IB Biology HL
Comprehensive flashcards for the IB Diploma Programme Biology, Higher Level (2025 syllabus, first exams 2025). Covers all four themes (A: Unity and diversity, B: Form and function, C: Interaction and interdependence, D: Continuity and change) including all Additional Higher Level (AHL) content.
Ämne: Biologi · Nivå: Gymnasium (16–19) · 497 kort
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- Water is a polar molecule: oxygen is more electronegative than hydrogen, giving partial negative charge (δ−) on O and partial positive (δ+) on the two H atoms.
- Hydrogen bonds form between water molecules when a δ+ hydrogen of one is attracted to the δ− oxygen of another. Individually weak, collectively they give water its emergent properties.
- Cohesion is attraction between water molecules; adhesion is attraction between water and other polar/charged surfaces. Together they enable capillary action and the transpiration stream in xylem.
- Water has high specific heat capacity (~4.18 J g⁻¹ °C⁻¹), high latent heat of vaporisation, and a wide liquid temperature range — properties that buffer temperature in cells and habitats.
- Water is the medium for metabolism and a reactant: it is a substrate in hydrolysis and in the light-dependent reactions (photolysis), and is produced in condensation reactions and aerobic respiration.
- Polar and ionic substances (glucose, amino acids, Na⁺, Cl⁻) are hydrophilic and dissolve in water; non-polar substances (lipids) are hydrophobic. Water is the universal transport medium in blood and sap.
- Surface tension arises from cohesion at the air-water interface and lets small organisms (e.g. pond skaters) walk on water. Ice is less dense than liquid water, so it floats and insulates water below.
- A DNA nucleotide has three components: a deoxyribose (pentose) sugar, a phosphate group, and one of four nitrogenous bases (adenine, thymine, guanine, cytosine).
- Nucleotides are joined by phosphodiester (covalent) bonds between the phosphate of one and the 3' carbon of the next sugar, forming a sugar-phosphate backbone with directionality (5' to 3').
- DNA is a double helix of two antiparallel strands (one runs 5'→3', the other 3'→5'), held together by hydrogen bonds between complementary bases and stabilised by base stacking.
- Complementary base pairing: adenine pairs with thymine via 2 hydrogen bonds; guanine pairs with cytosine via 3 hydrogen bonds. A purine always pairs with a pyrimidine, giving a uniform helix width.
- Purines (adenine, guanine) have a double-ring structure; pyrimidines (cytosine, thymine, uracil) have a single ring. This size difference underlies purine-pyrimidine pairing.
- RNA differs from DNA: ribose sugar (not deoxyribose), uracil instead of thymine, and it is single-stranded. Types include mRNA, tRNA and rRNA.
- DNA stores genetic information using a sequence of bases. Its two-strand structure allows accurate replication; the base sequence is conserved across generations and the genetic code is near-universal.
- Condensation reactions join monomers and release water; hydrolysis reactions split polymers using water. These reactions build and break carbohydrates, lipids, proteins and nucleic acids.
- Monosaccharides (e.g. glucose, fructose, galactose) are single sugar units. Two joined by a glycosidic bond form a disaccharide: glucose + glucose → maltose; glucose + fructose → sucrose; glucose + galactose → lactose.
- Polysaccharides: starch (amylose + amylopectin) stores glucose in plants; glycogen stores glucose in animals (highly branched); cellulose (β-glucose, straight chains) is a structural component of plant cell walls.
- Cellulose's β-1,4 glycosidic bonds force alternating glucose orientation, giving straight chains that hydrogen-bond into strong microfibrils. Starch's α-glucose bonds allow helical, easily hydrolysed chains.
- A triglyceride forms by condensation of one glycerol with three fatty acids, joined by ester bonds. Triglycerides are the main energy store in animals and store more energy per gram than carbohydrates.
- Saturated fatty acids have no C=C double bonds (solid at room temperature, e.g. animal fats); unsaturated have one or more C=C (liquid oils). Cis double bonds create kinks that lower the melting point.
- Phospholipids have a hydrophilic phosphate head and two hydrophobic fatty acid tails. This amphipathic nature drives spontaneous bilayer formation, the basis of cell membranes.
- Steroids (e.g. cholesterol, oestrogen, testosterone) are lipids with four fused carbon rings. Cholesterol modulates membrane fluidity and is a precursor for steroid hormones and bile.
- Amino acids share a central carbon bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen, and a variable R group. The R group determines whether an amino acid is polar, non-polar, acidic or basic.
- Peptide bonds form by condensation between the carboxyl group of one amino acid and the amino group of the next, releasing water. A chain of amino acids is a polypeptide.
- 20 different amino acids are used in protein synthesis. Their sequence (primary structure) is coded by genes, and the near-infinite possible sequences give proteins their enormous functional diversity.
- Protein structure levels: primary = amino acid sequence; secondary = α-helices and β-pleated sheets held by hydrogen bonds; tertiary = overall 3D fold; quaternary = two or more polypeptide subunits.
- Tertiary structure is stabilised by R-group interactions: hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges between cysteine residues. The fold determines protein function.
- Fibrous proteins (e.g. collagen, keratin) are long, insoluble and structural; globular proteins (e.g. haemoglobin, enzymes) are compact, often soluble and have metabolic/transport roles.
- Denaturation is loss of a protein's 3D conformation (not the primary sequence) due to heat or pH change breaking bonds in the tertiary/secondary structure. It usually causes irreversible loss of function.
- Viruses (HL) are non-cellular: a nucleic acid genome (DNA or RNA) inside a protein capsid, sometimes with a lipid envelope. They are obligate intracellular parasites with no metabolism of their own.
- Lytic cycle (HL): a virus injects its genome, hijacks host machinery to make new virions, then lyses the host cell. Lysogenic cycle: viral DNA integrates as a prophage and replicates with the host before later entering the lytic cycle.
- Retroviruses (HL), e.g. HIV, have an RNA genome and the enzyme reverse transcriptase, which makes DNA from RNA. This DNA integrates into the host genome.
- Viruses evolve rapidly (HL) because of short generation times, large populations, and high mutation rates (especially RNA viruses lacking proofreading). This drives antigenic drift/shift, e.g. in influenza.
- Cell theory states: all living organisms are made of one or more cells, the cell is the basic unit of life, and all cells arise from pre-existing cells. Atypical examples include striated muscle (multinucleate) and aseptate fungal hyphae.
- All cells share: a plasma membrane, cytoplasm, DNA as genetic material, and ribosomes for protein synthesis. These are common to prokaryotes and eukaryotes, suggesting a common origin.
- Prokaryotic cells (bacteria, archaea) lack a nucleus and membrane-bound organelles; DNA is a circular nucleoid plus plasmids. Eukaryotic cells have a nucleus and membrane-bound organelles.
- The first cells could have arisen from non-living matter through stages: formation of organic monomers, polymers, protocells with membranes, and self-replicating molecules. RNA may have been the first hereditary and catalytic molecule (RNA world).
- The last universal common ancestor (LUCA) is the inferred most recent organism from which all current life descends. Evidence includes the shared genetic code, ribosomes, ATP and core metabolic pathways.
- Endosymbiotic theory: mitochondria and chloroplasts originated from free-living prokaryotes engulfed by a host cell. Evidence: their own circular DNA, 70S ribosomes, double membranes, and binary-fission-like division.
- As a cell grows, volume increases faster than surface area, so the surface-area-to-volume ratio falls. A low ratio limits the rate of exchange of materials and heat, setting an upper limit on cell size.
- Magnification = image size ÷ actual size. A scale bar lets you calculate actual size. Typical sizes: ribosome ~25 nm, bacterium ~1–5 µm, animal cell ~10–100 µm.
- The nucleus is bounded by a double membrane (nuclear envelope) with pores. It contains chromatin (DNA + histones) and the nucleolus (ribosome subunit assembly), and controls gene expression.
- Mitochondria have a double membrane; the inner membrane is folded into cristae bearing the electron transport chain. The matrix is the site of the Krebs cycle. They are the site of aerobic respiration (ATP production).
- Chloroplasts have a double membrane, stacked thylakoids (grana) holding chlorophyll, and a fluid stroma. Light-dependent reactions occur on thylakoids; the Calvin cycle occurs in the stroma.
- Rough ER is studded with ribosomes and synthesises/transports proteins for secretion; smooth ER synthesises lipids and steroids and detoxifies. The Golgi apparatus modifies, sorts and packages proteins into vesicles.
- Ribosomes synthesise proteins. Eukaryotes have 80S ribosomes (free or on rough ER); prokaryotes, mitochondria and chloroplasts have smaller 70S ribosomes.
- Lysosomes contain hydrolytic (digestive) enzymes at low pH and break down worn organelles, pathogens and macromolecules. They form from the Golgi apparatus.
- Compartmentalisation (AHL) lets a eukaryotic cell maintain different conditions in each organelle: e.g. acidic lysosomes, separated metabolic pathways, and concentration of enzymes/substrates for efficiency.
- Plant cells additionally have a cellulose cell wall, a large central vacuole (turgor and storage), and chloroplasts. Animal cells have centrioles and lysosomes but no wall or chloroplasts.
- A species is often defined as a group of organisms that can interbreed and produce fertile, viable offspring. The biological species concept fails for asexual organisms, extinct species and ring species.