution, contrast.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on internal structure and compartmentalization.

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

• Eukaryotes: Nucleus, membrane-bound organelles.

• Plant Cells: Cell wall, chloroplasts, central vacuole.

• Animal Cells: Lysosomes, centrosomes.

• Surface-to-Volume Ratio: Limits cell size.

Nucleus and Ribosomes

The nucleus houses genetic material, and ribosomes synthesize proteins.

• Nucleus: Nuclear envelope, chromatin, nucleolus.

• Ribosomes: rRNA and proteins; free (cytosol) vs. bound (ER).

Endomembrane System

The endomembrane system regulates protein traffic and metabolic functions.

• Components: Nuclear envelope, ER (smooth and rough), Golgi apparatus, lysosomes, vacuoles, plasma membrane.

• ER: Smooth (lipid synthesis, detox), rough (protein synthesis).

• Golgi Apparatus: Cis (receiving), trans (shipping).

• Lysosomes: Digest macromolecules.

• Vacuoles: Food (animals), central (plants).

Mitochondria, Chloroplasts, and Peroxisomes

These organelles are involved in energy conversion and metabolic processes.

• Mitochondria: Cellular respiration; cristae increase surface area.

• Chloroplasts: Photosynthesis; compartments include stroma, thylakoid space.

• Peroxisomes: Break down fatty acids, detoxify.

Cytoskeleton and Cell Movement

The cytoskeleton provides structural support, motility, and transport within the cell.

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

• Microfilaments: Actin; shape, muscle contraction.

• Intermediate Filaments: Structural support.

• Motor Proteins: Move vesicles along cytoskeleton.

• Cilia/Flagella: Movement in cells and organisms.

Extracellular Matrix and Cell Junctions

Extracellular components and cell junctions coordinate cellular activities.

• Extracellular Matrix: Collagen, proteoglycans, fibronectin, integrins.

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

Membrane Transport and Cell Signaling

Membrane Structure and Fluidity

Cellular membranes are fluid mosaics of lipids and proteins, enabling selective permeability and cell communication.

• Phospholipid Bilayer: Amphipathic molecules form the basic structure.

• Fluid Mosaic Model: Proteins embedded in or attached to the bilayer.

• Cholesterol: Modulates membrane fluidity.

• Transmembrane Proteins: Integral proteins span the membrane.

Selective Permeability and Transport

Membranes allow selective passage of substances, using passive and active transport mechanisms.

• Passive Transport: Diffusion and osmosis; no energy required.

• Facilitated Diffusion: Channel/carrier proteins assist movement.

• Active Transport: Requires ATP; e.g., sodium-potassium pump.

• Bulk Transport: Exocytosis and endocytosis (phagocytosis, pinocytosis, receptor-mediated).

Cell Signaling

Cell signaling involves reception, transduction, and response, allowing cells to communicate and adapt.

• Reception: Ligand binds to receptor.

• Transduction: Signal transduction pathways amplify signals; protein kinases/phosphatases; second messengers (e.g., cAMP).

• Response: Activation of genes or cellular processes.

• Example: Epinephrine triggers glycogen breakdown in liver cells.

Cellular Respiration and Fermentation

Energy Flow and Catabolic Pathways

Cellular respiration extracts energy from organic molecules, storing it in ATP. Fermentation allows ATP production without oxygen.

• Fermentation: Partial degradation of sugars without oxygen.

• Aerobic Respiration: Uses O2 as final electron acceptor.

• Anaerobic Respiration: Uses alternative acceptors.

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

• Electron Transport Chain: Transfers electrons to O2, releasing energy.

Glycolysis and Citric Acid Cycle

Glycolysis and the citric acid cycle are central metabolic pathways producing ATP, NADH, and FADH2.

• Glycolysis: Occurs in cytosol; produces 2 ATP, 2 NADH, 2 pyruvate per glucose.

• Citric Acid Cycle: Completes glucose oxidation; produces CO2, NADH, FADH2, ATP.

Oxidative Phosphorylation and Chemiosmosis

Electron transport and chemiosmosis couple electron flow to ATP synthesis.

• ETC: Electrons from NADH/FADH2 to O2.

• Cytochromes: Proteins transferring electrons.

• Chemiosmosis: H+ gradient drives ATP synthase.

• ATP Yield: Glycolysis (2), Citric Acid Cycle (2), Oxidative Phosphorylation (~26–28).

Fermentation and Metabolic Pathways

Fermentation regenerates NAD+ for glycolysis; metabolic pathways interconnect with respiration.

• Alcoholic Fermentation: Yeast produces ethanol and CO2.

• Lactic Acid Fermentation: Humans produce lactate.

• Proteins/Lipids: Enter respiration via deamination and β-oxidation.

Photosynthesis

Overview and Structures

Photosynthesis converts light energy to chemical energy in glucose, occurring in chloroplasts of photoautotrophs.

• Chloroplasts: Contain thylakoids, stroma, and chlorophyll.

• Photosynthesis Equation:

• O2 Origin: From splitting water.

• Redox: CO2 reduced, H2O oxidized.

Light Reactions and Calvin Cycle

Light reactions produce ATP and NADPH; the Calvin cycle uses them to fix carbon into sugars.

• Photosynthetic Pigments: Chlorophyll a is primary.

• Electron Transport: Produces ATP and NADPH.

• Calvin Cycle: Anabolic; produces G3P (precursor to glucose).

• Rubisco: Enzyme for carbon fixation.

• C4 Plants: Adaptation for hot, arid climates.

The Cell Cycle: Mitosis and Meiosis

Roles and Key Terms

Cell division is essential for reproduction, growth, and repair. The cell cycle is tightly regulated.

• Genome: Complete genetic material.

• Chromatin: DNA + proteins.

• Chromosome: Condensed chromatin.

• Chromatid: Identical halves of a duplicated chromosome.

• Somatic Cells: Diploid; body cells.

• Gametes: Haploid; reproductive cells.

Cell Cycle Phases and Mitosis

The cell cycle alternates between interphase and mitotic phase. Mitosis produces identical cells; meiosis produces diverse gametes.

• Interphase: G1 (growth), S (DNA replication), G2 (prep).

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

• Cytokinesis: Cleavage furrow (animals), cell plate (plants).

• Binary Fission: Prokaryotic cell division.

Cell Cycle Regulation and Cancer

Cell cycle progression is controlled by checkpoints and growth factors. Cancer results from loss of regulation.

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

• Growth Factors: PDGF stimulates division.

• Cancer Cells: Bypass checkpoints, divide uncontrollably, metastasize.

Molecular Bases of Inheritance

DNA Structure and Replication

DNA is a double helix with antiparallel strands. Replication ensures genetic fidelity.

• Watson & Crick Model: Double helix, complementary base pairing (A-T, G-C).

• Semi-Conservative Replication: Each new molecule has one old and one new strand.

• Replication Fork: Y-shaped region where DNA is unwound.

• Key Enzymes: Primase, DNA polymerase, helicase, topoisomerase, ligase.

• Lagging Strand: Synthesized in Okazaki fragments.

• Repair Mechanisms: Mismatch repair, nucleotide excision repair.

Chromatin Organization

Chromatin is DNA packed with proteins; its structure affects gene expression.

• Euchromatin: Loosely packed, active.

• Heterochromatin: Densely packed, inactive.

Genetic Engineering Tools

Understanding DNA structure enables genetic engineering and molecular biology techniques.

• DNA Cloning: Inserting DNA into plasmids to create recombinant DNA.

• Restriction Enzymes: Cut DNA at specific sequences.

• Gel Electrophoresis: Separates DNA fragments by size.

• PCR: Amplifies DNA; steps are denaturation, annealing, extension.

• Next-Gen Sequencing: Rapid, high-throughput DNA sequencing.

• CRISPR-Cas9: Genome editing using guide RNA and Cas9 protein.

Regulation of Gene Expression

Prokaryotic Gene Regulation: Operons

Bacteria regulate gene expression via operons, responding to environmental changes.

• Trp Operon: Repressible; shut down by tryptophan (corepressor).

• Lac Operon: Inducible; activated by allolactose (inducer).

• Key Components: Operator, promoter, repressor, regulatory gene.

Eukaryotic Gene Regulation

Gene expression in eukaryotes is regulated at multiple stages, including chromatin modification, transcription, and post-transcriptional processes.

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

• Epigenetics: Heritable changes without DNA sequence alteration.

• Gene Control Elements: Introns, exons, transcription start site, poly-A signal.

• Alternative Splicing: Produces multiple proteins from one gene.

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

Noncoding RNAs and Gene Expression

Noncoding RNAs such as miRNAs and siRNAs regulate gene expression post-transcriptionally.

• miRNAs: Bind mRNA to degrade or block translation.

• siRNAs: Similar function; used in gene silencing.

Monitoring Gene Expression

Gene expression can be studied using hybridization, RT-PCR, and DNA microarrays.

• Nucleic Acid Hybridization: Detects specific sequences.

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

• DNA Microarrays: Measure expression of thousands of genes.

Gene Expression: Transcription and Translation

Central Dogma and Genetic Code

Genes specify proteins via transcription (DNA to RNA) and translation (RNA to protein). The genetic code is universal and consists of codons.

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

• RNA Polymerase: Synthesizes RNA 5’→3’.

• mRNA Modifications: 5’ cap, poly-A tail, splicing (removal of introns).

Translation and Protein Synthesis

Translation occurs at ribosomes, using tRNA to decode mRNA into polypeptides.

• tRNA: Carries amino acids; has anticodon.

• 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, modification, targeting.

Mutations and Their Effects

Mutations can alter protein structure and function, ranging from point mutations to large-scale chromosomal changes.

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

• Large-Scale Mutations: Chromosomal changes.

• Example: Sickle cell anemia (missense mutation).

Sample Table: Cytoskeleton Components

Sample Table: Types of Point Mutations

Additional info:

• Some context and examples were inferred to ensure completeness and academic quality.

• Tables were recreated based on referenced content and standard textbook knowledge.