# IB Biology HL/SL: Unity & Diversity, Molecular Biology, Cell Respiration, Photosynthesis & Genetics

> IBDP Biology (SL & HL) core revision covering Theme A–D: endosymbiotic theory, water & carbon macromolecules, enzyme kinetics, Davson-Danielli vs Singer-Nicolson membrane models, DNA replication, gene expression, cell respiration, photosynthesis, and inheritance.

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- **Level:** basic
- **Languages:** en → en
- **Total Cards:** 32

## Flashcards (32 Cards)

| # | Front (Term / Prompt) | Back (Definition / Answer) | Example | Multiple-Choice Options |
| :--- | :--- | :--- | :--- | :--- |
| 1 | Detail the HL stages of Aerobic Cell Respiration: Glycolysis, Link Reaction, Krebs Cycle, and Chemiosmosis / Oxidative Phosphorylation. | Glycolysis (cytoplasm): Glucose (6C) + 2 ATP → 2 TP (3C) → oxidised (2 NAD⁺ → 2 NADH) + 4 ATP → 2 Pyruvate (net 2 ATP, 2 NADH). Link Reaction (matrix): Pyruvate + CoA + NAD⁺ → Acetyl-CoA (2C) + CO2 + NADH. Krebs Cycle (matrix): Acetyl-CoA (2C) + Oxaloacetate (4C) → Citrate (6C) → 5C → 4C (per Acetyl-CoA: 2 CO2, 3 NADH, 1 FADH2, 1 ATP). Oxidative Phosphorylation (cristae): NADH and FADH2 donate electrons to ETC, pumping H⁺ into intermembrane space; H⁺ flows through ATP synthase to phosphorylate ADP; O2 is terminal electron acceptor forming H2O. |  |  |
| 2 | Explain the structure and function of Nucleosomes in eukaryotic chromatin packaging and gene expression regulation (HL). | A nucleosome consists of DNA (~147 base pairs) wrapped nearly twice around an octamer core of 8 positively charged histone proteins (2 each of H2A, H2B, H3, H4), secured by an H1 linker histone. Nucleosomes supercoil DNA to fit inside the nucleus, protect DNA from damage, and regulate transcription via acetylation/methylation of histone tails (euchromatin = loosely packed, active transcription; heterochromatin = tightly condensed, silent). |  |  |
| 3 | How did falsification of the Davson–Danielli (sandwich) model lead to the Singer–Nicolson Fluid Mosaic Model of membranes (IB Biology B2.1)? | Davson–Danielli proposed a phospholipid bilayer coated on both sides by continuous layers of hydrophilic globular proteins. It was falsified by: 1) Freeze-etched electron micrographs showing transmembrane globular proteins embedded inside the core of the bilayer; 2) Fluorescent antibody tagging (Frye & Edidin) showing membrane proteins drift laterally; 3) Amphipathic structure of isolated membrane proteins. |  |  |
| 4 | How does eukaryotic post-transcriptional modification of pre-mRNA (5' capping, 3' polyadenylation, and alternative splicing) work (HL)? | 1) A methylated guanine (7-methylguanosine) cap is added to the 5' end and a poly-A tail (100–250 adenine nucleotides) is added to the 3' end to protect mRNA from exonuclease degradation and aid nuclear export/ribosome binding; 2) Spliceosomes remove non-coding introns and join exons; 3) Alternative splicing splices different combinations of exons from the same gene, enabling one gene to code for multiple distinct polypeptide isoforms. |  |  |
| 5 | Differentiate between Continuous (polygenic) and Discontinuous (monogenic) variation with human examples. | Discontinuous variation is controlled by 1 or 2 genes (monogenic), shows discrete, non-overlapping phenotypic categories (represented by a bar chart), and is unaffected by the environment (e.g., ABO blood group). Continuous variation is controlled by the additive effect of many genes (polygenic inheritance) combined with environmental factors (e.g., sunlight, nutrition), producing a continuous normal distribution bell curve (e.g., human height and skin pigmentation). |  |  |
| 6 | How do xerophytes (desert plants) and halophytes (saline marsh plants) adapt structurally and physiologically to extreme water stress (B3.1 / C3.1)? | Xerophytes (e.g., marram grass, cacti): thick waxy cuticle, rolled leaves trapping humid air, sunken stomata in hairy pits, reduced leaf surface area (spines), and CAM physiology (opening stomata only at night to fix CO2 as malate). Halophytes (e.g., mangroves, Salicornia): compartmentalise toxic Na⁺ and Cl⁻ ions into large cell vacuoles (lowering solute potential Ψs so water enters roots from salty soil), succulent water-storing stems, and salt glands on leaves that actively excrete excess salt. |  |  |
| 7 | Compare how hydrophilic (polar/charged) and hydrophobic (non-polar) substances are transported in blood plasma. | Sodium chloride (NaCl) and amino acids are ionic/zwitterionic and dissolve directly in water. Glucose is polar and freely water-soluble. Oxygen (O2) is non-polar with low solubility in water, so it is bound to haemoglobin inside erythrocytes. Cholesterol and triglycerides (fats) are hydrophobic/non-polar and are transported inside lipoprotein complexes (monolayer of amphipathic phospholipids + proteins surrounding a hydrophobic core). |  |  |
| 8 | How do Rubisco (RuBP carboxylase) and Insulin exemplify globular protein functions, and how do Collagen and Spider Silk exemplify fibrous proteins? | Globular proteins (water-soluble, spherical tertiary structure): Rubisco fixes atmospheric CO2 in photosynthesis (most abundant enzyme on Earth); Insulin is a two-chain peptide hormone that binds to hepatocyte/muscle receptors to lower blood glucose. Fibrous proteins (insoluble, elongated structural roles): Collagen forms a triple helix of three polypeptides giving high tensile strength in skin, tendons, and blood vessel walls; Spider dragline silk combines β-pleated sheets and amorphous regions for extreme toughness. |  |  |
| 9 | Contrast the molecular structure and properties of saturated, monounsaturated, cis-polyunsaturated, and trans-unsaturated fatty acids. | Saturated fatty acids have no C=C double bonds, forming straight chains that pack tightly (strong London forces, high melting points, solid animal fats, linked to CHD). Cis-unsaturated fatty acids have H atoms on the same side of the C=C double bond, creating a 30° kink in the hydrocarbon chain so molecules cannot pack closely (liquid plant oils at room temperature). Trans-fatty acids (produced by partial industrial hydrogenation) have H atoms on opposite sides of C=C, forming straight chains with high melting points that raise LDL cholesterol and cause atherosclerosis. |  |  |
| 10 | Solve IB Genetics crosses involving ABO Blood Group Multiple Alleles (I^A, I^B, i) and X-linked recessive conditions (haemophilia, red-green colour blindness). | ABO blood groups involve 3 alleles of the isoagglutinogen gene: I^A and I^B are codominant, and both are dominant over recessive i (I^A I^A or I^A i = Group A; I^B I^B or I^B i = Group B; I^A I^B = Group AB; ii = Group O). Sex-linked recessive traits (X^h on the longer X chromosome, absent on the Y chromosome) are much more common in males (X^h Y) because males are hemizygous and only need one recessive allele from their carrier mother (X^H X^h) to express the condition. |  |  |
| 11 | How do Hershey and Chase's (1952) T2 bacteriophage experiment and Rosalind Franklin's X-ray diffraction support DNA structure and function (A1.2)? | Hershey & Chase labelled phage protein coats with radioactive sulfur (³⁵S) and phage DNA with radioactive phosphorus (³²P). After infection and centrifugation, ³²P was found inside the bacterial pellet while ³⁵S remained in the liquid supernatant, proving DNA (not protein) is the genetic material. Franklin's Photo 51 X-ray diffraction pattern showed a cross-shaped pattern proving a helical structure with regular 3.4 nm pitch and hydrophobic bases stacked inside. |  |  |
| 12 | Define the Mitotic Index and state the roles of Cyclins and CDKs in controlling the eukaryotic cell cycle (B2.3 / D2.1). | Mitotic Index = (Number of cells in mitosis: prophase, metaphase, anaphase, or telophase) ÷ (Total number of cells visible in field of view). A high mitotic index in adult tissue indicates rapid tissue repair or a malignant tumour. Cyclins are regulatory proteins whose concentrations rise and fall cyclically; they bind to Cyclin-Dependent Kinases (CDKs) to phosphorylate target proteins that trigger cell cycle checkpoints (G1/S, G2/M). |  |  |
| 13 | In IB Biology Internal Assessment (IA) and Paper 1B Data Analysis, how do you interpret error bars, standard deviation, and the coefficient of determination (R²)? | Standard deviation (s) measures the spread of data values around the sample mean (68% of normally distributed values fall within ±1 SD). If the SD (or 95% confidence interval) error bars of two group means overlap substantially, the difference between the means is likely NOT statistically significant (must be confirmed with a t-test or ANOVA at p < 0.05). R² (0 to 1) indicates the proportion of variance in the dependent variable explained by the independent variable. |  |  |
| 14 | Compare the carbon fluxes in the Global Carbon Cycle, including methanogenesis, peat formation, and ocean acidification on coral reefs. | Methanogenic archaeans in anaerobic wetlands/ruminant guts produce methane (CH4), which oxidises in the atmosphere to CO2 and H2O. In waterlogged, acidic, anaerobic bogs, saprotrophs cannot fully decompose dead organic matter, forming peat (which compresses under heat/pressure over millions of years into coal). Rising atmospheric CO2 dissolves in oceans to form carbonic acid (H2CO3 ⇌ H⁺ + HCO3⁻), lowering ocean pH and reducing carbonate ion (CO3²⁻) concentration needed by reef-building corals and molluscs to calcify their CaCO3 exoskeletons. |  |  |
| 15 | Why does the surface area-to-volume ratio (SA:V) limit maximum cell size, and how do cells adapt to high metabolic rates? | As a cell's radius r increases, volume increases cubically (∝ r³), determining the rate of metabolism/heat/waste production, whereas surface area increases quadratically (∝ r²), determining the rate of exchange across the membrane. Thus SA:V decreases as cells grow larger; cells with high metabolic rates adapt via microvilli, flattened shapes (erythrocytes), or invaginations (cristae/thylakoids). |  |  |
| 16 | Describe the flow of energy vs nutrients in an ecosystem, why food chains rarely exceed 4–5 trophic levels, and how Mesocosms test sustainability (C4.2). | Energy enters ecosystems as sunlight, flows unidirectionally through food chains with ~90% lost as respiratory heat, unconsumed parts, or egestion at each trophic level (pyramids of energy in kJ m⁻² yr⁻¹ are ALWAYS stepped), and leaves as heat—limiting food chains to 4–5 levels. Nutrients (C, N, P) are finite and are recycled endlessly by saprotrophic bacteria/fungi and detritivores. Sealed mesocosms demonstrate that an ecosystem is sustainable with only an autotroph, saprotrophs, nutrient cycling, and an external light source. |  |  |
| 17 | Compare Competitive vs Non-Competitive Enzyme Inhibition using Michaelis–Menten curves (Km and Vmax) and explain End-Product Inhibition (HL C1.1). | Km (Michaelis constant) is the substrate concentration at ½ Vmax (inverse measure of enzyme affinity for substrate). Competitive inhibition: increases Km (lowers apparent affinity) while Vmax remains unchanged at high [S]. Non-competitive (allosteric) inhibition: decreases Vmax while Km remains unchanged. End-product inhibition: the final product of a metabolic pathway binds to an allosteric site on the first enzyme in the pathway (e.g., isoleucine inhibiting threonine deaminase), providing negative feedback. |  |  |
| 18 | Explain the evidence for Evolution (fossil record, selective breeding, homologous pentadactyl limbs) and distinguish Gradualism from Punctuated Equilibrium (A4.1). | Homologous structures (such as the pentadactyl limb across mammals, birds, amphibians, and reptiles) have the same underlying evolutionary bone structure adapted to different functions via adaptive radiation (divergent evolution), whereas analogous structures evolve independently via convergent evolution. Gradualism proposes slow, continuous evolutionary change over geological time; Punctuated Equilibrium (Gould & Eldredge) proposes long periods of stasis interrupted by rapid speciation events following sudden environmental upheaval. |  |  |
| 19 | Contrast the structure and function of the mitochondrial compartments in HL Aerobic Cell Respiration (C1.2). | 1) Outer membrane: smooth, contains transport proteins (porins) permeable to pyruvate. 2) Intermembrane space: tiny volume so pumped H⁺ protons rapidly build up a steep proton motive force (electrochemical gradient). 3) Inner membrane: folded into cristae (huge surface area) containing electron transport chain complexes and ATP synthase. 4) Matrix: fluid containing enzymes and optimal pH for the Link Reaction and Krebs Cycle, plus 70S ribosomes and circular mitochondrial DNA. |  |  |
| 20 | In HL Genetics, how did Thomas Hunt Morgan's Drosophila crosses explain autosomal gene linkage and recombinant phenotypes? | Genes located on the same chromosome belong to the same linkage group (denoted with horizontal bars, e.g., GL // gl) and are inherited together rather than assorting independently, skewing a dihybrid test cross away from the expected 1:1:1:1 ratio. The small proportion of non-parental (recombinant) offspring arises from crossing over at chiasmata between non-sister chromatids during Prophase I of meiosis. |  |  |
| 21 | Compare Cyclic and Non-Cyclic Photophosphorylation in the Light-Dependent Reactions of HL Photosynthesis. | Non-cyclic photophosphorylation involves both Photosystem II (P680) and Photosystem I (P700): photolysis of water replaces electrons at PSII (producing O2), plastoquinone/cytochrome b6f pumps H⁺ into the thylakoid space to make ATP via chemiosmosis, and ferredoxin + NADP reductase reduce NADP⁺ to NADPH at PSI. Cyclic photophosphorylation (when NADP⁺ runs low) recycles excited electrons from PSI back to the cytochrome proton pump, producing additional ATP for the Calvin Cycle but NO NADPH and NO O2. |  |  |
| 22 | State the seven functions of life (MR H GREN / MR SHENG) demonstrated by unicellular organisms such as Paramecium and Chlorella. | Metabolism (enzyme-catalysed reactions in cytoplasm), Response (reacting to stimuli, e.g., cilia in Paramecium or eyespot in Chlamydomonas), Homeostasis (contractile vacuoles expelling excess water), Growth (increase in cell size/biomass), Reproduction (asexual mitosis or sexual), Excretion (diffusion of metabolic waste across plasma membrane), and Nutrition (heterotrophic endocytosis in Paramecium vs autotrophic photosynthesis in Chlorella). |  |  |
| 23 | Explain biomagnification vs bioaccumulation of persistent pollutants (e.g., DDT or methylmercury) and the ecological impact of macroplastic/microplastic pollution. | Bioaccumulation is the build-up of a non-biodegradable, fat-soluble toxin within the tissues of a single organism over its lifetime. Biomagnification is the progressive increase in concentration of the toxin at successively higher trophic levels in a food chain because predators consume large biomasses of contaminated prey (causing eggshell thinning in ospreys/peregrine falcons exposed to DDT). Microplastics (<5 mm) absorb persistent organic pollutants (POPs) and are ingested by filter feeders, entering marine food webs. |  |  |
| 24 | How did Melvin Calvin's 'lollipop' apparatus using ¹⁴CO2 and two-dimensional paper chromatography elucidate the Calvin Cycle? | Calvin grew unicellular Chlorella algae in a thin transparent 'lollipop' vessel illuminated with light, injected radioactive ¹⁴CO2, and killed algae in boiling ethanol at 1–5 second intervals. Separating radiolabelled compounds by 2D chromatography and autoradiography revealed that after 1–2 seconds the first radioactive compound formed was 3-carbon Glycerate 3-Phosphate (3-PGA / GP), followed by Triose Phosphate (G3P/TP) and regenerated Ribulose Bisphosphate (RuBP). |  |  |
| 25 | In IB Biology Cell Division, what is the role of supercoiling, kinetochores, and microtubules during mitosis, and how does cytokinesis differ in plant vs animal cells? | During prophase, DNA supercoils around histones into visible sister chromatids; spindle microtubules attach to protein kinetochores at each centromere and shorten during anaphase to pull sister chromatids to opposite poles. In animal cells, cytokinesis occurs via a contractile ring of actin and myosin microfilaments pinching the plasma membrane inwards (cleavage furrow). In plant cells, Golgi vesicles fuse along the equator to form a cell plate and new pectin middle lamella/cellulose cell wall from the inside out. |  |  |
| 26 | Describe the roles of all 7 enzymes/proteins in HL prokaryotic DNA replication (Helicase, Gyrase, Primase, DNA Pol III, DNA Pol I, Ligase, SSBs). | 1) DNA Gyrase (topoisomerase): relieves torsional strain ahead of replication fork; 2) DNA Helicase: unwinds double helix by breaking H-bonds; 3) Single-Stranded Binding proteins (SSBs): prevent strands re-annealing; 4) RNA Primase: synthesises short RNA primer; 5) DNA Polymerase III: synthesises new DNA strand 5'→3' (continuous on leading strand, discontinuous Okazaki fragments on lagging strand) using dNTPs; 6) DNA Polymerase I: removes RNA primers and replaces them with DNA; 7) DNA Ligase: seals nicks by forming covalent phosphodiester bonds. |  |  |
| 27 | What is the difference between an Action Spectrum and an Absorption Spectrum in photosynthesis, and how do you calculate Rf in TLC of leaf pigments (C1.3)? | An Absorption Spectrum plots the percentage of light absorbed by each photosynthetic pigment (chlorophyll a, chlorophyll b, carotenoids) against wavelength (peaks in blue ~450 nm and red ~670 nm; green ~500–550 nm reflected). An Action Spectrum plots the overall rate of photosynthesis at each wavelength of light. In Thin-Layer Chromatography (TLC): Rf = (distance travelled by pigment spot) ÷ (distance travelled by solvent front). |  |  |
| 28 | What is a Clade in Cladistics (A3.2), why are molecular clocks (base/amino acid sequences) preferred over morphology, and what happened to the Figwort family? | A clade is a monophyletic group consisting of a single common ancestral species and ALL of its evolutionary descendants. DNA base sequences and amino acid sequences accumulate neutral mutations at a roughly constant rate (molecular clock), avoiding errors caused by convergent evolution (analogous traits). Chloroplast gene sequencing showed the traditional Figwort family (Scrophulariaceae) was polyphyletic, leading taxonomists to reclassify its genera into several distinct families. |  |  |
| 29 | In IB Biology Water Potential & Osmolarity Required Practical, how do you determine the isotonic osmolarity of plant tissue (e.g., potato tubers)? | Cut uniform cylinders of potato tissue, measure initial mass, and immerse in a series of sucrose or NaCl solutions of known molarity (0.0 to 1.0 mol dm⁻³) for a fixed time. Blot dry, measure final mass, and plot Percentage Change in Mass against Solution Concentration. The x-intercept (where % change in mass = 0%) gives the isotonic concentration where net water movement is zero and solute potential equals that of the tissue. |  |  |
| 30 | How does the CRISPR-Cas9 gene editing system target and modify specific DNA sequences in living cells (IB Biology 2023+ syllabus A1.2 / D1.3)? | CRISPR-Cas9 uses a synthetic single guide RNA (sgRNA) engineered to be complementary to a specific 20-base target DNA sequence adjacent to a PAM (Protospacer Adjacent Motif) site. The Cas9 endonuclease binds the guide RNA, scans genomic DNA until the sgRNA hybridises with the target sequence, and makes a precise double-stranded cut in the DNA, allowing the cell's repair machinery to knock out a faulty gene or knock in a healthy donor sequence. |  |  |
| 31 | What three pieces of evidence support the Endosymbiotic Theory for the origin of eukaryotic mitochondria and chloroplasts (IB Biology A2.2)? | Mitochondria and chloroplasts evolved from engulfed independent prokaryotes: 1) Both possess their own single circular naked DNA molecule (like bacteria); 2) Both contain 70S ribosomes (smaller than eukaryotic 80S cytoplasmic ribosomes); 3) Both have a double membrane (inner membrane from prokaryote, outer from host food vacuole) and divide independently by binary fission. |  |  |
| 32 | Compare the sigmoid (S-shaped) population growth curve (natality, mortality, immigration, emigration) and density-dependent vs density-independent limiting factors (C4.1). | Population change = (Natality + Immigration) − (Mortality + Emigration). Sigmoid growth curve has 3 phases: 1) Exponential growth phase: abundant resources, minimal competition/predation, N + I ≫ M + E; 2) Transitional phase: intraspecific competition for food/space and predation increase as population approaches environmental limits; 3) Plateau (stationary) phase: population stabilises at the Carrying Capacity (K) where N + I = M + E, regulated by negative feedback from density-dependent factors (disease, competition, predation). |  |  |

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