IB Biology Exam Notes

Genetics

• Sex-linked inheritance:
  • Traits located on the X chromosome (e.g., hemophilia).
  • Males are more likely to express these traits.
  • In pedigrees: If mother has it, sons are likely affected; X-linked dominant if all sons of affected mother have the disease.
• Punnett Squares:
  • Monohybrid and dihybrid crosses (e.g., AaBb x AaBb → 9:3:3:1 ratio).
  • Use to calculate genotypic and phenotypic ratios.
  • Understand linked genes vs unlinked (different ratios observed).
• Codominance:
  • Both alleles are expressed equally (e.g., AB blood).
• Mendelian Ratios:
  • Expected: 3:1 (monohybrid), 9:3:3:1 (dihybrid), 1:1, etc.
• Pedigree Charts:
  • Identify autosomal vs sex-linked, dominant vs recessive.
  • X-linked recessive often shows up in males only.
• Cytokinesis:
  • Equal (usual mitosis) vs unequal (e.g., budding).
• Cell Cycle & Cyclins:
  • Cyclins regulate timing; bind to CDK (cyclin-dependent kinase) to push cycle forward.
  • Phases: G1 → S → G2 → Mitosis.
• Non-disjunction & Trisomy:
  • Occurs during anaphase I or II → leads to gametes with abnormal chromosome numbers (e.g., trisomy 21).
• Recombination & Linked Genes:
  • Linked genes are inherited together unless crossing over occurs (producing recombinants).

Cell Biology

• Mitosis vs Meiosis:
  • Mitosis = 2 identical cells; Meiosis = 4 varied gametes.
  • Includes synapsis, crossing over (prophase I), and segregation.
• Ultrastructure (Electron Micrograph):
  • Identify organelles (mitochondria, ER, etc.) in high-resolution images.
• Membrane Transport:
  • Passive: simple/facilitated diffusion, osmosis.
  • Active: uses ATP (e.g., sodium-potassium pump).
• Types & Classes of Cells:
  • Prokaryotes vs Eukaryotes.
  • Endosymbiosis theory (mitochondria/chloroplasts).
• Magnification: Magnification = \frac{Image Size}{Actual Size}
  • Be sure to convert units (mm ↔ µm).

Molecular Biology

• DNA Structure:
  • Double helix; complementary base pairing (A-T, C-G).
• DNA Technology:
  • Gel electrophoresis (separates DNA fragments).
  • PCR (amplifies DNA).
• Hershey-Chase Experiment:
  • Proved DNA is genetic material (used radioactive isotopes).
• Miller-Urey Experiment:
  • Simulated early Earth; showed how amino acids (organic compounds) could form.
  • Supports abiogenesis.
• Virus Reproduction:
  • Uses host machinery.
  • Lytic cycle (immediate replication) vs Lysogenic (integrates into host genome).

Evolution

• Types of Selection:
  • Stabilizing – favors average
  • Directional – favors one extreme
  • Disruptive – favors both extremes (e.g., guppies with bright colors vs predators)
• Natural Selection:
  • Variation → overproduction → survival of the fittest → favorable alleles passed on.
• Selection Patterns on Graphs:
  • Be able to label and interpret bell curves for different selection types.
  • Use examples like color intensity in guppies.

Human Physiology

• Circulatory System:
  • Structure/function of arteries, veins, capillaries.
  • Plasma, RBCs, WBCs.
  • Oncotic pressure and hydrostatic forces → cause fluid exchange in capillaries (tissue fluid).
• Ventilation:
  • Role of diaphragm/intercostal muscles.
  • Bohr Shift: pH drops → hemoglobin releases more O₂ → ventilation increases.
• Genetic Disease – Hemophilia:
  • X-linked recessive.
  • Mostly affects males; females can be carriers.

Math Notes

• No Math Except:
  • Percent change: \frac{New - Old}{Old} \times 100
  • Magnification (as above)

Enzymes

• Active site: Region on the enzyme where the substrate binds.
• Lock and key model vs Induced fit model: Substrate fits precisely vs enzyme adjusts shape slightly.
• Factors affecting enzyme activity:
  • Temperature (optimum, denaturation)
  • pH (specific for each enzyme)
  • Substrate concentration (saturation point)

Cellular Respiration

• Aerobic respiration: Occurs in mitochondria; includes glycolysis, link reaction, Krebs cycle, and oxidative phosphorylation. Requires oxygen.
• Anaerobic respiration: In absence of oxygen; produces lactic acid (humans) or ethanol + CO₂ (yeast).
• ATP yield:
  • Glycolysis: 2 ATP
  • Total (aerobic): ~36–38 ATP

Photosynthesis

• Light-dependent reactions: Occur in thylakoid membrane; produce ATP and NADPH.
• Light-independent reactions (Calvin cycle): Occur in stroma; use ATP and NADPH to fix CO₂ into glucose.
• Limiting factors: Light intensity, CO₂ concentration, temperature.

DNA & RNA

• Structure: DNA = double helix; RNA = single strand.
• Base pairing:
  • DNA: A–T, C–G
  • RNA: A–U, C–G
• Processes:
  • Replication (DNA → DNA)
  • Transcription (DNA → mRNA)
  • Translation (mRNA → protein)

Genetic Engineering & Biotechnology

• Gel electrophoresis: Separates DNA fragments by size.
• PCR (Polymerase Chain Reaction): Amplifies DNA.
• GMO example: Bt corn contains a gene to produce insecticidal protein.

Evolution & Natural Selection

• Variation: Caused by mutation, meiosis, and sexual reproduction.
• Natural selection: Best-adapted individuals survive and reproduce.
• Speciation: Can be due to geographic isolation (allopatric).

Updated IB Biology Review Sheet

Genetics

• ✅ Sex-linked inheritance
• ✅ Punnett squares
• ✅ Codominance
• ✅ Mendelian ratios
• ✅ Pedigrees
• ✅ Linked genes & recombination
• ✅ Diagrams and crosses (dihybrid, sex-linked)
• 🧠 Likely to be on the exam
  • 🔹 Test cross – Used to determine the unknown genotype
  • 🔹 Chi-squared test – May appear in HL or Paper 3, to compare observed vs expected outcomes
  • 🔹 Mutations – Types (silent, missense, nonsense) and their effect on amino acid sequences
  • 🔹 Sickle cell anemia – Example of co-dominant inheritance and natural selection (malaria resistance)
  • 🔹 Gene linkage evidence – Ratios that don't match Mendelian expectations

Cell Biology

• ✅ Mitosis vs meiosis
• ✅ Ultrastructure & microscopy
• ✅ Membrane transport
• ✅ Types of cells (euk/prok)
• ✅ Endosymbiosis
• 🧠 Likely to be on the exam
  • 🔹 Surface area to volume ratio – Affects diffusion and limits cell size
  • 🔹 Stem cells – Therapeutic use (Stargardt’s disease)
  • 🔹 Cell cycle control – Cyclins and checkpoints
  • 🔹 Electron micrograph skills – Identifying specific organelles
  • 🔹 Experimental design for osmosis – Plasmolysis in plant cells, estimating osmolarity

Molecular Biology

• ✅ DNA structure
• ✅ DNA technology (PCR, gel electrophoresis)
• ✅ Miller-Urey
• ✅ Hershey-Chase
• ✅ Viral reproduction
• 🧠 Likely to be on the exam
  • 🔹 Transcription & translation – Understand codons, tRNA, ribosome roles
  • 🔹 Enzymes – Factors affecting rate, denaturation, induced fit model
  • 🔹 Lactose intolerance – Human evolution example (lactase persistence)
  • 🔹 Structure of nucleotides & base pairing – You may be asked to draw or label these
  • 🔹 Structure of DNA vs RNA – Including sugar types and number of strands

Evolution

• ✅ Types of selection
• ✅ Natural selection
• ✅ Graphs
• 🧠 Likely to be on the exam
  • 🔹 Evidence for evolution – Fossils, homologous structures, selective breeding
  • 🔹 Adaptive radiation – E.g., Darwin’s finches
  • 🔹 Speciation – Allopatric vs sympatric
  • 🔹 Antibiotic resistance – A great real-world natural selection case
  • 🔹 Cladograms – Interpreting or constructing basic phylogenetic trees

Human Physiology

• ✅ Circulatory system
• ✅ Ventilation & Bohr shift
• ✅ Hemophilia
• 🧠 Likely to be on the exam
  • 🔹 Gas exchange surfaces – Alveoli structure & function
  • 🔹 Heart – Structure, blood flow path, valves
  • 🔹 Ventilation rate control – Medulla detects CO₂ levels → affects breathing
  • 🔹 Blood composition – Plasma, RBCs, WBCs, platelets
  • 🔹 Immune system basics – Antigens, antibodies, role of lymphocytes
  • 🔹 Hormones – Insulin/glucagon from pancreas; homeostasis connection

Math & Graphing

• ✅ Percent change
• ✅ Magnification
• 🧠 Likely to be on the exam
  • 🔹 Graphing skills – Especially in data-based questions
  • 🔹 Calculating actual size from a scale bar or image
  • 🔹 Using ratio and unit conversions (e.g., mm to µm)

Experimental / Data-Based Skills (Paper 3-style)

• 🧪 You may be asked to…
  • ● Identify variables (IV/DV/control)
  • ● Comment on data reliability or validity
  • ● Calculate mean, range, or percentage error
  • ● Interpret unfamiliar experiment setups and analyze trends

IB Biology Exam-Ready Guide

• 💡 = Commonly tested
• 🧪 = Application/experiment
• 📊 = Data/graph/analysis

GENETICS

• 🔹 Sex-linked Inheritance 💡
  • ● X-linked recessive/dominant traits (e.g., hemophilia, color blindness).
  • ● Affected males inherit from mothers.
  • ● Carrier females: heterozygous (X^AX^a).
  • ● Pedigree recognition: More males affected = X-linked.
• 🔹 Punnett Squares 💡
  • ● Monohybrid (Aa x Aa → 3:1)
  • ● Dihybrid (AaBb x AaBb → 9:3:3:1)
  • ● Sex-linked (e.g., X^AX^a x X^AY)
  • ● Test cross to determine genotype of dominant individual.
• 🔹 Codominance & Multiple Alleles 💡
  • ● Codominance: Both alleles expressed (e.g., AB blood type).
  • ● Multiple alleles: ABO system (I^A, I^B, i).
• 🔹 Mendelian Ratios 💡
  • ● 3:1 = heterozygous monohybrid
  • ● 9:3:3:1 = dihybrid cross
  • ● Modified ratios if linkage or codominance is involved.
• 🔹 Pedigrees 📊
  • ● Determine inheritance pattern (dominant, recessive, autosomal, X-linked).
  • ● Squares = males; circles = females.
  • ● Shaded = affected.
• 🔹 Linked Genes & Recombination 💡📊
  • ● Genes on the same chromosome = inherited together.
  • ● Recombinant phenotypes occur less frequently.
  • ● Use observed vs expected ratios to infer linkage (Chi-squared test HL).
• 🔹 Mutations and Genetic Disorders
  • ● Point mutation, frameshift, silent/missense/nonsense.
  • ● Example: Sickle cell anemia (GAG → GTG on DNA → valine instead of glutamic acid).
• 🧪 Techniques
  • ● PCR, gel electrophoresis, DNA profiling, gene transfer.

CELL BIOLOGY

• 🔹 Mitosis vs Meiosis 💡
  • ● Mitosis: 2 diploid cells, identical.
  • ● Meiosis: 4 haploid gametes, variation.
  • ● Know all phases and key events:
    • ○ Crossing over (prophase I)
    • ○ Independent assortment (metaphase I)
• 🔹 Cell Ultrastructure 💡
  • ● Know organelles: nucleus, ER, Golgi, mitochondria, chloroplast, lysosome, ribosome.
  • ● Be able to label from micrographs or electron images.
• 🔹 Membrane Transport 💡
  • ● Passive: Diffusion, facilitated, osmosis.
  • ● Active: Sodium-potassium pump, endocytosis/exocytosis.
  • ● Osmosis experiments: Estimating osmolarity using plant tissue. 🧪
• 🔹 Types of Cells
  • ● Prokaryotes: No nucleus, small, binary fission.
  • ● Eukaryotes: Compartmentalized.
  • ● Stem cells: Therapeutic use (e.g., Stargardt’s disease).
• 🔹 Endosymbiotic Theory 💡
  • ● Mitochondria & chloroplasts were once free-living prokaryotes.
  • ● Evidence: Double membrane, DNA, 70S ribosomes.
• 🔹 Magnification Magnification = \frac{Image Size}{Actual Size}
  • ● Be comfortable converting units (mm ↔ µm ↔ nm).

MOLECULAR BIOLOGY

• 🔹 DNA Structure & Replication 💡
  • ● Double helix: A-T, C-G.
  • ● DNA replication: Semi-conservative, uses DNA polymerase.
• 🔹 Transcription & Translation 💡
  • ● Transcription: DNA → mRNA (in nucleus).
  • ● Translation: mRNA → protein (in ribosome, with tRNA).
  • ● Know codons, anticodons, and the ribosome.
• 🔹 Enzymes 💡🧪
  • ● Substrate binds active site (induced fit).
  • ● Factors: Temperature, pH, substrate concentration.
  • ● Denaturation: Shape of active site changes.
• 🔹 Miller-Urey Experiment 🧪
  • ● Simulated early Earth.
  • ● Organic molecules formed → supports abiogenesis.
• 🔹 Hershey-Chase Experiment 💡
  • ● DNA, not protein, is genetic material.
  • ● Used radioactive isotopes with bacteriophages.
• 🔹 Virus Structure & Reproduction
  • ● Not living: No metabolism, need host.
  • ● Reproduce via lytic or lysogenic cycle.

EVOLUTION

• 🔹 Natural Selection 💡
  • ● More offspring than the environment supports.
  • ● Variation exists.
• ● Best-adapted survive & reproduce.
• 🔹 Types of Selection 📊
  • ● Stabilizing: Favors average (e.g., birth weight)
  • ● Directional: One extreme (e.g., antibiotic resistance)
  • ● Disruptive: Both extremes (e.g., guppy coloration)
• 🔹 Speciation 💡
  • ● Geographic (allopatric) or behavioral (sympatric).
  • ● Adaptive radiation: One ancestor → many new niches (e.g., finches).
• 🔹 Evidence for Evolution
  • ● Fossils, homologous structures, molecular similarities.
• 📊 Graphs of Selection
  • ● Interpret shifts in bell curves.
  • ● Label axes and explain the direction of selection.

HUMAN PHYSIOLOGY

• 🔹 Circulatory System 💡
  • ● Heart structure: chambers, valves, flow path.
  • ● Arteries: thick, high pressure.
  • ● Veins: valves, low pressure.
  • ● Capillaries: exchange site (tissue fluid).
• 🔹 Blood Composition
  • ● Plasma, red/white blood cells, platelets.
  • ● Know functions of each component.
• 🔹 Ventilation 💡
  • ● Inhalation = diaphragm contracts down, thoracic cavity expands.
  • ● Controlled by the medulla (detects CO₂).
• 🔹 Bohr Shift 📊
  • ● CO₂ ↑ → pH ↓ → hemoglobin releases more O₂.
  • ● Causes a shift in the oxygen dissociation curve.
• 🔹 Genetic Disease: Hemophilia
  • ● X-linked recessive.
  • ● Males more affected, females carriers.
• 📊 IB-Style Graphing & Data Skills
  • ● Draw/label line and bar graphs
  • ● State IV/ DV / controls
  • ● Calculate mean, range, percent change \frac{New - Old}{Old} \times 100
  • ● Evaluate trends in unfamiliar data

Lytic vs Lysogenic Cycles

• Viruses are not alive → no metabolism, no growth → they hijack host cells.
• 🔹 Lytic Cycle 💥 (Quick + Destructive)
  1. Virus injects DNA/RNA into the host.
  2. The viral genome is replicated using host machinery.
  3. The host makes viral proteins and new virus particles.
  4. Cell bursts (lysis) → releases viruses to infect other cells.
  • 🔑 Key term: Active infection
  • 🔍 Example: Influenza, SARS-CoV-2
• 🔹 Lysogenic Cycle 💤 (Hidden + Long-term)
  1. Viral DNA integrates into host DNA → becomes provirus.
  2. Every time the cell divides, it copies viral DNA too.
  3. No symptoms – virus is dormant.
  4. Can switch to the lytic cycle later (due to stress, radiation, etc.)
  • 🔑 Key term: Dormancy, provirus
  • 🔍 Example: HIV, Herpes simplex
• 🧠 You should be able to:
  • ● Compare both cycles in a table/diagram
  • ● Explain which one causes immediate symptoms
  • ● State which one causes long-term viral presence
  • ● Label them in a diagram
  • ● Recognize real-life examples

Natural Selection

• 🧬 IB Definition (in a nutshell): Individuals with beneficial inherited traits are more likely to survive and reproduce, passing on those traits to the next generation.
• 🔹 Steps of Natural Selection:
  1. Variation in a population (mutation/sexual reproduction).
  2. Overproduction of offspring → struggle for survival.
  3. Differential survival (best adapted survive).
  4. Reproduction and passing on favorable alleles.
  5. Change in allele frequency over generations.
• 🔹 Example: Antibiotic Resistance 💊
  • ● A random mutation gives bacteria resistance.
  • ● Antibiotic kills all but the resistant bacteria.
  • ● These reproduce → now the population is resistant.
• 📊 Selection Types (Be graph-ready):
  • ● Stabilizing: Favors average (e.g., human birth weight).
  • ● Directional: Favors one extreme (e.g., dark moths in pollution).
  • ● Disruptive: Favors both extremes (e.g., bright vs dull guppies).

Leaf Structure

• 🍃 Parts of a Leaf (Structure & Function)
  • 🔹 Cross-section of a leaf (from top to bottom):

• 🌬 Pathway of Gases in the Leaf (CO₂ for Photosynthesis)
  1. CO₂ enters through stomata in the lower epidermis.
  2. Diffuses through the air spaces in the spongy mesophyll.
  3. Reaches the palisade mesophyll → enters cells → used in photosynthesis in chloroplasts.
     • At the same time:
       • ● O₂ (produced as a byproduct) exits the same way.
       • ● Water vapor also exits via transpiration.
• 💧 Pathway of Water into the Leaf (for Photosynthesis)
  1. Absorbed by root hairs → through xylem vessels (via transpiration pull, adhesion, cohesion).
  2. Reaches the vein in the leaf → moves to mesophyll cells.
  3. Used in light-dependent reactions of photosynthesis.
• 🍬 Pathway of Glucose (after Photosynthesis)
  1. Made in palisade mesophyll.
  2. Moves into phloem (in vein).
  3. Transported to other parts of the plant (e.g., roots, fruits) via translocation.

Handwritten Notes Breakdown

Genetics

• Hemophilia is sex-linked recessive → sons of affected mothers likely to have it (mentioned 2x).
• Males are more likely to express sex-linked traits (X-linked).
• Dihybrid cross: AaBb x AaBb → 9:3:3:1
• Pedigree analysis: autosomal vs sex-linked; recessive vs dominant
• X-linked recessive = mostly males affected
• Linked genes:
  • Recombination → recombinants
  • Linked genes inherited together
• Unequal vs equal cytokinesis
• Cyclins regulate the cell cycle by binding to CDKs
• Trisomy (e.g., 21) due to non-disjunction
• ✅ Repeated ideas:
  • ● Hemophilia pattern (sex-linked recessive + male expression) = emphasized 2x
  • ● Sex-linked → more males affected = emphasized 2x

Cell Biology

• Osmosis & water potential: movement from high to low water potential
• Membrane transport: diffusion, active transport
• Membrane model: fluid mosaic (noted visually)
• Enzyme activity:
  • Active site + substrate
  • Temperature, pH, substrate concentration affect rate
  • Denaturation occurs at high temps/pH extremes
• Electron micrographs:
  • Identify: mitochondria, rough/smooth ER, chloroplasts
  • Surface area to volume ratio → cell efficiency
• ✅ Repeated ideas:
  • ● Osmosis → discussed in more than one part
  • ● Enzyme factors → shown clearly with three variables repeated

Molecular Biology

• PCR: makes many DNA copies
• Gel electrophoresis: separates DNA by size
• Miller-Urey experiment: simulated early Earth, showed organic compounds could form (abiogenesis)
• Hershey-Chase: proved DNA is genetic material (used viruses & radioactive isotopes)
• Viral reproduction:
  • Lytic = immediate replication + cell bursts
  • Lysogenic = viral DNA integrates; can later switch to the lytic
• ✅ Repeated ideas:
  • ● Lytic vs lysogenic detail is well spaced out and clearly important
  • ● PCR + gel electrophoresis appear together in relation to DNA processing

Evolution

• Natural selection:
  • Variation → overproduction → struggle → survival → reproduction
  • Leads to increased frequency of favorable traits
• Types of selection:
  • Stabilizing = average favored
  • Directional = one extreme favored
  • Disruptive = both extremes favored
• Graphs of selection: bell curve changes
• Examples:
  • Guppies (predator = less color; no predator = more color)
• ✅ Repeated ideas:
  • ● Types of selection with graphs and examples — emphasized clearly

Human Physiology

• Bohr Shift:
  • ↑ CO₂ → ↓ pH → O₂ released from hemoglobin
  • Results in increased breathing rate (mentioned twice)
• Blood:
  • Veins, arteries, capillaries → structure and function
  • Oncotic pressure in capillaries causes fluid movement → tissue fluid
• Heart + circulation:
  • Right & left sides, valves
  • Hemophilia again under blood/genetic context
• ✅ Repeated ideas:
  • ● Bohr shift and its result (mentioned more than once)
  • ● Hemophilia mentioned again — bridging genetics and physiology

Math & Measurements

• Percent change formula: \frac{New - Old}{Old} \times 100
• Magnification formula: Magnification = \frac{Image size}{Actual size}
• ✅ Repeated ideas:
  • ● Magnification appears in diagram examples and formula

Unlinked Genes

• 🧬 Unlinked Genes = On different chromosomes
  • ● They assort independently during meiosis.
  • ● So their combinations follow Mendel’s law of independent assortment.
    • 👉 Result: When you do a dihybrid cross (e.g., AaBb x AaBb) with unlinked genes, you get the classic 9:3:3:1 ratio in the offspring phenotypes:

• This only works when:
  • ● The genes are on separate chromosomes
  • ● Or are very far apart on the same chromosome (so crossing over randomizes them anyway)
• 🔗 Linked Genes = On the same chromosome
  • ● They tend to be inherited together
  • ● You get ratios that don’t match the 9:3:3:1
  • ● Crossing over can still happen → makes recombinants (less frequent)
• 🧠 TL;DR: Unlinked genes = normal Mendelian ratios (9:3:3:1) Linked genes = weird ratios (fewer recombinants)

Cytokinesis

• ✂️ Cytokinesis = Splitting the Cytoplasm
  • Cytokinesis is the final step of cell division, when the cytoplasm divides to form two separate cells.
• ⚖️ Equal vs Unequal Cytokinesis
  • ✅ Equal Cytokinesis
    • Occurs in mitosis (most cells):
      • ● The cytoplasm divides evenly between the two daughter cells.
      • ● Both cells are usually the same size.
      • ● Common in body (somatic) cells of animals and plants.
    • 🧬 Example:
      • ● Your skin cells dividing during growth or healing.
  • ⚠️ Unequal Cytokinesis
    • Occurs in special cases like:
      • ● Budding in yeast or hydra
      • ● Oogenesis (egg formation in animals)
      • ➡️ One cell gets most of the cytoplasm, the other gets very little.
    • 🧬 Example:
      • ● In human egg production, only one large ovum is made + 3 small polar bodies (which usually die off).
• 🧠 TL;DR: Equal = Two similar daughter cells Unequal = One main cell + tiny cell(s) (like polar bodies or buds)

Budding

• 🌟 Budding – A Type of Asexual Reproduction
  • Definition: Budding is when a new organism grows off the side of a parent, eventually detaching and becoming its own independent organism.
  • 🔹 Key Characteristics:
    • ● Happens in unicellular organisms (like yeast) and simple multicellular ones (like hydra).
    • ● Unequal cytokinesis – the parent cell divides its cytoplasm unevenly.
    • ● The "bud" is smaller at first but genetically identical to the parent.
    • ● It's a form of asexual reproduction – so no gametes or genetic variation involved.
• 🧬 Examples:

• 🧠 TL;DR: Budding = new organism grows from the parent’s body via unequal cytokinesis, creating a genetically identical clone.

Miller-Urey Experiment

• 🧪 The Miller-Urey experiment is an IB Biology classic — super testable and surprisingly easy to remember once you break it down.
• 🌍 Miller-Urey Experiment (1953) 🔬
  • Goal: To test whether organic molecules (like amino acids) could form spontaneously under conditions thought to exist on early Earth.
• 🧪 What They Did:
  1. Created a closed system that mimicked early Earth’s atmosphere and oceans:
     • ○ Gases: Methane (CH₄), ammonia (NH₃), hydrogen (H₂), water vapor (H₂O)
  2. Added electrical sparks to simulate lightning (energy source)
  3. Cooled the system so compounds would condense like rain.
• 📌 What They Found: After a week, the liquid contained amino acids and other organic compounds — basic building blocks of life!
• 💡 IB Exam Relevance:
  • ● Supports the theory of abiogenesis = life originated from non-living matter.
  • ● Shows simple chemicals → complex organic molecules given the right conditions.
  • ● Frequently appears in questions about the origin of life.
• 🧠 TL;DR: Miller-Urey showed that life’s building blocks could form from gases + lightning on early Earth 🌩