IB Biology SL Complete Revision Guide

TOPIC 1 — Cell Biology\n\n## Cell Theory and Exceptions\nCell theory is the foundational model of biology, consisting of three primary statements that must be learned verbatim:\n1. All living organisms are composed of $one$ or more cells.\n2. The cell is the basic unit of life.\n3. All cells come from pre-existing cells (referred to as Omnis cellula e cellula).\n\n### Exam Trap: Viruses\nViruses are categorized as NOT being cells and NOT living. This is because they cannot reproduce independently. Therefore, they do NOT support cell theory.\n\n### Exceptions to Cell Theory\n- Giant Algae (Acetabularia): Exist as $one$ giant cell, challenging the idea that large organisms are multicellular.\n- Skeletal Muscle Fibers: These are multinucleate (having many nuclei within a single long membrane), challenging the idea that cells are simple discrete units.\n\n## Prokaryotes vs. Eukaryotes\nProkaryotic cells (e.g., Bacteria, Archaea) and Eukaryotic cells (e.g., Animals, Plants, Fungi, Protists) share common features: both possess ribosomes, DNA, a cell membrane, and cytoplasm.\n\n| Feature | Prokaryote | Eukaryote |\n| :--- | :--- | :--- |\n| Nucleus | No membrane-bound nucleus (nucleoid region) | True nucleus with nuclear envelope |\n| DNA | Circular, in cytoplasm | Linear, in nucleus, with histones |\n| Ribosomes | $70S$ (smaller) | $80S$ (larger) |\n| Membrane Organelles | None | Mitochondria, ER, Golgi, etc. |\n| Cell Wall | Peptidoglycan (bacteria) | Cellulose (plants) / chitin (fungi) / none (animals) |\n| Size | $1-10\, \mu \text{m}$ | $10-100\, \mu \text{m}$ |\n\n### Note on Ribosomes\nThe difference between $70S$ and $80S$ ribosomes is the reason antibiotics can target bacterial cells without causing harm to human (host) cells.\n\n## Fluid Mosaic Model — Cell Membrane\nThe cell membrane is structured as a dynamic phospholipid bilayer.\n- Phospholipid Bilayer: Consists of hydrophilic (water-loving) heads facing outward toward water and hydrophobic (water-fearing) tails facing inward away from water. This arrangement is spontaneous and self-sealing.\n- Fluidity: Phospholipids and proteins can move laterally; they are not fixed in place.\n- Mosaic: Proteins are scattered throughout the bilayer in a mosaic pattern.\n- Integral Proteins: Span the full bilayer; they function as channel and carrier proteins for transport.\n- Peripheral Proteins: Attached to the surface; involved in cell signaling.\n- Cholesterol: Sits between phospholipids to stabilize fluidity; prevents the membrane from becoming too rigid in cold or too fluid in heat.\n- Glycoproteins: Proteins with carbohydrate chains used for cell recognition, receptor sites, and immune response.\n\n## Membrane Transport Mechanisms\n| Type | Direction | ATP? | Protein? | What Moves | Example |\n| :--- | :--- | :--- | :--- | :--- | :--- |\n| Simple Diffusion | High to Low | No | No | Small nonpolar molecules | $O_2$, $CO_2$, lipids |\n| Facilitated Diffusion | High to Low | No | Yes (channel/carrier) | Polar molecules, ions | Glucose, $Na^+$, $K^+$ |\n| Osmosis | High to Low Water Potential | No | Aquaporins (optional) | Water ONLY | Water across membranes |\n| Active Transport | Low to High | YES | Yes (carrier) | Ions, molecules | $Na^+/K^+$ pump |\n| Endocytosis | INTO the cell | YES | N/A (engulfing) | Large particles | Phagocytosis |\n| Exocytosis | OUT of cell | YES | N/A (fusion) | Large molecules | Neurotransmitters |\n\n### Verbatim Definition of Osmosis\n"The movement of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential."\n\n### Exam Trap: Passive vs. Active\nFacilitated diffusion is PASSI VE because it moves molecules DOWN the concentration gradient. ATP is ONLY required for active transport against the gradient. This is a yearly exam focus.\n\n## Cell Division — Mitosis (PMAT)\nMitosis results in $2$ genetically IDENTICAL diploid cells used for growth and repair.\n1. Prophase: Chromosomes condense; spindle fibers form from centrioles; nuclear envelope breaks down. Chromosomes consist of two sister chromatids joined at a centromere.\n2. Metaphase: Chromosomes align at the equator (metaphase plate). Spindle fibers attach to the centromeres. Best stage for counting chromosomes.\n3. Anaphase: Sister chromatids are pulled to opposite poles as fibers shorten. Cell elongates.\n4. Telophase: Nuclear envelopes reform; chromosomes decondense. Cytokinesis begins.\n\n# TOPIC 2 — Molecular Biology\n\n## DNA Structure\nDNA is a double helix made of nucleotides containing deoxyribose sugar, a phosphate group, and a nitrogenous base. Strands are antiparallel: one runs $5' \text{ to } 3'$, the other $3' \text{ to } 5'$. \n- Base Pairing Rules: Adenine (A) pairs with Thymine (T) via $2$ hydrogen bonds. Guanine (G) pairs with Cytosine (C) via $3$ hydrogen bonds. The $G-C$ bond is stronger.\n- Purines vs. Pyrimidines: Purines (A, G) have a double ring; Pyrimidines (C, T, U) have a single ring. A purine always pairs with a pyrimidine to maintain constant helix width.\n- RNA vs. DNA: RNA has ribose sugar, Uracil instead of Thymine, and is single-stranded.\n\n## DNA Replication (Semi-Conservative)\n1. Helicase: Unwinds/unzips the double helix by breaking hydrogen bonds at the replication fork.\n2. Primase: Adds an RNA primer to give DNA polymerase a start point.\n3. DNA Polymerase: Reads template $3' \text{ to } 5'$ and builds the new strand $5' \text{ to } 3'$ only.\n4. Ligase: Joins Okazaki fragments on the lagging strand.\n5. Result: Two identical DNA molecules each with $one$ original and $one$ new strand.\n\n## Protein Synthesis\n- Transcription: Occurs in the nucleus. RNA polymerase uses a DNA template strand (read $3' \text{ to } 5'$) to build an mRNA strand ($5' \text{ to } 3'$).\n- RNA Processing: Spliceosomes remove introns and join exons.\n- Translation: Occurs at the ribosome. mRNA codons (triplets) determine amino acid sequences. tRNA carries anticodons. \n- Codons: $64$ possible codons for $20$ amino acids. Start codon is AUG (methionine). Stop codons (UAA, UAG, UGA) terminate translation.\n- Code Properties: The genetic code is UNIVERSAL (shared ancestry) and DEGENERATE (multiple codons for the same amino acid reducer mutation effects).\n\n## Enzymes\n- Induced Fit Model: The active site changes shape slightly to mould around the substrate.\n- Factors: \n - Temperature: Rate increases with kinetic energy until optimum; above optimum results in denaturation.\n - pH: Optimal pH varies (Pepsin = $2$, Trypsin = $8$). Extreme pH denatures.\n - Substrate Concentration: Rate increases until enzyme saturation (plateau).\n- Inhibitors:\n - Competitive: Similar shape to substrate; binds to active site. Effect reduced by increasing substrate concentration.\n - Non-competitive: Binds allosteric site; changes active site shape. Adding substrate has no effect.\n\n## Respiration\n- Aerobic: $\text{Glucose} + \text{Oxygen} \rightarrow CO_2 + \text{Water} + \text{ATP}$. Yields $36-38\, \text{ATP}$. Occurs in Glycolysis (cytoplasm) → Krebs (matrix) → ETC (inner mitochondrial membrane/cristae).\n- Anaerobic (Animals): $\text{Glucose} \rightarrow \text{Lactic acid} + \text{ATP}$. Yields $2\, \text{ATP}$.\n- Anaerobic (Yeast): $\text{Glucose} \rightarrow \text{Ethanol} + CO_2 + \text{ATP}$. Yields $2\, \text{ATP}$.\n\n# TOPIC 3 — Genetics\n\n## Key Definitions\n- Gene: Heritable factor controlling a specific characteristic (DNA sequence coding for a polypeptide).\n- Allele: Specific form of a gene at the same locus.\n- Dominant: Expressed even in heterozygotes (masks recessive).\n- Recessive: Only expressed when homozygous.\n- Codominance: Both alleles expressed simultaneously (e.g., $I^A I^B$ blood type AB).\n\n## Meiosis vs. Mitosis\nMeiosis involves $2$ divisions producing $4$ genetically different haploid ($n$) cells. \n- Genetic Variation Sources: \n 1. Crossing over (Prophase I) at chiasmata.\n 2. Independent assortment (Metaphase I).\n 3. Random fertilization.\n\n## Inheritence and Sex Linkage\n- X-Linked Traits: Genes on X chromosome. Males ($XY$) are hemizygous and always express recessive X-linked alleles (e.g., hemophilia). Females ($XX$) can be carriers.\n- Test Cross: Breeding a dominant phenotype individual with a homozygous recessive ($bb$) to determine genotype.\n- Mutations:\n - Substitution: Silent (no change), Missense (one amino acid change), Nonsense (premature stop).\n - Deletion/Insertion: Frameshift; changes all downstream codons; usually non-functional protein.\n- Down Syndrome: Trisomy $21$ caused by non-disjunction; risk increases with maternal age.\n\n# TOPIC 4 — Ecology\n\n## Definitions and Energy\n- Species: Can interbreed to produce fertile offspring.\n- Community: Different species interacting in an area.\n- Ecosystem: Community plus abiotic environment.\n- Autotroph: Produces own organic matter (Producers).\n- Heterotroph: Consumes other organisms.\n- Detritivore: Internal digestion of dead matter (e.g., earthworms).\n- Saprotroph: External digestion via secreted enzymes (e.g., fungi).\n\n### Energy Flow\n- $10\%$ Rule: Only $10\%$ of energy transfers to the next level. $90\%$ is lost as heat, faeces, or used for movement.\n- Pyramids: Pyramids of energy are ALWAYS upright.\n\n## Nutrient Cycles\n### Nitrogen Cycle\n1. Nitrogen Fixation: Rhizobium (root nodules) or Azotobacter convert $N_2 \rightarrow NH_4^+$.\n2. Nitrification: $NH_4^+ \rightarrow NO_2^- \rightarrow NO_3^-$.\n3. Denitrification: $NO_3^- \rightarrow N_2$ (occurs in anaerobic/waterlogged conditions).\n\n## Population Growth\n- J-curve: Exponential; no limiting factors.\n- S-curve: Sigmoid; stabilizes at Carrying Capacity ($K$) due to limiting factors.\n- Factors: Density-dependent (disease, food) vs. Density-independent (natural disasters).\n\n# TOPIC 5 — Evolution & Biodiversity\n\n## Evidence for Evolution\n1. Fossil Record: Shows transitional forms (e.g., Archaeopteryx).\n2. Selective Breeding: Demonstrates phenotypic change over generations.\n3. Homologous Structures: Pentadactyl limb (human hand, whale flipper, bat wing) sharing bone structures (humerus, radius, ulna, carpals, phalanges) due to common ancestry.\n4. Comparative Biochemistry: DNA and amino acid sequence similarities (e.g., Cytochrome c).\n5. Antibiotic Resistance: Natural selection observed in real time.\n\n## Natural Selection (Darwin's Steps)\n1. Overproduction: Leads to competition.\n2. Heritable Variation: Arises from mutation and meiosis.\n3. Differential Survival: "Survival of the fittest" (reproductive fitness).\n4. Change in Allele Frequency: Advantageous alleles increase over generations.\n\n## Classification\n- Hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species (Dear King Philip Came Over For Good Soup).\n- Three Domains: Bacteria, Archaea, Eukarya.\n- Speciation: Allopatric (geographical barrier) leads to genetic divergence and reproductive isolation.\n\n# TOPIC 6 — Human Physiology\n\n## Digestion\n- Amylase: Starch → Maltose (Salivary glands/Pancreas).\n- Pepsin: Proteins → Polypeptides (Stomach, pH $2$).\n- Trypsin: Polypeptides → Shorter peptides (Pancreas/Small intestine, pH $8$).\n- Villi: One cell thick, large surface area, capillaries (glucose/AA) and lacteals (lipids).\n\n## Blood System\n- Arteries: Thick walls, high pressure, away from heart.\n- Veins: Thin walls, valves, toward heart.\n- Capillaries: One cell thick for exchange.\n- Pulmonary Exceptions: Pulmonary Artery (deoxygenated), Pulmonary Vein (oxygenated).\n- Heart: Left ventricle wall is thicker because it pumps to the entire body (systemic circuit) vs. only the lungs.\n\n## Immune Response\n1. Non-specific: Phagocytes (neutrophils/macrophages) engulf pathogens.\n2. Specific: B lymphocytes (antibodies) and T lymphocytes (cell death). Memory cells allow for much faster/stronger secondary responses.\n\n## Neurons and Synapses\n1. Action potential reaches pre-synaptic knob.\n2. $Ca^{2+}$ enters.\n3. Vesicles fuse and release neurotransmitter (exocytosis).\n4. Neurotransmitter diffuses across synaptic cleft.\n5. Binds to post-synaptic receptors.\n6. New action potential generated; neurotransmitter broken down by enzymes.\n\n## Blood Glucose Regulation\n- Insulin: From Beta cells; triggered by HIGH glucose; promotes glycogenesis (glucose → glycogen).\n- Glucagon: From Alpha cells; triggered by LOW glucose; promotes glycogenolysis (glycogen → glucose).\n- Diabetes: Type $1$ (No insulin/Beta cells destroyed) vs. Type $2$ (Insulin resistance).\n\n# QUESTIONS & DISCUSSION\n\nQ: State two features shared by both prokaryotic and eukaryotic cells.\nA: Ribosomes, DNA, cell membrane, or cytoplasm. (Both can carry out protein synthesis and cellular respiration.)\n\nQ: Explain why facilitated diffusion does not require ATP.\nA: Facilitated diffusion moves molecules DOWN the concentration gradient (high to low). This is the direction molecules naturally move; no energy input is needed. ATP is only needed for active transport against the gradient.\n\nQ: Explain why males are more likely to be affected by X-linked recessive conditions than females.\nA: Males only have one X chromosome ($XY$), so they only need $one$ copy of the recessive allele to express the trait. Females ($XX$) need $two$ copies to show the trait; females with $one$ copy are carriers.\n\nQ: Distinguish between a detritivore and a saprotroph.\nA: Detritivore: Ingests dead organic matter and digests it internally (e.g., earthworm). Saprotroph: Secretes digestive enzymes EXTERNALLY onto dead matter and then absorbs the products (e.g., fungi).", "title": "IB Biology SL Complete Revision Guide"}