DP Biology (first assessment 2025)

Syllabus sections

A. Unity and diversity
A1.1. Water
A1.1.1. Water as the medium for life
A1.1.2. Hydrogen bonds as a consequence of the polar covalent bonds within water molecules
A1.1.3. Cohesion of water molecules due to hydrogen bonding and consequences for organisms
A1.1.4. Adhesion of water to materials that are polar or charged and impacts for organisms
A1.1.5. Solvent properties of water linked to its role as a medium for metabolism and for transport in plants and animals
A1.1.6. Physical properties of water and the consequences for animals in aquatic habitats
A1.1.7. Extraplanetary origin of water on Earth and reasons for its retention
A1.1.8. Relationship between the search for extraterrestrial life and the presence of water
A1.2. Nucleic acids
A1.2.1. DNA as the genetic material of all living organisms
A1.2.2. Components of a nucleotide
A1.2.3. Sugar–phosphate bonding and the sugar–phosphate “backbone” of DNA and RNA
A1.2.4. Bases in each nucleic acid that form the basis of a code
A1.2.5. RNA as a polymer formed by condensation of nucleotide monomers
A1.2.6. DNA as a double helix made of two antiparallel strands of nucleotides with two strands linked by hydrogen bonding between complementary base pairs
A1.2.7. Differences between DNA and RNA
A1.2.8. Role of complementary base pairing in allowing genetic information to be replicated and expressed
A1.2.9. Diversity of possible DNA base sequences and the limitless capacity of DNA for storing i
A1.2.10. Conservation of the genetic code across all life forms as evidence of universal common ancestry
A1.2.11. Directionality of RNA and DNA
A1.2.12. Purine-to-pyrimidine bonding as a component of DNA helix stability
A1.2.13. Structure of a nucleosome
A1.2.14. Evidence from the Hershey–Chase experiment for DNA as the genetic material
A1.2.15. Chargaff’s data on the relative amounts of pyrimidine and purine bases across diverse life forms
A2.1. Origins of cells (HL only)
A2.1.1. Conditions on early Earth and the pre-biotic formation of carbon compounds
A2.1.2. Cells as the smallest units of self-sustaining life
A2.1.3. Challenge of explaining the spontaneous origin of cells
A2.1.4. Evidence for the origin of carbon compounds
A2.1.5. Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers
A2.1.6. RNA as a presumed first genetic material
A2.1.7. Evidence for a last universal common ancestor
A2.1.8. Approaches used to estimate dates of the first living cells and the last universal common ancestor
A2.1.9. Evidence for the evolution of the last universal common ancestor in the vicinity of hydrothermal vents
A2.2. Cell structure
A2.2.2. Microscopy skills
A2.2.1. Cells as the basic structural unit of all living organisms
A2.2.4. Structures common to cells in all living organisms
A2.2.5. Prokaryote cell structure
A2.2.3. Developments in microscopy
A2.2.6. Eukaryote cell structure
A2.2.7. Processes of life in unicellular organisms
A2.2.8. Differences in eukaryotic cell structure between animals, fungi and plants
A2.2.9. Atypical cell structure in eukaryotes
A2.2.10. Cell types and cell structures viewed in light and electron micrographs
A2.2.11. Drawing and annotation based on electron micrographs
A2.2.12. Origin of eukaryotic cells by endosymbiosis
A2.2.13. Cell differentiation as the process for developing specialized tissues in multicellular organisms
A2.2.14. Evolution of multicellularity
A2.3. Viruses (HL only)
A2.3.1. Structural features common to viruses
A2.3.2. Diversity of structure in viruses
A2.3.3. Lytic cycle of a virus
A2.3.4. Lysogenic cycle of a virus
A2.3.5. Evidence for several origins of viruses from other organisms
A2.3.6. Rapid evolution in viruses
A3.1. Diversity of organisms
A3.1.1. Variation between organisms as a defining feature of life
A3.1.2. Species as groups of organisms with shared traits
A3.1.3. Binomial system for naming organisms
A3.1.4. Biological species concept
A3.1.5. Difficulties distinguishing between populations and species due to divergence of noninterbreeding populations during speciation
A3.1.6. Diversity in chromosome numbers of plant and animal species
A3.1.7. Karyotyping and karyograms
A3.1.8. Unity and diversity of genomes within species
A3.1.9. Diversity of eukaryote genomes
A3.1.10. Comparison of genome sizes
A3.1.11. Current and potential future uses of whole genome sequencing
A3.1.12. Difficulties applying the biological species concept to asexually reproducing species and to bacteria that have horizontal gene transfer
A3.1.13. Chromosome number as a shared trait within a species
A3.1.14. Engagement with local plant or animal species to develop a dichotomous key
A3.1.15. Identification of species from environmental DNA in a habitat using barcodes
A3.2. Classification and cladistics (HL only)
A3.2.1. Need for classification of organisms
A3.2.2. Difficulties classifying organisms into the traditional hierarchy of taxa
A3.2.3. Advantages of classification corresponding to evolutionary relationships
A3.2.4. Clades as groups of organisms with common ancestry and shared characteristics
A3.2.5. Gradual accumulation of sequence differences as the basis for estimates of when clades diverged from a common ancestor
A3.2.6. Base sequences of genes or amino acid sequences of proteins as the basis for constructing cladograms
A3.2.7. Analysing cladograms
A3.2.8. Using cladistics to investigate whether the classification of groups corresponds to evolutionary relationships
A3.2.9. Classification of all organisms into three domains using evidence from rRNA base sequences
A4.1. Evolution and speciation
A4.1.1. Evolution as change in the heritable characteristics of a population
A4.1.2. Evidence for evolution from base sequences in DNA or RNA and amino acid sequences in proteins
A4.1.3. Evidence for evolution from selective breeding of domesticated animals and crop plants
A4.1.4. Evidence for evolution from homologous structures
A4.1.5. Convergent evolution as the origin of analogous structures
A4.1.6. Speciation by splitting of pre-existing species
A4.1.7. Roles of reproductive isolation and differential selection in speciation
A4.1.8. Differences and similarities between sympatric and allopatric speciation
A4.1.9. Adaptive radiation as a source of biodiversity
A4.1.10. Barriers to hybridization and sterility of interspecific hybrids as mechanisms for of preventing the mixing of alleles between species
A4.1.11. Abrupt speciation in plants by hybridization and polyploidy
A4.2. Conservation of biodiversity
A4.2.1. Biodiversity as the variety of life in all its forms, levels and combinations
A4.2.2. Comparisons between current number of species on Earth and past levels of biodiversity
A4.2.3. Causes of anthropogenic species extinction
A4.2.4. Causes of ecosystem loss
A4.2.5. Evidence for a biodiversity crisis
A4.2.6. Causes of the current biodiversity crisis
A4.2.7. Need for several approaches to conservation of biodiversity
A4.2.8. Selection of evolutionarily distinct and globally endangered species for conservation prioritization in the EDGE of Existence programme
B. Form and function
B1.1. Carbohydrates and lipids
B1.1.1. Chemical properties of a carbon atom allowing for the formation of diverse compounds upon which life is based
B1.1.2. Production of macromolecules by condensation reactions that link monomers to form a polymer
B1.1.3. Digestion of polymers into monomers by hydrolysis reactions
B1.1.4. Form and function of monosaccharides
B1.1.5. Polysaccharides as energy storage compounds
B1.1.6. Structure of cellulose related to its function as a structural polysaccharide in plants
B1.1.7. Role of glycoproteins in cell–cell recognition
B1.1.8. Hydrophobic properties of lipids
B1.1.9. Formation of triglycerides and phospholipids by condensation reactions
B1.1.10. Difference between saturated, monounsaturated and polyunsaturated fatty acids
B1.1.11. Triglycerides in adipose tissues for energy storage and thermal insulation
B1.1.12. Formation of phospholipid bilayers as a consequence of the hydrophobic and hydrophilic regions
B1.1.13. Ability of non-polar steroids to pass through the phospholipid bilayer
B1.2. Proteins
B1.2.1. Generalized structure of an amino acid
B1.2.2. Condensation reactions forming dipeptides and longer chains of amino acids
B1.2.3. Dietary requirements for amino acids
B1.2.4. Infinite variety of possible peptide chains
B1.2.5. Effect of pH and temperature on protein structure
B1.2.6. Chemical diversity in the R-groups of amino acids as a basis for the immense diversity in protein form and function
B1.2.7. Impact of primary structure on the conformation of proteins
B1.2.8. Pleating and coiling of secondary structure of proteins
B1.2.9. Dependence of tertiary structure on hydrogen bonds, ionic bonds, disulfide covalent bonds and hydrophobic interactions
B1.2.10. Effect of polar and non-polar amino acids on tertiary structure of proteins
B1.2.11. Quaternary structure of non-conjugated and conjugated proteins
B1.2.12. Relationship of form and function in globular and fibrous proteins
B2.1. Membranes and membrane transport
B2.1.1. Lipid bilayers as the basis of cell membranes
B2.1.2. Lipid bilayers as barriers
B2.1.3. Simple diffusion across membranes
B2.1.4. Integral and peripheral proteins in membranes
B2.1.5. Movement of water molecules across membranes by osmosis and the role of aquaporins
B2.1.6. Channel proteins for facilitated diffusion
B2.1.7. Pump proteins for active transport
B2.1.8. Selectivity in membrane permeability
B2.1.9. Structure and function of glycoproteins and glycolipids
B2.1.10. Fluid mosaic model of membrane structure
B2.1.11. Relationships between fatty acid composition of lipid bilayers and their fluidity
B2.1.12. Cholesterol and membrane fluidity in animal cells
B2.1.13. Membrane fluidity and the fusion and formation of vesicles
B2.1.14. Gated ion channels in neurons
B2.1.15. Sodium–potassium pumps as an example of exchange transporters
B2.1.16. Sodium-dependent glucose cotransporters as an example of indirect active transport
B2.1.17. Adhesion of cells to form tissues
B2.2. Organelles and compartmentalization
B2.2.1. Organelles as discrete subunits of cells that are adapted to perform specific functions
B2.2.2. Advantage of the separation of the nucleus and cytoplasm into separate compartments
B2.2.3. Advantages of compartmentalization in the cytoplasm of cells
B2.2.4. Adaptations of the mitochondrion for production of ATP by aerobic cell respiration
B2.2.5. Adaptations of the chloroplast for photosynthesis
B2.2.6. Functional benefits of the double membrane of the nucleus
B2.2.7. Structure and function of free ribosomes and of the rough endoplasmic reticulum
B2.2.8. Structure and function of the Golgi apparatus
B2.2.9. Structure and function of vesicles in cells
B2.3. Cell specialization
B2.3.1. Production of unspecialized cells following fertilization and their development into specialized cells by differentiation
B2.3.2. Properties of stem cells
B2.3.3. Location and function of stem cell niches in adult humans
B2.3.4. Differences between totipotent, pluripotent and multipotent stem cells
B2.3.5. Cell size as an aspect of specialization
B2.3.6. Surface area-to-volume ratios and constraints on cell size
B2.3.7. Adaptations to increase surface area-to-volume ratios of cells
B2.3.8. Adaptations of type I and type II pneumocytes in alveoli
B2.3.9. Adaptations of cardiac muscle cells and striated muscle fibres
B2.3.10. Adaptations of sperm and egg cells
B3.1. Gas exchange
B3.1.1. Gas exchange as a vital function in all organisms
B3.1.2. Properties of gas-exchange surfaces
B3.1.3. Maintenance of concentration gradients at exchange surfaces in animals
B3.1.4. Adaptations of mammalian lungs for gas exchange
B3.1.5. Ventilation of the lungs
B3.1.6. Measurement of lung volumes
B3.1.7. Adaptations for gas exchange in leaves
B3.1.8. Distribution of tissues in a leaf
B3.1.9. Transpiration as a consequence of gas exchange in a leaf
B3.1.10. Stomatal density
B3.1.11. Adaptations of foetal and adult haemoglobin for the transport of oxygen
B3.1.12. Bohr shift
B3.1.13. Oxygen dissociation curves as a means of representing the affinity of haemoglobin for oxygen at different oxygen concentrations
B3.2. Transport
B3.2.1. Adaptations of capillaries for exchange of materials between blood and the internal or external environment
B3.2.2. Structure of arteries and veins
B3.2.3. Adaptations of arteries for the transport of blood away from the heart
B3.2.4. Measurement of pulse rates
B3.2.5. Adaptations of veins for the return of blood to the heart
B3.2.6. Causes and consequences of occlusion of the coronary arteries
B3.2.7. Transport of water from roots to leaves during transpiration
B3.2.8. Adaptations of xylem vessels for transport of water
B3.2.9. Distribution of tissues in a transverse section of the stem of a dicotyledonous plant
B3.2.10. Distribution of tissues in a transverse section of the root of a dicotyledonous plant
B3.2.11. Release and reuptake of tissue fluid in capillaries
B3.2.12. Exchange of substances between tissue fluid and cells in tissues
B3.2.13. Drainage of excess tissue fluid into lymph ducts
B3.2.14. Differences between the single circulation of bony fish and the double circulation of mammals
B3.2.15. Adaptations of the mammalian heart for delivering pressurized blood to the arteries
B3.2.16. Stages in the cardiac cycle
B3.2.17. Generation of root pressure in xylem vessels by active transport of mineral ions
B3.2.18. Adaptations of phloem sieve tubes and companion cells for translocation of sap
B3.3. Muscle and motility (HL only)
B3.3.1. Adaptations for movement as a universal feature of living organisms
B3.3.2. Sliding filament model of muscle contraction
B3.3.3. Role of the protein titin and antagonistic muscles in muscle relaxation
B3.3.4. Structure and function of motor units in skeletal muscle
B3.3.5. Roles of skeletons as anchorage for muscles and as levers
B3.3.6. Movement at a synovial joint
B3.3.7. Range of motion of a joint
B3.3.8. Internal and external intercostal muscles as an example of antagonistic muscle action to facilitate internal body movements
B3.3.9. Reasons for locomotion
B3.3.10. Adaptations for swimming in marine mammals
B4.1. Adaptation to environment
B4.1.1. Habitat as the place in which a community, species, population or organism lives
B4.1.2. Adaptations of organisms to the abiotic environment of their habitat
B4.1.3. Abiotic variables affecting species distribution
B4.1.4. Range of tolerance of a limiting factor
B4.1.5. Conditions required for coral reef formation
B4.1.6. Abiotic factors as the determinants of terrestrial biome distribution
B4.1.7. Biomes as groups of ecosystems with similar communities due to similar abiotic conditions and convergent evolution
B4.1.8. Adaptations to life in hot deserts and tropical rainforest
B4.2. Ecological niches
B4.2.1. Ecological niche as the role of a species in an ecosystem
B4.2.2. Differences between organisms that are obligate anaerobes, facultative anaerobes and obligate aerobes
B4.2.3. Photosynthesis as the mode of nutrition in plants, algae and several groups of photosynthetic prokaryotes
B4.2.4. Holozoic nutrition in animals
B4.2.5. Mixotrophic nutrition in some protists
B4.2.6. Saprotrophic nutrition in some fungi and bacteria
B4.2.7. Diversity of nutrition in archaea
B4.2.8. Relationship between dentition and the diet of omnivorous and herbivorous representative members of the family Hominidae
B4.2.9. Adaptations of herbivores for feeding on plants and of plants for resisting herbivory
B4.2.10. Adaptations of predators for finding, catching and killing prey and of prey animals for resisting predation
B4.2.11. Adaptations of plant form for harvesting light
B4.2.12. Fundamental and realized niches
B4.2.13. Competitive exclusion and the uniqueness of ecological niches
C. Interaction and interdependence
C1.1. Enzymes and metabolism
C1.1.1. Enzymes as catalysts
C1.1.2. Role of enzymes in metabolism
C1.1.3. Anabolic and catabolic reactions
C1.1.4. Enzymes as globular proteins with an active site for catalysis
C1.1.5. Interactions between substrate and active site to allow induced-fit binding
C1.1.6. Role of molecular motion and substrate-active site collisions in enzyme catalysis
C1.1.7. Relationships between the structure of the active site, enzyme–substrate specificity and denaturation
C1.1.8. Effects of temperature, pH and substrate concentration on the rate of enzyme activity
C1.1.9. Measurements in enzyme-catalysed reactions
C1.1.10. Effect of enzymes on activation energy
C1.1.11. Intracellular and extracellular enzyme-catalysed reactions
C1.1.12. Generation of heat energy by the reactions of metabolism
C1.1.13. Cyclical and linear pathways in metabolism
C1.1.14. Allosteric sites and non-competitive inhibition
C1.1.15. Competitive inhibition as a consequence of an inhibitor binding reversibly to an active site
C1.1.16. Regulation of metabolic pathways by feedback inhibition
C1.1.17. Mechanism-based inhibition as a consequence of chemical changes to the active site caused by the irreversible binding of an inhibitor
C1.2. Cell respiration
C1.2.1. ATP as the molecule that distributes energy within cells
C1.2.2. Life processes within cells that ATP supplies with energy
C1.2.3. Energy transfers during interconversions between ATP and ADP
C1.2.4. Cell respiration as a system for producing ATP within the cell using energy released from carbon compounds
C1.2.5. Differences between anaerobic and aerobic cell respiration in humans
C1.2.6. Variables affecting the rate of cell respiration
C1.2.7. Role of NAD as a carrier of hydrogen and oxidation by removal of hydrogen during cell respiration
C1.2.8. Conversion of glucose to pyruvate by stepwise reactions in glycolysis with a net yield of ATP and reduced NAD
C1.2.9. Conversion of pyruvate to lactate as a means of regenerating NAD in anaerobic cell respiration
C1.2.10. Anaerobic cell respiration in yeast and its use in brewing and baking
C1.2.11. Oxidation and decarboxylation of pyruvate as a link reaction in aerobic cell respiration
C1.2.12. Oxidation and decarboxylation of acetyl groups in the Krebs cycle with a yield of ATP and reduced NAD
C1.2.13. Transfer of energy by reduced NAD to the electron transport chain in the mitochondrion
C1.2.14. Generation of a proton gradient by flow of electrons along the electron transport chain
C1.2.15. Chemiosmosis and the synthesis of ATP in the mitochondrion
C1.2.16. Role of oxygen as terminal electron acceptor in aerobic cell respiration
C1.2.17. Differences between lipids and carbohydrates as respiratory substrates
C1.3. Photosynthesis
C1.3.1. Transformation of light energy to chemical energy when carbon compounds are produced in photosynthesis
C1.3.2. Conversion of carbon dioxide to glucose in photosynthesis using hydrogen obtained by splitting water
C1.3.3. Oxygen as a by-product of photosynthesis in plants, algae and cyanobacteria
C1.3.4. Separation and identification of photosynthetic pigments by chromatography
C1.3.5. Absorption of specific wavelengths of light by photosynthetic pigments
C1.3.6. Similarities and differences of absorption and action spectra
C1.3.7. Techniques for varying concentrations of carbon dioxide, light intensity or temperature experimentally to investigate the effects of limiting factors on the rate of photosynthesis
C1.3.8. Carbon dioxide enrichment experiments as a means of predicting future rates of photosynthesis and plant growth
C1.3.9. Photosystems as arrays of pigment molecules that can generate and emit excited electrons
C1.3.10. Advantages of the structured array of different types of pigment molecules in a photosystem
C1.3.11. Generation of oxygen by the photolysis of water in photosystem II
C1.3.12. ATP production by chemiosmosis in thylakoids
C1.3.13. Reduction of NADP by photosystem I
C1.3.14. Thylakoids as systems for performing the light-dependent reactions of photosynthesis
C1.3.15. Carbon fixation by Rubisco
C1.3.16. Synthesis of triose phosphate using reduced NADP and ATP
C1.3.17. Regeneration of RuBP in the Calvin cycle using ATP
C1.3.18. Synthesis of carbohydrates, amino acids and other carbon compounds using the products of the Calvin cycle and mineral nutrients
C1.3.19. Interdependence of the light-dependent and light-independent reactions
C2.1. Chemical signalling [HL only]
C2.1.1. Receptors as proteins with binding sites for specific signalling chemicals
C2.1.2. Cell signalling by bacteria in quorum sensing
C2.1.3. Hormones, neurotransmitters, cytokines and calcium ions as examples of functional categories of signalling chemicals in animals
C2.1.4. Chemical diversity of hormones and neurotransmitters
C2.1.5. Localized and distant effects of signalling molecules
C2.1.6. Differences between transmembrane receptors in a plasma membrane and intracellular receptors in the cytoplasm or nucleus
C2.1.7. Initiation of signal transduction pathways by receptors
C2.1.8. Transmembrane receptors for neurotransmitters and changes to membrane potential
C2.1.9. Transmembrane receptors that activate G proteins
C2.1.10. Mechanism of action of epinephrine (adrenaline) receptors
C2.1.11. Transmembrane receptors with tyrosine kinase activity
C2.1.12. Intracellular receptors that affect gene expression
C2.1.13. Effects of the hormones oestradiol and progesterone on target cells
C2.1.14. Regulation of cell signalling pathways by positive and negative feedback
C2.2. Neural signalling
C2.2.1. Neurons as cells within the nervous system that carry electrical impulses
C2.2.2. Generation of the resting potential by pumping to establish and maintain concentration gradients of sodium and potassium ions
C2.2.3. Nerve impulses as action potentials that are propagated along nerve fibres
C2.2.4. Variation in the speed of nerve impulses
C2.2.5. Synapses as junctions between neurons and between neurons and effector cells
C2.2.6. Release of neurotransmitters from a presynaptic membrane
C2.2.7. Generation of an excitatory postsynaptic potential
C2.2.8. Depolarization and repolarization during action potentials
C2.2.9. Propagation of an action potential along a nerve fibre/axon as a result of local currents
C2.2.10. Oscilloscope traces showing resting potentials and action potentials
C2.2.11. Saltatory conduction in myelinated fibres to achieve faster impulses
C2.2.12. Effects of exogenous chemicals on synaptic transmission
C2.2.13. Inhibitory neurotransmitters and generation of inhibitory postsynaptic potentials
C2.2.14. Summation of the effects of excitatory and inhibitory neurotransmitters in a postsynaptic neuron
C2.2.15. Perception of pain by neurons with free nerve endings in the skin
C2.2.16. Consciousness as a property that emerges from the interaction of individual neurons in the brain
C3.1. Integration of body systems
C3.1.1. System integration
C3.1.2. Cells, tissues, organs and body systems as a hierarchy of subsystems that are integrated in a multicellular living organism
C3.1.3. Integration of organs in animal bodies by hormonal and nervous signalling and by transport of materials and energy
C3.1.4. The brain as a central information integration organ
C3.1.5. The spinal cord as an integrating centre for unconscious processes
C3.1.6. Input to the spinal cord and cerebral hemispheres through sensory neurons
C3.1.7. Output from the cerebral hemispheres to muscles through motor neurons
C3.1.8. Nerves as bundles of nerve fibres of both sensory and motor neurons
C3.1.9. Pain reflex arcs as an example of involuntary responses with skeletal muscle as the effector
C3.1.10. Role of the cerebellum in coordinating skeletal muscle contraction and balance
C3.1.11. Modulation of sleep patterns by melatonin secretion as a part of circadian rhythms
C3.1.12. Epinephrine (adrenaline) secretion by the adrenal glands to prepare the body for vigorous activity
C3.1.13. Control of the endocrine system by the hypothalamus and pituitary gland
C3.1.14. Feedback control of heart rate following sensory input from baroreceptors and chemoreceptors
C3.1.15. Feedback control of ventilation rate following sensory input from chemoreceptors
C3.1.16. Control of peristalsis in the digestive system by the central nervous system and enteric nervous system
C3.1.17. Observations of tropic responses in seedlings
C3.1.18. Positive phototropism as a directional growth response to lateral light in plant shoots
C3.1.19. Phytohormones as signalling chemicals controlling growth, development and response to stimuli in plants
C3.1.20. Auxin efflux carriers as an example of maintaining concentration gradients of phytohormones
C3.1.21. Promotion of cell growth by auxin
C3.1.22. Interactions between auxin and cytokinin as a means of regulating root and shoot growth
C3.1.23. Positive feedback in fruit ripening and ethylene production
C3.2. Defence against disease
C3.2.1. Pathogens as the cause of infectious diseases
C3.2.2. Skin and mucous membranes as a primary defence
C3.2.3. Sealing of cuts in skin by blood clotting
C3.2.4. Differences between the innate immune system and the adaptive immune system
C3.2.5. Infection control by phagocytes
C3.2.6. Lymphocytes as cells in the adaptive immune system that cooperate to produce antibodies
C3.2.7. Antigens as recognition molecules that trigger antibody production
C3.2.8. Activation of B-lymphocytes by helper T-lymphocytes
C3.2.9. Multiplication of activated B-lymphocytes to form clones of antibody-secreting plasma cells
C3.2.10. Immunity as a consequence of retaining memory cells
C3.2.11. Transmission of HIV in body fluids
C3.2.12. Infection of lymphocytes by HIV with AIDS as a consequence
C3.2.13. Antibiotics as chemicals that block processes occurring in bacteria but not in eukaryotic cells
C3.2.14. Evolution of resistance to several antibiotics in strains of pathogenic bacteria
C3.2.15. Zoonoses as infectious diseases that can transfer from other species to humans
C3.2.16. Vaccines and immunization
C3.2.17. Herd immunity and the prevention of epidemics
C3.2.18. Evaluation of data related to the COVID-19 pandemic
C4.1. Populations and communities
C4.1.1. Populations as interacting groups of organisms of the same species living in an area
C4.1.2. Estimation of population size by random sampling
C4.1.3. Random quadrat sampling to estimate population size for sessile organisms
C4.1.4. Capture–mark–release–recapture and the Lincoln index to estimate population size for motile organisms
C4.1.5. Carrying capacity and competition for limited resources
C4.1.6. Negative feedback control of population size by density-dependent factors
C4.1.7. Population growth curves
C4.1.8. Modelling of the sigmoid population growth curve
C4.1.9. Competition versus cooperation in intraspecific relationships
C4.1.10. A community as all of the interacting organisms in an ecosystem
C4.1.11. Herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity as categories of interspecific relationship within communities
C4.1.12. Mutualism as an interspecific relationship that benefits both species
C4.1.13. Resource competition between endemic and invasive species
C4.1.14. Tests for interspecific competition
C4.1.15. Use of the chi-squared test for association between two species
C4.1.16. Predator–prey relationships as an example of density-dependent control of animal populations
C4.1.17. Top-down and bottom-up control of populations in communities
C4.1.18. Allelopathy and secretion of antibiotics
C4.2. Transfers of energy and matter
C4.2.1. Ecosystems as open systems in which both energy and matter can enter and exit
C4.2.2. Sunlight as the principal source of energy that sustains most ecosystems
C4.2.3. Flow of chemical energy through food chains
C4.2.4. Construction of food chains and food webs to represent feeding relationships in a community
C4.2.5. Supply of energy to decomposers as carbon compounds in organic matter coming from dead organisms
C4.2.6. Autotrophs as organisms that use external energy sources to synthesize carbon compounds from simple inorganic substances
C4.2.7. Use of light as the external energy source in photoautotrophs and oxidation reactions as the energy source in chemoautotrophs
C4.2.8. Heterotrophs as organisms that use carbon compounds obtained from other organisms to synthesize the carbon compounds that they require
C4.2.9. Release of energy in both autotrophs and heterotrophs by oxidation of carbon compounds in cell respiration
C4.2.10. Classification of organisms into trophic levels
C4.2.11. Construction of energy pyramids
C4.2.12. Reductions in energy availability at each successive stage in food chains due to large energy losses between trophic levels
C4.2.13. Heat loss to the environment in both autotrophs and heterotrophs due to conversion of chemical energy to heat in cell respiration
C4.2.14. Restrictions on the number of trophic levels in ecosystems due to energy losses
C4.2.15. Primary production as accumulation of carbon compounds in biomass by autotrophs
C4.2.16. Secondary production as accumulation of carbon compounds in biomass by heterotrophs
C4.2.17. Constructing carbon cycle diagrams
C4.2.18. Ecosystems as carbon sinks and carbon sources
C4.2.19. Release of carbon dioxide into the atmosphere during combustion of biomass, peat, coal, oil and natural gas
C4.2.20. Analysis of the Keeling Curve in terms of photosynthesis, respiration and combustion
C4.2.21. Dependence of aerobic respiration on atmospheric oxygen produced by photosynthesis, and of photosynthesis on atmospheric carbon dioxide produced by respiration
C4.2.22. Recycling of all chemical elements required by living organisms in ecosystems
D. Continuity and Change
D1.1. DNA replication
D1.1.1. DNA replication as production of exact copies of DNA with identical base sequences
D1.1.2. Semi-conservative nature of DNA replication and role of complementary base pairing
D1.1.3. Role of helicase and DNA polymerase in DNA replication
D1.1.4. Polymerase chain reaction and gel electrophoresis as tools for amplifying and separating DNA
D1.1.5. Applications of polymerase chain reaction and gel electrophoresis
D1.1.6. Directionality of DNA polymerases
D1.1.7. Differences between replication on the leading strand and the lagging strand
D1.1.8. Functions of DNA primase, DNA polymerase I, DNA polymerase III and DNA ligase in replication
D1.1.9. DNA proofreading
D1.2. Protein synthesis
D1.2.1. Transcription as the synthesis of RNA using a DNA template
D1.2.2. Role of hydrogen bonding and complementary base pairing in transcription
D1.2.3. Stability of DNA templates
D1.2.4. Transcription as a process required for the expression of genes
D1.2.5. Translation as the synthesis of polypeptides from mRNA
D1.2.6. Roles of mRNA, ribosomes and tRNA in translation
D1.2.7. Complementary base pairing between tRNA and mRNA
D1.2.8. Features of the genetic code
D1.2.9. Using the genetic code expressed as a table of mRNA codons
D1.2.10. Stepwise movement of the ribosome along mRNA and linkage of amino acids by peptide bonding to the growing polypeptide chain
D1.2.11. Mutations that change protein structure
D1.2.12. Directionality of transcription and translation
D1.2.13. Initiation of transcription at the promoter
D1.2.14. Non-coding sequences in DNA do not code for polypeptides
D1.2.15. Post-transcriptional modification in eukaryotic cells
D1.2.16. Alternative splicing of exons to produce variants of a protein from a single gene
D1.2.17. Initiation of translation
D1.2.18. Modification of polypeptides into their functional state
D1.2.19. Recycling of amino acids by proteasomes
D1.3. Mutations and gene editing
D1.3.1. Gene mutations as structural changes to genes at the molecular level
D1.3.2. Consequences of base substitutions
D1.3.3. Consequences of insertions and deletions
D1.3.4. Causes of gene mutation
D1.3.5. Randomness in mutation
D1.3.6. Consequences of mutation in germ cells and somatic cells
D1.3.7. Mutation as a source of genetic variation
D1.3.8. Gene knockout as a technique for investigating the function of a gene by changing it to make it inoperative
D1.3.9. Use of the CRISPR sequences and the enzyme Cas9 in gene editing
D1.3.10. Hypotheses to account for conserved or highly conserved sequences in genes
D2.1 Cell and nuclear division
D2.1.1. Generation of new cells in living organisms by cell division
D2.1.2. Cytokinesis as splitting of cytoplasm in a parent cell between daughter cells
D2.1.3. Equal and unequal cytokinesis
D2.1.4. Roles of mitosis and meiosis in eukaryotes
D2.1.5. DNA replication as a prerequisite for both mitosis and meiosis
D2.1.6. Condensation and movement of chromosomes as shared features of mitosis and meiosis
D2.1.7. Phases of mitosis
D2.1.8. Identification of phases of mitosis
D2.1.9. Meiosis as a reduction division
D2.1.10. Down syndrome and non-disjunction
D2.1.11. Meiosis as a source of variation
D2.1.12. Cell proliferation for growth, cell replacement and tissue repair
D2.1.13. Phases of the cell cycle
D2.1.14. Cell growth during interphase
D2.1.15. Control of the cell cycle using cyclins
D2.1.16. Consequences of mutations in genes that control the cell cycle
D2.1.17. Differences between tumours in rates of cell division and growth and in the capacity for metastasis and invasion of neighbouring tissue
D2.2 Gene expression [HL only]
D2.2.1. Gene expression as the mechanism by which information in genes has effects on the phenotype
D2.2.2. Regulation of transcription by proteins that bind to specific base sequences in DNA
D2.2.3. Control of the degradation of mRNA as a means of regulating translation
D2.2.4. Epigenesis as the development of patterns of differentiation in the cells of a multicellular organism
D2.2.5. Differences between the genome, transcriptome and proteome of individual cells
D2.2.6. Methylation of the promoter and histones in nucleosomes as examples of epigenetic tags
D2.2.7. Epigenetic inheritance through heritable changes to gene expression
D2.2.8. Examples of environmental effects on gene expression in cells and organisms
D2.2.9. Consequences of removal of most but not all epigenetic tags from the ovum and sperm
D2.2.10. Monozygotic twin studies
D2.2.11. External factors impacting the pattern of gene expression
D2.3 Water potential
D2.3.1. Solvation with water as the solvent
D2.3.2. Water movement from less concentrated to more concentrated solutions
D2.3.3. Water movement by osmosis into or out of cells
D2.3.4. Changes due to water movement in plant tissue bathed in hypotonic and those bathed in hypertonic solutions
D2.3.5. Effects of water movement on cells that lack a cell wall
D2.3.6. Effects of water movement on cells with a cell wall
D2.3.7. Medical applications of isotonic solutions
D2.3.8. Water potential as the potential energy of water per unit volume
D2.3.9. Movement of water from higher to lower water potential
D2.3.10. Contributions of solute potential and pressure potential to the water potential of cells with walls
D2.3.11. Water potential and water movements in plant tissue
D3.1 Reproduction
D3.1.1. Differences between sexual and asexual reproduction
D3.1.2. Role of meiosis and fusion of gametes in the sexual life cycle
D3.1.3. Differences between male and female sexes in sexual reproduction
D3.1.4. Anatomy of the human male and female reproductive systems
D3.1.5. Changes during the ovarian and uterine cycles and their hormonal regulation
D3.1.6. Fertilization in humans
D3.1.7. Use of hormones in in vitro fertilization (IVF) treatment
D3.1.8. Sexual reproduction in flowering plants
D3.1.9. Features of an insect-pollinated flower
D3.1.10. Methods of promoting cross-pollination
D3.1.11. Self-incompatibility mechanisms to increase genetic variation within a species
D3.1.12. Dispersal and germination of seeds
D3.1.13. Control of the developmental changes of puberty by gonadotropin-releasing hormone and steroid sex hormones
D3.1.14. Spermatogenesis and oogenesis in humans
D3.1.15. Mechanisms to prevent polyspermy
D3.1.16. Development of a blastocyst and implantation in the endometrium
D3.1.17. Pregnancy testing by detection of human chorionic gonadotropin secretion
D3.1.18. Role of the placenta in foetal development inside the uterus
D3.1.19. Hormonal control of pregnancy and childbirth
D3.1.20. Hormone replacement therapy and the risk of coronary heart disease
D3.2 Inheritance
D3.2.1. Production of haploid gametes in parents and their fusion to form a diploid zygote as the means of inheritance
D3.2.2. Methods for conducting genetic crosses in flowering plants
D3.2.3. Genotype as the combination of alleles inherited by an organism
D3.2.4. Phenotype as the observable traits of an organism resulting from genotype and environmental factors
D3.2.5. Effects of dominant and recessive alleles on phenotype
D3.2.6. Phenotypic plasticity as the capacity to develop traits suited to the environment experienced by an organism, by varying patterns of gene expression
D3.2.7. Phenylketonuria as an example of a human disease due to a recessive allele
D3.2.8. Single-nucleotide polymorphisms and multiple alleles in gene pools
D3.2.9. ABO blood groups as an example of multiple alleles
D3.2.10. Incomplete dominance and codominance
D3.2.11. Sex determination in humans and inheritance of genes on sex chromosomes
D3.2.12. Haemophilia as an example of a sex-linked genetic disorder
D3.2.13. Pedigree charts to deduce patterns of inheritance of genetic disorders
D3.2.14. Continuous variation due to polygenic inheritance and/or environmental factors
D3.2.15. Box-and-whisker plots to represent data for a continuous variable such as student height
D3.2.16. Segregation and independent assortment of unlinked genes in meiosis
D3.2.17. Punnett grids for predicting genotypic and phenotypic ratios in dihybrid crosses involving pairs of unlinked autosomal genes
D3.2.18. Loci of human genes and their polypeptide products
D3.2.19. Autosomal gene linkage
D3.2.20. Recombinants in crosses involving two linked or unlinked genes
D3.2.21. Use of a chi-squared test on data from dihybrid crosses
D3.3 Homeostasis
D3.3.1. Homeostasis as maintenance of the internal environment of an organism
D3.3.2. Negative feedback loops in homeostasis
D3.3.3. Regulation of blood glucose as an example of the role of hormones in homeostasis
D3.3.4. Physiological changes that form the basis of type 1 and type 2 diabetes
D3.3.5. Thermoregulation as an example of negative feedback control
D3.3.6. Thermoregulation mechanisms in humans
D3.3.7. Role of the kidney in osmoregulation and excretion
D3.3.8. Role of the glomerulus, Bowman’s capsule and proximal convoluted tubule in excretion
D3.3.9. Role of the loop of Henle
D3.3.10. Osmoregulation by water reabsorption in the collecting ducts
D3.3.11. Changes in blood supply to organs in response to changes in activity
D4.1 Natural selection
D4.1.1. Natural selection as the mechanism driving evolutionary change
D4.1.2. Roles of mutation and sexual reproduction in generating the variation on which natural selection acts
D4.1.3. Overproduction of offspring and competition for resources as factors that promote natural selection
D4.1.4. Abiotic factors as selection pressures
D4.1.5. Differences between individuals in adaptation, survival and reproduction as the basis for natural selection
D4.1.6. Requirement that traits are heritable for evolutionary change to occur
D4.1.7. Sexual selection as a selection pressure in animal species
D4.1.8. Modelling of sexual and natural selection based on experimental control of selection pressures
D4.1.9. Concept of the gene pool
D4.1.10. Allele frequencies of geographically isolated populations
D4.1.11. Changes in allele frequency in the gene pool as a consequence of natural selection between individuals according to differences in their heritable traits
D4.1.12. Differences between directional, disruptive and stabilizing selection
D4.1.13. Hardy–Weinberg equation and calculations of allele or genotype frequencies
D4.1.14. Hardy–Weinberg conditions that must be maintained for a population to be in genetic equilibrium
D4.1.15. Artificial selection by deliberate choice of traits
D4.2 Stability and change
D4.2.1. Stability as a property of natural ecosystems
D4.2.2. Requirements for stability in ecosystems
D4.2.3. Deforestation of Amazon rainforest as an example of a possible tipping point in ecosystem stability
D4.2.4. Use of a model to investigate the effect of variables on ecosystem stability
D4.2.5. Role of keystone species in the stability of ecosystems
D4.2.6. Assessing sustainability of resource harvesting from natural ecosystems
D4.2.7. Factors affecting the sustainability of agriculture
D4.2.8. Eutrophication of aquatic and marine ecosystems due to leaching
D4.2.9. Biomagnification of pollutants in natural ecosystems
D4.2.10. Effects of microplastic and macroplastic pollution of the oceans
D4.2.11. Restoration of natural processes in ecosystems by rewilding
D4.2.12. Ecological succession and its causes
D4.2.13. Changes occurring during primary succession
D4.2.14. Cyclical succession in ecosystems
D4.2.15. Climax communities and arrested succession
D4.3 Climate Change
D4.3.1. Anthropogenic causes of climate change
D4.3.2. Positive feedback cycles in global warming
D4.3.3. Change from net carbon accumulation to net loss in boreal forests as an example of a tipping point
D4.3.4. Melting of landfast ice and sea ice as examples of polar habitat change
D4.3.5. Changes in ocean currents altering the timing and extent of nutrient upwelling
D4.3.6. Poleward and upslope range shifts of temperate species
D4.3.7. Threats to coral reefs as an example of potential ecosystem collapse
D4.3.8. Afforestation, forest regeneration and restoration of peat-forming wetlands as approaches to carbon sequestration
D4.3.9. Phenology as research into the timing of biological events
D4.3.10. Disruption to the synchrony of phenological events by climate change
D4.3.11. Increases to the number of insect life cycles within a year due to climate change
D4.3.12. Evolution as a consequence of climate change
Nature of Science